Fix viewport dimension acquisition and preserve measurement scaling

docs/viewport-dimensions.md

@@ -0,0 +1,48 @@
+# Dimensions through paper-space viewports
+
+Paper-space point commands can snap to displayed model geometry through a planar,
+orthographic layout viewport. The acquired result retains the original model
+coordinate, its paper coordinate, the viewport frame, and feature identity.
+Ordinary selection continues to operate on the active drawing space.
+
+`DIMLINEAR`, `DIMALIGNED`, `DIMANGULAR`, `DIMRADIUS`, and `DIMDIAMETER` use this
+acquisition context. The supported object-pick modes also query viewport geometry.
+Snapping and object picking share viewport draw order, visibility, and clipping
+rules. Object picking uses the resident interaction index and applies block
+instance transforms. Nonuniformly transformed circles and circular arcs are
+declined as radial object picks because their displayed geometry is elliptical.
+
+## Input and measurement rules
+
+- Free and typed sheet points measure paper units, including points inside a
+  viewport rectangle.
+- Measuring points must all belong to the same space and, when applicable, the
+  same viewport. An incompatible pick leaves the command at its current step.
+- Dimension-line and text placement do not change the measurement's space.
+- Paper geometry takes priority over viewport geometry at an object pick.
+- The final dimension style is applied before computing measurement scaling.
+  Length dimensions keep paper-space definition points and a negative `DIMLFAC`
+  override containing the effective user factor multiplied by `1 / viewport scale`.
+  Angular measurements do not use this length factor.
+- `DIMASSOC=0` uses the same compensated value before exploding the annotation.
+
+## Persistent measurement data
+
+`OCS_VIEWPORT_MEASUREMENT` XData stores three values: version `1` (integer16),
+the positive user factor (real), and viewport compensation (real). This keeps the
+user's unit conversion separate from viewport scale. The ordinary `DIMLFAC`
+override remains sufficient for displaying the saved measurement in other CAD
+readers. The data survives DXF/DWG saves and undo/redo.
+
+The dependent association change uses this metadata when viewport scale changes.
+This creation change alone creates nonassociative viewport dimensions and leaves
+the drawing's `DIMASSOC` setting unchanged. Direct model/paper associations retain
+their existing behavior.
+
+## Validation
+
+`src/app/viewport_dimension_tests.rs` exercises snap-engine acquisition, command
+commits for all five commands, object picks, input retries, active positive and
+negative style factors, exploded dimensions, undo/redo, DXF/DWG persistence,
+clipping, overlapping viewports, pan, scale, and twist. These are headless tests;
+they do not establish visual acceptance in the desktop UI.

src/app/command_driver.rs

@@ -388,7 +388,9 @@ impl OpenCADStudio {
             // accepted-snap list must stay index-parallel with the points the
             // command collects (PR1). Interactive picks record themselves in
             // the click handler and never reach here.
-            self.record_accepted_snap(None, None, *point);
+            if !self.record_accepted_snap(i, None, None, *point) {
+                return Task::none();
+            }
         }
         if default_start {
             let StepInput::Point(point) = &input else {
@@ -402,6 +404,7 @@ impl OpenCADStudio {
             self.push_ucs_to_cmd(i);
         }
         if let StepInput::EntityPick(handle, point) = &input {
+            if !self.dimension_acquisition_allowed(i, None) { return Task::none(); }
             let solid_pick = matches!(
                 self.tabs[i].scene.document.get_entity(*handle),
                 Some(
@@ -474,6 +477,7 @@ impl OpenCADStudio {
         }
         let ctrl = self.ctrl_down;
         let shift = self.shift_down;
+        let picked_handle = if let StepInput::EntityPick(h, _) = &input { Some(*h) } else { None };
         let result: Option<CmdResult> = {
             let Some(cmd) = self.tabs[i].active_cmd.as_mut() else {
                 return Task::none();
@@ -492,6 +496,10 @@ impl OpenCADStudio {
                 StepInput::Escape => Some(cmd.on_escape()),
             }
         };
+        if let Some(handle) = picked_handle {
+            self.record_dimension_entity_points(i, None, handle, Vec::new());
+        }
+        self.sync_dimension_snaps(i);
         match result {
             Some(r) => self.apply_cmd_result(r),
             None => Task::none(),
@@ -2357,7 +2365,12 @@ impl OpenCADStudio {
                 // A dimension placed on the sheet but measuring model geometry
                 // through a viewport carries the compensation as a negative
                 // DIMLFAC override.
-                self.apply_viewport_dimension_measurement(i, &mut entity);
+                if !preserve_base_style {
+                    crate::scene::creation_style::apply_current_creation_styles(&self.tabs[i].scene.document, &mut entity);
+                }
+                if !self.apply_viewport_dimension_measurement(i, &mut entity) {
+                    return Task::none();
+                }
                 // ...and is deliberately *not* associated in paper space: its
                 // definition points are projected paper coordinates, so a
                 // paper-space association would bind it to geometry that is

src/app/dim_viewport.rs

@@ -1,262 +1,162 @@
-//! Viewport-aware dimension measurement (PR2).
-//!
-//! A dimension created in a paper-space layout while measuring geometry seen
-//! through a content viewport must *place* itself in paper coordinates but
-//! *report* the model measurement. Because a viewport frame is a similarity
-//! (uniform scale plus in-plane twist), the model measurement of any linear,
-//! aligned or radial span is exactly the raw paper measurement times the
-//! viewport compensation `1 / scale` -- projecting the model span onto the
-//! rotated paper axis and compensating is the same number. So no geometry is
-//! rewritten: the compensation is persisted once, as a negative DIMLFAC
-//! override on the dimension (see
-//! [`MeasurementScale::viewport_dimlfac_override`]), and applied once, by the
-//! linear formatter.
-//!
-//! Angular dimensions are never compensated -- a similarity preserves angles.
-//!
-//! ## Where the snaps come from
-//! The per-point [`AcceptedSnap`] list is PR1's
-//! ([`OpenCADStudio::accepted_snaps`]): the click handler records the real
-//! snap for every interactive pick, and `apply_step_input` records a free
-//! snap for typed / dynamic-input / headless points. There is exactly one
-//! source of truth. Two things are layered on top here:
-//!
-//! * the explicit object-pick path ([`Scene::dimension_pick_through_viewport`])
-//!   is not a point step at all, so it records its own snaps
-//!   ([`OpenCADStudio::record_dimension_viewport_pick`]);
-//! * a *typed* coordinate carries no snap, so PR1 records it as a free point
-//!   with no viewport. [`OpenCADStudio::measure_snap`] re-attaches the
-//!   content viewport under such a point, which is what makes
-//!   `DIMLINEAR` + typed coordinates inside a 1:10 viewport measure the model.
-//!   It is deliberately limited to snaps that are free *and* source-less, so a
-//!   genuine paper-sheet snap that happens to sit over a viewport is never
-//!   reinterpreted.
-
-use acadrust::entities::Dimension;
-use acadrust::types::Handle;
-use acadrust::xdata::XDataValue;
-use acadrust::EntityType;
-use glam::DVec3;
+//! Paper-space dimension acquisition and measurement. Only acquired geometry
+//! carries viewport compensation; free and typed sheet coordinates stay on the sheet.
 
+use super::OpenCADStudio;
 use crate::command::DimensionAssociationSource;
 use crate::entities::dim_override;
 use crate::scene::viewport_ref::{AcceptedSnap, MeasurementScale, SnapSourceRef, ViewportFrame};
+use acadrust::entities::Dimension;
+use acadrust::types::Handle;
+use acadrust::EntityType;
+use glam::DVec3;
 
-use super::OpenCADStudio;
-
-/// How a pending dimension should be measured.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub(crate) enum DimensionMeasureSpace {
-    /// Plain measurement in the space the definition points live in.
     Direct,
-    /// Every measuring point came through one viewport; compensate by its
-    /// scale.
     Viewport(ViewportFrame),
-    /// The points came from different viewports, or from a viewport and the
-    /// bare sheet. Falls back to `Direct` with a warning — never a silently
-    /// wrong number.
     Mixed,
 }
 
 impl OpenCADStudio {
-    /// Record the accepted snaps produced by the explicit object-pick path (a
-    /// dimension "select object" pick resolved through a content viewport).
-    ///
-    /// An object pick is not a point step, so PR1's click handler never sees
-    /// it. One pick supplies *both* extension origins, so it is recorded
-    /// twice: that keeps the list index-parallel with the definition points
-    /// the command derives from the entity, and lets the "every measuring
-    /// point came through one viewport" classification see a complete set.
-    pub(crate) fn record_dimension_viewport_pick(
+    pub(crate) fn finish_command_click(&mut self, i: usize) {
+        let mut selection = self.tabs[i].scene.selection.borrow_mut();
+        selection.left_down = false;
+        selection.left_press_pos = None;
+        selection.left_press_time = None;
+        selection.left_dragging = false;
+    }
+
+    #[allow(dead_code)] // Consumed by the dependent association change.
+    pub(crate) fn dimension_measuring_snaps(&self, _i: usize) -> Vec<AcceptedSnap> {
+        self.accepted_snaps().to_vec()
+    }
+
+    pub(crate) fn dimension_measure_space(&self, i: usize) -> DimensionMeasureSpace {
+        let scene = &self.tabs[i].scene;
+        if scene.current_layout == "Model" || scene.active_viewport.is_some() {
+            return DimensionMeasureSpace::Direct;
+        }
+        let mut frame: Option<ViewportFrame> = None;
+        let mut direct = false;
+        for snap in self.accepted_snaps() {
+            match snap.frame {
+                Some(f) => match frame {
+                    None => frame = Some(f),
+                    Some(previous) if previous.viewport == f.viewport => {}
+                    Some(_) => return DimensionMeasureSpace::Mixed,
+                },
+                None => direct = true,
+            }
+        }
+        match (frame, direct) {
+            (None, _) => DimensionMeasureSpace::Direct,
+            (Some(f), false) => DimensionMeasureSpace::Viewport(f),
+            _ => DimensionMeasureSpace::Mixed,
+        }
+    }
+
+    /// Reject an incompatible measuring input before it advances the command.
+    pub(crate) fn dimension_acquisition_allowed(
         &mut self,
-        frame: ViewportFrame,
-        paper_point: DVec3,
-        model_point: DVec3,
+        i: usize,
+        viewport: Option<Handle>,
+    ) -> bool {
+        if !self.tabs[i]
+            .active_cmd
+            .as_ref()
+            .is_some_and(|c| c.measures_through_viewports())
+        {
+            return true;
+        }
+        if self.accepted_snaps().iter().any(|s| s.viewport != viewport) {
+            self.command_line.push_error(crate::t!("Pick geometry from the same viewport, or use paper-space points for the whole dimension.").as_ref());
+            return false;
+        }
+        true
+    }
+
+    /// A command can reject a degenerate point without advancing its step.
+    pub(crate) fn sync_dimension_snaps(&mut self, i: usize) {
+        if let Some(cmd) = self.tabs[i]
+            .active_cmd
+            .as_ref()
+            .filter(|c| c.measures_through_viewports())
+        {
+            self.accepted_snaps
+                .truncate(cmd.dimension_acquired_points().len());
+        }
+    }
+
+    /// Record each definition point actually acquired by an object pick.
+    /// Arcs may supply three angular slots, lines two, and radial picks one.
+    pub(crate) fn record_dimension_entity_points(
+        &mut self,
+        i: usize,
+        frame: Option<ViewportFrame>,
         entity: Handle,
         block_path: Vec<Handle>,
     ) {
-        for _ in 0..2 {
+        let Some(cmd) = self.tabs[i]
+            .active_cmd
+            .as_ref()
+            .filter(|c| c.measures_through_viewports())
+        else {
+            return;
+        };
+        let points = cmd.dimension_acquired_points();
+        for paper_point in points.into_iter().skip(self.accepted_snaps.len()) {
             self.push_accepted_snap(AcceptedSnap {
                 paper_point,
-                model_point,
-                viewport: Some(frame.viewport),
-                frame: Some(frame),
+                model_point: frame.map_or(paper_point, |f| f.paper_to_model(paper_point)),
+                viewport: frame.map(|f| f.viewport),
+                frame,
                 source: Some(SnapSourceRef {
                     source: DimensionAssociationSource::inferred(entity),
                     block_path: block_path.clone(),
-                    // An object pick acquires the whole curve, not one
-                    // feature; the per-slot feature point is derived from the
-                    // committed dimension's own definition points.
-                    snap_type: crate::snap::SnapType::Nearest,
+                    snap_type: crate::snap::SnapType::ObjectPick,
+                    intersection: None,
                 }),
             });
         }
     }
 
-    /// The accepted snap for the `idx`-th collected point of the active
-    /// command, with the typed-coordinate viewport fallback applied.
-    ///
-    /// Reads PR1's list -- there is no second store.
-    #[allow(dead_code)]
-    pub(crate) fn accepted_snap_for_point(&self, i: usize, idx: usize) -> Option<AcceptedSnap> {
-        self.accepted_snaps().get(idx).map(|snap| self.measure_snap(i, snap))
-    }
-
-    /// One accepted snap as *measurement* sees it.
-    ///
-    /// Identical to the recorded snap except that a point which carries no
-    /// snap at all (a typed coordinate, a dynamic-input point, a headless
-    /// script point) and lands inside a content viewport is re-attached to
-    /// that viewport, so typing `100,50` inside a 1:10 viewport measures the
-    /// model just like clicking there would. A snap that landed on real
-    /// paper-sheet geometry keeps `viewport: None` and is never reinterpreted.
-    pub(crate) fn measure_snap(&self, i: usize, snap: &AcceptedSnap) -> AcceptedSnap {
-        if snap.frame.is_some() || snap.source.is_some() {
-            return snap.clone();
-        }
-        let scene = &self.tabs[i].scene;
-        if scene.current_layout == "Model" || scene.active_viewport.is_some() {
-            return snap.clone();
-        }
-        match scene.viewport_frame_at_paper_point(snap.paper_point) {
-            Some(frame) => AcceptedSnap {
-                paper_point: snap.paper_point,
-                model_point: frame.paper_to_model(snap.paper_point),
-                viewport: Some(frame.viewport),
-                frame: Some(frame),
-                source: None,
-            },
-            None => snap.clone(),
-        }
-    }
-
-    /// The accepted snaps that say what the pending dimension *measures*, in
-    /// collection order.
-    ///
-    /// The final collected point of every dimension command in scope is the
-    /// dimension-line / text placement click, which says nothing about what is
-    /// being measured -- it routinely lands on bare sheet even for a viewport
-    /// measurement, and inside a viewport rectangle even for a plain paper
-    /// one. It is dropped here, which is also why association inference is
-    /// guarded on this set rather than on every recorded snap.
-    pub(crate) fn dimension_measuring_snaps(&self, i: usize) -> Vec<AcceptedSnap> {
-        let all = self.accepted_snaps();
-        let measuring = match all.len() {
-            0 | 1 => all,
-            n => &all[..n - 1],
-        };
-        measuring.iter().map(|snap| self.measure_snap(i, snap)).collect()
-    }
-
-    /// Classify how the pending dimension should be measured from the snaps
-    /// recorded for it.
-    pub(crate) fn dimension_measure_space(&self, i: usize) -> DimensionMeasureSpace {
-        let scene = &self.tabs[i].scene;
-        // Only a dimension being created *on the sheet* can measure through a
-        // viewport. Inside MSPACE the command already works in model space.
-        if scene.current_layout == "Model" || scene.active_viewport.is_some() {
-            return DimensionMeasureSpace::Direct;
-        }
-        let measuring = self.dimension_measuring_snaps(i);
-        let measuring = measuring.as_slice();
-        if measuring.is_empty() {
-            return DimensionMeasureSpace::Direct;
-        }
-        let mut frame: Option<ViewportFrame> = None;
-        let mut any_free = false;
-        for snap in measuring {
-            match snap.frame {
-                Some(f) => match frame {
-                    None => frame = Some(f),
-                    Some(existing) if existing.viewport == f.viewport => {}
-                    Some(_) => return DimensionMeasureSpace::Mixed,
-                },
-                None => any_free = true,
-            }
-        }
-        match (frame, any_free) {
-            (None, _) => DimensionMeasureSpace::Direct,
-            (Some(f), false) => DimensionMeasureSpace::Viewport(f),
-            // One origin through a viewport and the other on the sheet: the
-            // two points are not in a common measurable space.
-            (Some(_), true) => DimensionMeasureSpace::Mixed,
-        }
-    }
-
-    /// Apply viewport compensation to a dimension about to be committed.
-    ///
-    /// Whether the dimension may also take a paper-space association is a
-    /// separate question with the same answer source:
-    /// [`OpenCADStudio::dimension_association_allowed`].
     pub(crate) fn apply_viewport_dimension_measurement(
         &mut self,
         i: usize,
         entity: &mut EntityType,
-    ) {
+    ) -> bool {
         let EntityType::Dimension(dimension) = entity else {
-            return;
+            return true;
         };
-        let is_angular = matches!(
+        let angular = matches!(
             dimension,
             Dimension::Angular2Ln(_) | Dimension::Angular3Pt(_)
         );
         let frame = match self.dimension_measure_space(i) {
-            DimensionMeasureSpace::Direct => return,
+            DimensionMeasureSpace::Direct => return true,
             DimensionMeasureSpace::Mixed => {
-                self.command_line.push_warning(
-                    crate::t!(
-                        "Mixed-space measurement is not supported: the extension origins come from different viewports, or from a viewport and the sheet. Measuring in paper space instead (no viewport scale compensation)."
-                    )
-                    .as_ref(),
-                );
-                return;
+                self.command_line.push_error(crate::t!("Pick geometry from the same viewport, or use paper-space points for the whole dimension.").as_ref());
+                return false;
             }
-            DimensionMeasureSpace::Viewport(frame) => frame,
+            DimensionMeasureSpace::Viewport(f) => f,
         };
-
-        // A similarity preserves angles, so an angular dimension reads the
-        // model angle already and needs no override.
-        if is_angular {
-            return;
+        if angular {
+            return true;
         }
-
-        let compensation = frame.paper_to_model_length_factor();
-        let style_dimlfac = self.pending_dimension_style_dimlfac(i, entity);
-        if let Some(value) =
-            MeasurementScale::viewport_dimlfac_override(style_dimlfac, compensation)
-        {
-            // The negative DIMLFAC convention is resolved from the dimension's
-            // owner block, and the commit path (or, with DIMASSOC = 0, the
-            // explode path) reads the entity before `add_entity_to_layout`
-            // assigns one. Stamping the layout block here — the same handle
-            // the add would set — keeps the entity self-describing, so the
-            // text is compensated whichever path consumes it.
-            let layout_block = self.tabs[i].scene.current_layout_block_handle_pub();
-            if layout_block.is_valid() {
-                entity.common_mut().owner_handle = layout_block;
-            }
-            dim_override::set_on_entity(
-                entity,
-                dim_override::DIMLFAC,
-                Some(XDataValue::Real(value)),
-            );
+        let dimlfac = self.pending_dimension_style_dimlfac(i, entity);
+        let scale = MeasurementScale {
+            user_lfac: MeasurementScale::user_lfac_for_space(dimlfac, true),
+            viewport_compensation: frame.paper_to_model_length_factor(),
+        };
+        if !scale.paper_factor().is_finite() || scale.paper_factor() <= 0.0 {
+            return false;
         }
+        // Owner is needed even by DIMASSOC=0's explode path, before insertion.
+        entity.common_mut().owner_handle = self.tabs[i].scene.current_layout_block_handle_pub();
+        scale.write_to_entity(entity);
+        true
     }
 
-    /// Explicit "select object" pick for a dimension command, resolved through
-    /// a paper-space content viewport.
-    ///
-    /// Paper-space hit testing only sees sheet entities, so a click on model
-    /// geometry displayed inside a viewport finds nothing. This maps the
-    /// cursor into model space through the viewport frame, hit-tests the
-    /// viewport's resident model wires (descending into block instances and
-    /// baking the instance transform), then hands the command a copy of the
-    /// picked entity projected onto the sheet. The commands therefore keep
-    /// working entirely in paper coordinates, and the measurement is
-    /// compensated at commit time like any other viewport measurement.
-    ///
-    /// Returns `None` when the click is not inside a content viewport or hits
-    /// nothing, so the caller falls back to the ordinary paper-space pick.
     pub(crate) fn try_dimension_viewport_entity_pick(
         &mut self,
         i: usize,
@@ -266,42 +166,38 @@ impl OpenCADStudio {
         let pick = self.tabs[i]
             .scene
             .dimension_pick_through_viewport(paper, aperture_paper)?;
-        self.record_dimension_viewport_pick(
-            pick.frame,
-            pick.paper_point,
-            pick.model_point,
-            pick.entity_handle,
-            pick.block_path.clone(),
-        );
+        if !self.dimension_acquisition_allowed(i, Some(pick.frame.viewport)) {
+            return Some(crate::command::CmdResult::NeedPoint);
+        }
         let command = self.tabs[i].active_cmd.as_mut()?;
         command.inject_picked_entity(pick.paper_entity);
-        Some(command.on_entity_pick(pick.entity_handle, pick.paper_point))
+        let result = command.on_entity_pick(pick.entity_handle, pick.paper_point);
+        self.record_dimension_entity_points(
+            i,
+            Some(pick.frame),
+            pick.entity_handle,
+            pick.block_path,
+        );
+        Some(result)
     }
 
-    /// The DIMLFAC the pending dimension would inherit *without* a viewport
-    /// override: its own existing override if it has one, otherwise its named
-    /// dimension style's value.
     fn pending_dimension_style_dimlfac(&self, i: usize, entity: &EntityType) -> f64 {
         let EntityType::Dimension(dimension) = entity else {
             return 1.0;
         };
-        if let Some(value) = dim_override::real(
+        dim_override::real(
             &dimension.base().common.extended_data,
             dim_override::DIMLFAC,
-        ) {
-            return value;
-        }
-        let name = dimension.base().style_name.as_str();
-        self.tabs[i]
-            .scene
-            .document
-            .dim_styles
-            .iter()
-            .find(|style| {
-                style.name.eq_ignore_ascii_case(name)
-                    || (name.trim().is_empty() && style.name.eq_ignore_ascii_case("Standard"))
-            })
-            .map(|style| style.dimlfac)
-            .unwrap_or(1.0)
+        )
+        .or_else(|| {
+            self.tabs[i]
+                .scene
+                .document
+                .dim_styles
+                .iter()
+                .find(|s| s.name.eq_ignore_ascii_case(&dimension.base().style_name))
+                .map(|s| s.dimlfac)
+        })
+        .unwrap_or(1.0)
     }
 }

src/app/mod.rs

@@ -11,6 +11,8 @@ mod annotation_data;
 mod command_driver;
 pub(crate) mod commands;
 pub(crate) mod dim_viewport;
+#[cfg(test)]
+mod viewport_dimension_tests;
 mod document;
 mod drafting_settings;
 pub(crate) mod expr_eval;

src/app/update/viewport.rs

@@ -2374,6 +2374,8 @@ impl OpenCADStudio {
                         extension_dir: None,
                         viewport: None,
                         source: None,
+                        secondary_source: None,
+                        model_point: None,
                     });
                     if let Some(cmd) = self.tabs[i].active_cmd.as_mut() {
                         cmd.set_acquisition_hint(Some(pick.label));
@@ -3853,29 +3855,7 @@ impl OpenCADStudio {
                 command.set_ctrl(self.ctrl_down);
                 command.set_shift(self.shift_down);
             }
-            // PR2: a dimension "select object" pick that lands inside a paper
-            // content viewport is resolved against the model geometry that
-            // viewport displays. Paper hit testing only sees sheet entities,
-            // so this runs first and falls through when it finds nothing.
-            let dimension_viewport_pick = if self.tabs[i].active_cmd.as_ref().is_some_and(
-                |command| command.needs_entity_pick() && command.measures_through_viewports(),
-            ) {
-                let aperture_paper = {
-                    let camera = self.tabs[i].scene.camera.borrow();
-                    let per_px = if bounds.height > 0.0 {
-                        camera.ortho_size() as f64 * 2.0 / bounds.height as f64
-                    } else {
-                        0.0
-                    };
-                    crate::ui::overlay::pick_box_aperture_px(self.pick_box) as f64 * per_px
-                };
-                self.try_dimension_viewport_entity_pick(i, pick_wcs, aperture_paper)
-            } else {
-                None
-            };
-            let result = if let Some(result) = dimension_viewport_pick {
-                Some(result)
-            } else if self.tabs[i]
+            let result = if self.tabs[i]
                 .active_cmd
                 .as_ref()
                 .map(|c| c.needs_structure_point_pick())
@@ -4065,6 +4045,7 @@ impl OpenCADStudio {
                         }
                     }
 
+                    if !self.dimension_acquisition_allowed(i, None) { self.finish_command_click(i); return Task::none(); }
                     let shift = self.shift_down;
                     let result = self.tabs[i].active_cmd.as_mut().map(|c| {
                         // Shift-swap state for TRIM/EXTEND (#336).
@@ -4072,6 +4053,7 @@ impl OpenCADStudio {
                         c.set_entity_pick_direction(entity_pick_direction);
                         c.on_entity_pick(handle, entity_pick_point)
                     });
+                    self.record_dimension_entity_points(i, None, handle, Vec::new());
                     // HATCHEDIT: after pick, inject hatch model data into the command.
                     if self.tabs[i]
                         .active_cmd
@@ -4142,11 +4124,24 @@ impl OpenCADStudio {
                         }
                     }
                     result
+                } else if let Some(result) = {
+                    let dimension = self.tabs[i].active_cmd.as_ref().is_some_and(|c| c.measures_through_viewports());
+                    if dimension {
+                        let per_px = self.tabs[i].scene.camera.borrow().ortho_size() as f64 * 2.0 / (bounds.height as f64).max(1.0);
+                        self.try_dimension_viewport_entity_pick(i, pick_wcs, crate::ui::overlay::pick_box_aperture_px(self.pick_box) as f64 * per_px)
+                    } else { None }
+                } {
+                    Some(result)
                 } else if self.tabs[i]
                     .active_cmd
                     .as_ref()
                     .is_some_and(|command| command.entity_pick_accepts_points())
                 {
+                    let pending = self.pending_click_snap.take();
+                    if !self.record_accepted_snap(i, pending.map(|(hit, _)| hit), pending.and_then(|(_, frame)| frame), pick_wcs) {
+                        self.finish_command_click(i);
+                        return Task::none();
+                    }
                     self.refresh_command_point_pick_context(i);
                     self.tabs[i]
                         .active_cmd
@@ -4215,17 +4210,18 @@ impl OpenCADStudio {
                         self.tabs[i].dyn_active = 0;
                     }
                 }
-                self.last_point = Some(world_pt);
                 // Record what this point step accepted, alongside the point
                 // itself (PR1): paper point, model point, viewport, frame and
                 // geometry identity. Commands that don't care keep consuming
                 // `world_pt` exactly as before.
                 let pending = self.pending_click_snap.take();
-                self.record_accepted_snap(
+                if !self.record_accepted_snap(
+                    i,
                     pending.map(|(hit, _)| hit),
                     pending.and_then(|(_, frame)| frame),
                     world_pt,
-                );
+                ) { self.finish_command_click(i); return Task::none(); }
+                self.last_point = Some(world_pt);
                 // The one-shot snap override is spent by this pick —
                 // restore the running osnap configuration (#337).
                 self.snapper.clear_override();
@@ -4260,6 +4256,7 @@ impl OpenCADStudio {
                 }
             };
 
+            self.sync_dimension_snaps(i);
             if let Some(r) = result {
                 let task = self.apply_cmd_result(r);
                 let mut sel = self.tabs[i].scene.selection.borrow_mut();

src/app/update/viewport_snap.rs

@@ -102,10 +102,11 @@ impl OpenCADStudio {
     /// after ortho / polar / axis-lock / dynamic-input resolution.
     pub(crate) fn record_accepted_snap(
         &mut self,
+        tab: usize,
         snap: Option<SnapResult>,
         frame: Option<ViewportFrame>,
         committed: glam::DVec3,
-    ) {
+    ) -> bool {
         let accepted = match snap {
             // A viewport snap keeps its frame only when the hit really came
             // through it, so a paper-sheet snap never claims a model point.
@@ -115,7 +116,24 @@ impl OpenCADStudio {
             }
             None => AcceptedSnap::free(committed),
         };
+        if self.tabs[tab]
+            .active_cmd
+            .as_ref()
+            .is_some_and(|cmd| cmd.measures_through_viewports())
+        {
+            if self.tabs[tab]
+                .active_cmd
+                .as_ref()
+                .is_some_and(|cmd| cmd.dimension_placement_pending())
+            {
+                return true;
+            }
+            if !self.dimension_acquisition_allowed(tab, accepted.viewport) {
+                return false;
+            }
+        }
         self.push_accepted_snap(accepted);
+        true
     }
 
     /// Append an already-built accepted snap, keeping the retention cap.
@@ -165,30 +183,12 @@ impl OpenCADStudio {
             return None;
         }
 
-        let handles = self.tabs[i].scene.layout_content_viewports();
-        // Draw order puts the last-drawn viewport on top; probe top-most first
-        // so overlapping viewports resolve the same way they render.
-        for &handle in handles.iter().rev() {
-            if !self.tabs[i].scene.viewport_displays_content(handle) {
-                continue;
-            }
-            let Some(frame) = self.tabs[i].scene.viewport_frame(handle) else {
-                continue; // oblique / perspective viewport: unsupported
-            };
-            // Clip first (rectangle + non-rectangular clip boundary), in paper
-            // coordinates, so a cursor over the cut-away part of a clipped
-            // viewport sees nothing.
-            if !self
-                .tabs[i]
-                .scene
-                .viewport_displays_paper_point(handle, cursor_paper.truncate())
-            {
-                continue;
-            }
-            let Some((cam, rect)) = self
-                .tabs[i]
-                .scene
-                .viewport_edit_frame_for(handle, canvas)
+        for frame in self.tabs[i]
+            .scene
+            .viewport_frames_at_paper_point(cursor_paper)
+        {
+            let handle = frame.viewport;
+            let Some((cam, rect)) = self.tabs[i].scene.viewport_edit_frame_for(handle, canvas)
             else {
                 continue;
             };
@@ -208,8 +208,7 @@ impl OpenCADStudio {
             // the viewport camera projects back onto `local`.
             let model_cursor = frame.paper_to_model(cursor_paper);
 
-            let wires = self
-                .tabs[i]
+            let wires = self.tabs[i]
                 .scene
                 .model_wires_for_viewport_arc(handle, bounds.height);
             let candidates = self.tabs[i].scene.interaction_candidates_near(
@@ -221,14 +220,14 @@ impl OpenCADStudio {
                 self.snapper.osnap_radius_px,
             );
 
-            // Object snap only. The grid belongs to the paper sheet, and the
-            // rubber-band origin / tracking points are paper-space points, so
-            // perpendicular and extension feet would be nonsense in model
-            // space. Save and restore, since `Snapper` is shared state.
+            // The grid belongs to the sheet. Translate the construction
+            // anchor for perpendicular/tangent snaps into this viewport's
+            // model coordinates, then restore the shared snapper state.
             let saved_grid = self.snapper.grid_snap_on;
             let saved_from = self.snapper.from_point;
             self.snapper.grid_snap_on = false;
-            self.snapper.from_point = None;
+            self.snapper.from_point =
+                saved_from.map(|p| frame.paper_to_model(p.as_dvec3()).as_vec3());
             let hit = self.snapper.snap(
                 model_cursor,
                 local,
@@ -251,8 +250,7 @@ impl OpenCADStudio {
             // the aperture reaching past the clip. Reject those: a snap must be
             // a real feature the viewport actually displays.
             let paper = frame.model_to_paper(hit.world);
-            if !self
-                .tabs[i]
+            if !self.tabs[i]
                 .scene
                 .viewport_displays_paper_point(handle, paper.truncate())
             {
@@ -291,5 +289,7 @@ fn project_hit_to_paper(
         }),
         viewport: Some(viewport),
         source: hit.source,
+        secondary_source: hit.secondary_source,
+        model_point: Some(hit.world),
     }
 }

src/app/viewport_dimension_tests.rs

@@ -0,0 +1,379 @@
+use super::OpenCADStudio;
+use crate::command::{DimensionAssociationSource, StepInput};
+use crate::entities::dim_override;
+use crate::scene::viewport_ref::{AcceptedSnap, MeasurementScale, ViewportFrame};
+use crate::scene::Scene;
+use crate::snap::{SnapResult, SnapType};
+use acadrust::entities::{Circle, Dimension, Line, Viewport};
+use acadrust::types::{Handle, Vector3};
+use acadrust::{CadDocument, EntityType};
+use glam::DVec3;
+
+fn fixture() -> (OpenCADStudio, Handle, ViewportFrame) {
+    let mut app = OpenCADStudio::new_for_test();
+    app.automation_op(r#"{"op":"new"}"#);
+    let mut scene = Scene::new();
+    let line = scene.add_entity(EntityType::Line(Line::from_points(
+        Vector3::ZERO,
+        Vector3::new(100.0, 0.0, 0.0),
+    )));
+    scene.document.add_layout("Dimensions").unwrap();
+    scene.set_current_layout("Dimensions".into());
+    let mut viewport = Viewport::new();
+    viewport.id = 2;
+    viewport.center = Vector3::new(50.0, 50.0, 0.0);
+    viewport.width = 100.0;
+    viewport.height = 100.0;
+    viewport.view_height = 1000.0;
+    viewport.custom_scale = 0.1;
+    viewport.view_direction = Vector3::UNIT_Z;
+    viewport.status.is_on = true;
+    let viewport = scene.add_entity(EntityType::Viewport(viewport));
+    let frame = scene.viewport_frame(viewport).unwrap();
+    scene.document.header.dimension_associativity = 2;
+    let i = app.active_tab;
+    app.tabs[i].scene = scene;
+    (app, line, frame)
+}
+
+fn hit(frame: ViewportFrame, source: Handle, model: DVec3) -> SnapResult {
+    SnapResult {
+        world: frame.model_to_paper(model),
+        model_point: Some(model),
+        screen: iced::Point::ORIGIN,
+        snap_type: SnapType::Endpoint,
+        tangent_obj: None,
+        extension_base: None,
+        extension_base2: None,
+        extension_origin: None,
+        extension_dir: None,
+        viewport: Some(frame.viewport),
+        source: Some(DimensionAssociationSource::inferred(source)),
+        secondary_source: None,
+    }
+}
+
+fn point(app: &mut OpenCADStudio, frame: ViewportFrame, source: Handle, model: DVec3) {
+    let i = app.active_tab;
+    let hit = hit(frame, source, model);
+    let paper = hit.world.with_z(0.0);
+    assert!(app.record_accepted_snap(i, Some(hit), Some(frame), paper));
+    let result = app.tabs[i].active_cmd.as_mut().unwrap().on_point(paper);
+    app.sync_dimension_snaps(i);
+    let _ = app.apply_cmd_result(result);
+}
+
+fn dimension(doc: &CadDocument) -> &Dimension {
+    doc.entities()
+        .find_map(|e| match e {
+            EntityType::Dimension(d) => Some(d),
+            _ => None,
+        })
+        .unwrap()
+}
+
+fn displayed(doc: &CadDocument) -> f64 {
+    let dim = dimension(doc);
+    if matches!(dim, Dimension::Angular2Ln(_) | Dimension::Angular3Pt(_)) {
+        return dim.measurement();
+    }
+    let factor = dim_override::real(&dim.base().common.extended_data, dim_override::DIMLFAC)
+        .or_else(|| {
+            doc.dim_styles
+                .get(&dim.base().style_name)
+                .map(|s| s.dimlfac)
+        })
+        .unwrap_or(1.0);
+    dim.measurement() * MeasurementScale::user_lfac_for_space(factor, true)
+}
+
+fn finish_aligned(app: &mut OpenCADStudio, line: Handle, frame: ViewportFrame) {
+    let _ = app.dispatch_command("DIMALIGNED");
+    point(app, frame, line, DVec3::ZERO);
+    point(app, frame, line, DVec3::X * 100.0);
+    let _ = app.feed_command(StepInput::Point(DVec3::new(250.0, 200.0, 0.0)));
+}
+
+#[test]
+fn viewport_dimension_projected_snap_keeps_identity_and_elevation() {
+    let (_, line, frame) = fixture();
+    let model = DVec3::new(100.0, 0.0, 35.0);
+    let hit = hit(frame, line, model);
+    let accepted =
+        AcceptedSnap::from_snap(&hit, Some(frame)).with_paper_point(hit.world.with_z(0.0));
+    assert_eq!(accepted.model_point, model);
+    assert_eq!(accepted.source.as_ref().unwrap().source.handle, line);
+    let moved = accepted.with_paper_point(hit.world + DVec3::X);
+    assert!(moved.source.is_none());
+    assert!((moved.model_point - frame.paper_to_model(moved.paper_point)).length() < 1e-9);
+}
+
+#[test]
+fn viewport_dimension_creation_preserves_active_units_and_roundtrips() {
+    for factor in [25.4, -25.4] {
+        let (mut app, line, frame) = fixture();
+        let i = app.active_tab;
+        let mut style = acadrust::tables::DimStyle::new("Millimetres");
+        style.dimlfac = factor;
+        app.tabs[i].scene.document.dim_styles.add(style).unwrap();
+        app.tabs[i].scene.document.header.current_dimstyle_name = "Millimetres".into();
+        finish_aligned(&mut app, line, frame);
+        let doc = &app.tabs[i].scene.document;
+        assert_eq!(dimension(doc).base().style_name, "Millimetres");
+        assert!((displayed(doc) - 2540.0).abs() < 1e-5, "{}", displayed(doc));
+        let data = MeasurementScale::read(&dimension(doc).base().common.extended_data).unwrap();
+        assert_eq!(data.user_lfac, 25.4);
+        assert_eq!(doc.header.dimension_associativity, 2);
+        for ext in ["dxf", "dwg"] {
+            let bytes = crate::io::save_to_bytes(doc, ext, doc.version).unwrap();
+            let loaded = crate::io::load_bytes(&format!("viewport.{ext}"), bytes).unwrap();
+            assert!(
+                (displayed(&loaded) - 2540.0).abs() < 1e-4,
+                "{ext}: {}",
+                displayed(&loaded)
+            );
+            assert_eq!(
+                MeasurementScale::read(&dimension(&loaded).base().common.extended_data),
+                Some(data)
+            );
+        }
+        app.undo_steps(1);
+        assert!(!app.tabs[i]
+            .scene
+            .document
+            .entities()
+            .any(|e| matches!(e, EntityType::Dimension(_))));
+        app.redo_steps(1);
+        assert!((displayed(&app.tabs[i].scene.document) - 2540.0).abs() < 1e-5);
+    }
+}
+
+#[test]
+fn viewport_dimension_mixed_and_degenerate_picks_allow_retry() {
+    let (mut app, line, frame) = fixture();
+    let i = app.active_tab;
+    let _ = app.dispatch_command("DIMALIGNED");
+    point(&mut app, frame, line, DVec3::ZERO);
+    point(&mut app, frame, line, DVec3::ZERO);
+    assert_eq!(app.accepted_snaps().len(), 1);
+    let _ = app.feed_command(StepInput::Point(DVec3::new(60.0, 50.0, 0.0)));
+    assert_eq!(app.accepted_snaps().len(), 1);
+    assert_eq!(
+        app.tabs[i]
+            .active_cmd
+            .as_ref()
+            .unwrap()
+            .dimension_acquired_points()
+            .len(),
+        1
+    );
+    let other = ViewportFrame {
+        viewport: Handle::new(0xFFFF),
+        ..frame
+    };
+    assert!(!app.record_accepted_snap(
+        i,
+        Some(hit(other, line, DVec3::X * 100.0)),
+        Some(other),
+        DVec3::new(60.0, 50.0, 0.0)
+    ));
+    point(&mut app, frame, line, DVec3::X * 100.0);
+    let _ = app.feed_command(StepInput::Point(DVec3::new(250.0, 200.0, 0.0)));
+    assert!((displayed(&app.tabs[i].scene.document) - 100.0).abs() < 1e-5);
+}
+
+#[test]
+fn viewport_dimension_typed_sheet_points_keep_paper_units() {
+    let (mut app, _, _) = fixture();
+    let result = app.automation_op(r#"{"op":"run","cmd":"DIMALIGNED 50,50 60,50 60,55"}"#);
+    assert_eq!(result["ok"], true);
+    let doc = &app.tabs[app.active_tab].scene.document;
+    assert!((displayed(doc) - 10.0).abs() < 1e-9);
+    assert!(MeasurementScale::read(&dimension(doc).base().common.extended_data).is_none());
+}
+
+#[test]
+fn viewport_dimension_linear_aligned_and_radial_object_picks() {
+    for (name, expected) in [
+        ("DIMLINEAR", 100.0),
+        ("DIMALIGNED", 100.0),
+        ("DIMRADIUS", 100.0),
+        ("DIMDIAMETER", 200.0),
+    ] {
+        let (mut app, _, frame) = fixture();
+        let i = app.active_tab;
+        let radial = name == "DIMRADIUS" || name == "DIMDIAMETER";
+        let model = if radial {
+            let mut circle = Circle::new();
+            circle.center = Vector3::new(0.0, 200.0, 0.0);
+            circle.radius = 100.0;
+            app.tabs[i].scene.set_current_layout("Model".into());
+            app.tabs[i].scene.add_entity(EntityType::Circle(circle));
+            app.tabs[i].scene.set_current_layout("Dimensions".into());
+            DVec3::new(100.0, 200.0, 0.0)
+        } else {
+            DVec3::new(50.0, 0.0, 0.0)
+        };
+        let _ = app.dispatch_command(name);
+        if !radial {
+            let _ = app.feed_command(StepInput::Enter);
+        }
+        let result = app
+            .try_dimension_viewport_entity_pick(i, frame.model_to_paper(model), 0.5)
+            .unwrap();
+        let _ = app.apply_cmd_result(result);
+        assert_eq!(
+            app.accepted_snaps().len(),
+            if radial { 1 } else { 2 },
+            "{name}"
+        );
+        let _ = app.feed_command(StepInput::Point(DVec3::new(55.0, 80.0, 0.0)));
+        assert!(
+            (displayed(&app.tabs[i].scene.document) - expected).abs() < 1e-4,
+            "{name}: {}",
+            displayed(&app.tabs[i].scene.document)
+        );
+    }
+}
+
+#[test]
+fn viewport_dimension_angular_preserves_angle_without_length_factor() {
+    let (mut app, line, frame) = fixture();
+    let _ = app.dispatch_command("DIMANGULAR");
+    let _ = app.feed_command(StepInput::Enter);
+    point(&mut app, frame, line, DVec3::ZERO);
+    point(&mut app, frame, line, DVec3::X * 100.0);
+    point(&mut app, frame, line, DVec3::Y * 100.0);
+    let _ = app.feed_command(StepInput::Point(DVec3::new(70.0, 70.0, 0.0)));
+    let doc = &app.tabs[app.active_tab].scene.document;
+    assert!((dimension(doc).measurement() - 90.0).abs() < 1e-5);
+    assert!(MeasurementScale::read(&dimension(doc).base().common.extended_data).is_none());
+}
+
+#[test]
+fn viewport_dimension_snap_query_matches_pan_scale_and_twist() {
+    for (height, twist, target) in [
+        (1000.0, 0.0, Vector3::ZERO),
+        (500.0, 0.45, Vector3::new(80.0, 20.0, 0.0)),
+    ] {
+        let (mut app, line, initial) = fixture();
+        let i = app.active_tab;
+        let Some(EntityType::Viewport(vp)) =
+            app.tabs[i].scene.document.get_entity_mut(initial.viewport)
+        else {
+            panic!()
+        };
+        vp.view_height = height;
+        vp.custom_scale = vp.height / height;
+        vp.twist_angle = twist;
+        vp.view_target = target;
+        let frame = app.tabs[i].scene.viewport_frame(initial.viewport).unwrap();
+        let model = DVec3::X * 100.0;
+        let paper = frame.model_to_paper(model);
+        let bounds = iced::Rectangle {
+            x: 0.0,
+            y: 0.0,
+            width: 1000.0,
+            height: 1000.0,
+        };
+        let screen = {
+            let cam = app.tabs[i].scene.camera.borrow();
+            let ndc = cam
+                .view_proj_rte(bounds)
+                .project_point3((paper - cam.eye()).as_vec3());
+            iced::Point::new((ndc.x + 1.0) * 500.0, (1.0 - ndc.y) * 500.0)
+        };
+        app.snapper.snap_enabled = true;
+        let (hit, acquired_frame) = app
+            .paper_viewport_snap(i, screen, (1000.0, 1000.0), paper)
+            .unwrap();
+        assert_eq!(hit.source.unwrap().handle, line);
+        assert!((hit.world - paper).length() < 1e-5, "{hit:?}");
+        let accepted = AcceptedSnap::from_snap(&hit, Some(acquired_frame));
+        assert!((accepted.model_point - model).length() < 1e-6);
+    }
+}
+
+#[test]
+fn viewport_dimension_eligibility_is_shared_and_uses_clipping() {
+    use acadrust::entities::LwPolyline;
+    use acadrust::types::Vector2;
+    let (mut app, _, first) = fixture();
+    let scene = &mut app.tabs[app.active_tab].scene;
+    let mut overlay = scene.document.get_entity(first.viewport).unwrap().clone();
+    if let EntityType::Viewport(vp) = &mut overlay {
+        vp.id = 3;
+        vp.width = 200.0;
+    }
+    overlay.common_mut().handle = Handle::NULL;
+    let top = scene.add_entity(overlay);
+    let paper = DVec3::new(55.0, 50.0, 0.0);
+    assert_eq!(
+        scene.viewport_frame_at_paper_point(paper).unwrap().viewport,
+        top
+    );
+    assert_eq!(
+        scene
+            .dimension_pick_through_viewport(paper, 0.2)
+            .unwrap()
+            .frame
+            .viewport,
+        top
+    );
+    let mut clip = LwPolyline::from_points(vec![
+        Vector2::new(0.0, 0.0),
+        Vector2::new(100.0, 0.0),
+        Vector2::new(0.0, 100.0),
+    ]);
+    clip.is_closed = true;
+    let clip = scene.add_entity(EntityType::LwPolyline(clip));
+    if let Some(EntityType::Viewport(vp)) = scene.document.get_entity_mut(top) {
+        vp.clip_boundary_handle = clip;
+    }
+    assert_eq!(
+        scene.viewport_frame_at_paper_point(paper).unwrap().viewport,
+        first.viewport
+    );
+    assert_eq!(
+        scene
+            .dimension_pick_through_viewport(paper, 0.2)
+            .unwrap()
+            .frame
+            .viewport,
+        first.viewport
+    );
+    if let Some(EntityType::Viewport(vp)) = scene.document.get_entity_mut(first.viewport) {
+        vp.status.is_on = false;
+    }
+    assert!(scene.viewport_frame_at_paper_point(paper).is_none());
+    assert!(scene.dimension_pick_through_viewport(paper, 0.2).is_none());
+}
+
+#[test]
+fn viewport_dimension_exploded_commit_uses_compensated_text_and_undo() {
+    let (mut app, line, frame) = fixture();
+    let i = app.active_tab;
+    app.tabs[i].scene.document.header.dimension_associativity = 0;
+    finish_aligned(&mut app, line, frame);
+    let doc = &app.tabs[i].scene.document;
+    assert!(!doc
+        .entities()
+        .any(|e| matches!(e, EntityType::Dimension(_))));
+    let texts: Vec<_> = doc
+        .entities()
+        .filter_map(|e| match e {
+            EntityType::Text(t) => Some(t.value.clone()),
+            EntityType::MText(t) => Some(t.value.clone()),
+            _ => None,
+        })
+        .collect();
+    assert!(texts.iter().any(|t| t.contains("100")), "{texts:?}");
+    app.undo_steps(1);
+    assert_eq!(app.tabs[i].scene.document.header.dimension_associativity, 0);
+    app.redo_steps(1);
+    assert!(app.tabs[i]
+        .scene
+        .document
+        .entities()
+        .any(|e| matches!(e, EntityType::Text(_) | EntityType::MText(_))));
+}

src/command.rs

@@ -2254,6 +2254,14 @@ pub trait CadCommand: Send {
         false
     }
 
+    /// Definition points acquired so far, in the command's working space.
+    /// Used to retain references only for accepted measuring inputs.
+    fn dimension_acquired_points(&self) -> Vec<DVec3> { Vec::new() }
+
+    /// The next point places annotation rather than acquiring geometry.
+    fn dimension_placement_pending(&self) -> bool { false }
+
+
     fn needs_entity_pick(&self) -> bool {
         false
     }

src/entities/dimension.rs

@@ -1638,25 +1638,11 @@ pub(crate) fn dimension_in_paper_space(dimension: &Dimension, document: &CadDocu
     if !owner.is_valid() {
         return false;
     }
-    let model_block = document
-        .block_records
-        .get("*Model_Space")
-        .map(|record| record.handle)
-        .filter(|handle| handle.is_valid())
-        .or_else(|| {
-            document.objects.values().find_map(|object| match object {
-                acadrust::objects::ObjectType::Layout(layout)
-                    if layout.name == "Model" && !layout.block_record.is_null() =>
-                {
-                    Some(layout.block_record)
-                }
-                _ => None,
-            })
-        });
-    match model_block {
-        Some(model) => owner != model,
-        None => false,
-    }
+    document.objects.values().any(|object| matches!(object,
+        acadrust::objects::ObjectType::Layout(layout)
+            if layout.name != "Model" && layout.block_record == owner
+    ))
+
 }
 
 /// The value a dimension's text reports for its measurement: the raw

src/modules/annotate/aligned_dim.rs

@@ -167,6 +167,17 @@ impl CadCommand for AlignedDimensionCommand {
         true
     }
 
+    fn dimension_acquired_points(&self) -> Vec<DVec3> {
+        match self.step {
+            Step::First => vec![], Step::Second(p) => vec![p],
+            Step::DimLine { p1, p2 } => vec![p1, p2],
+        }
+    }
+
+    fn dimension_placement_pending(&self) -> bool {
+        matches!(self.step, Step::DimLine { .. })
+    }
+
     fn input_kind(&self) -> InputKind {
         if self.awaiting_text {
             InputKind::FreeText

src/modules/annotate/angular_dim.rs

@@ -488,6 +488,20 @@ impl CadCommand for AngularDimensionCommand {
         true
     }
 
+    fn dimension_acquired_points(&self) -> Vec<DVec3> {
+        match self.step {
+            Step::Vertex => vec![], Step::FirstRay(p) => vec![p],
+            Step::SecondRay { vertex, first } | Step::CircleSecondRay { vertex, first, .. } => vec![vertex, first],
+            Step::SecondLine { first_start, first_end, .. } => vec![first_start, first_end],
+            Step::ArcPoint3 { vertex, first, second } => vec![vertex, first, second],
+            Step::ArcPoint2 { first_start, first_end, second_start, second_end } => vec![first_start, first_end, second_start, second_end],
+        }
+    }
+
+    fn dimension_placement_pending(&self) -> bool {
+        matches!(self.step, Step::ArcPoint3 { .. } | Step::ArcPoint2 { .. })
+    }
+
     fn on_escape(&mut self) -> CmdResult {
         CmdResult::Cancel
     }

src/modules/annotate/diameter_dim.rs

@@ -138,6 +138,16 @@ impl CadCommand for DiameterDimensionCommand {
         true
     }
 
+    fn dimension_acquired_points(&self) -> Vec<DVec3> {
+        match self.step {
+            Step::SelectObject => vec![], Step::DimLine(source) => vec![dvec(source.point_at_angle(source.start_angle))],
+        }
+    }
+
+    fn dimension_placement_pending(&self) -> bool {
+        matches!(self.step, Step::DimLine(_))
+    }
+
     fn on_escape(&mut self) -> CmdResult {
         CmdResult::Cancel
     }

src/modules/annotate/linear_dim.rs

@@ -223,6 +223,17 @@ impl CadCommand for LinearDimensionCommand {
         true
     }
 
+    fn dimension_acquired_points(&self) -> Vec<DVec3> {
+        match self.step {
+            Step::FirstPoint => vec![], Step::SecondPoint(p) => vec![p],
+            Step::DimensionLine { first, second } => vec![first, second],
+        }
+    }
+
+    fn dimension_placement_pending(&self) -> bool {
+        matches!(self.step, Step::DimensionLine { .. })
+    }
+
     fn on_escape(&mut self) -> CmdResult {
         CmdResult::Cancel
     }

src/modules/annotate/radius_dim.rs

@@ -136,6 +136,16 @@ impl CadCommand for RadiusDimensionCommand {
         true
     }
 
+    fn dimension_acquired_points(&self) -> Vec<DVec3> {
+        match self.step {
+            Step::SelectObject => vec![], Step::DimLine(source) => vec![dvec(source.point_at_angle(source.start_angle))],
+        }
+    }
+
+    fn dimension_placement_pending(&self) -> bool {
+        matches!(self.step, Step::DimLine(_))
+    }
+
     fn on_escape(&mut self) -> CmdResult {
         CmdResult::Cancel
     }

src/scene/mspace.rs

@@ -160,6 +160,18 @@ impl Scene {
         })
     }
 
+    /// Visible, supported content viewports at a sheet point, topmost first.
+    /// Shared by snapping and explicit dimension object picking.
+    pub fn viewport_frames_at_paper_point(&self, paper: glam::DVec3) -> Vec<crate::scene::viewport_ref::ViewportFrame> {
+        if self.current_layout == "Model" || self.active_viewport.is_some() { return Vec::new(); }
+        self.layout_content_viewports().iter().rev().filter_map(|&handle| {
+            if !self.viewport_displays_content(handle) || !self.viewport_displays_paper_point(handle, paper.truncate()) {
+                return None;
+            }
+            self.viewport_frame(handle)
+        }).collect()
+    }
+
     /// Does `paper` lie inside the viewport's displayed area?
     ///
     /// Always bounded by the viewport's paper rectangle; a viewport with a
@@ -184,9 +196,8 @@ impl Scene {
         }
         let poly = self.clip_boundary_polygon(vp.clip_boundary_handle, vp.center.z as f32);
         if poly.len() < 3 {
-            // A clip entity we cannot tessellate: fall back to the rectangle
-            // rather than refusing every snap in the viewport.
-            return true;
+            // An unsupported boundary cannot establish snap eligibility.
+            return false;
         }
         point_in_polygon_xy(paper, &poly)
     }

src/scene/viewport_dim_seam.rs

@@ -21,7 +21,9 @@ use super::*;
 use crate::command::EntityTransform;
 use crate::scene::viewport_ref::ViewportFrame;
 use acadrust::types::{Matrix4, Transform};
-use glam::{DVec2, DVec3};
+#[cfg(test)]
+use glam::DVec2;
+use glam::DVec3;
 
 /// A model entity resolved by clicking inside a paper-space content viewport.
 #[derive(Clone, Debug)]
@@ -44,36 +46,16 @@ pub struct ViewportDimPick {
 }
 
 impl Scene {
-
-    /// The smallest *on* content viewport of the current layout whose paper
-    /// rectangle contains `paper`. `None` on the Model tab, while editing
-    /// inside a viewport (MSPACE), or when the point is on bare sheet.
     pub fn content_viewport_at_paper_point(&self, paper: DVec3) -> Option<Handle> {
-        if self.current_layout == "Model" || self.active_viewport.is_some() {
-            return None;
-        }
-        let (_, _, handles) = self.paper_viewport_handles();
-        handles
-            .iter()
-            .filter_map(|handle| {
-                let Some(EntityType::Viewport(vp)) = self.document.get_entity(*handle) else {
-                    return None;
-                };
-                if !vp.status.is_on || vp.width <= 0.0 || vp.height <= 0.0 {
-                    return None;
-                }
-                let dx = (paper.x - vp.center.x).abs();
-                let dy = (paper.y - vp.center.y).abs();
-                (dx <= vp.width * 0.5 && dy <= vp.height * 0.5)
-                    .then(|| (*handle, vp.width * vp.height))
-            })
-            .min_by(|a, b| a.1.total_cmp(&b.1))
-            .map(|(handle, _)| handle)
+        self.viewport_frames_at_paper_point(paper)
+            .first()
+            .map(|f| f.viewport)
     }
 
-    /// [`Scene::content_viewport_at_paper_point`] resolved to a frame.
     pub fn viewport_frame_at_paper_point(&self, paper: DVec3) -> Option<ViewportFrame> {
-        self.viewport_frame(self.content_viewport_at_paper_point(paper)?)
+        self.viewport_frames_at_paper_point(paper)
+            .into_iter()
+            .next()
     }
 
     /// Resolve an explicit dimension object pick made on the sheet but landing
@@ -86,28 +68,42 @@ impl Scene {
         paper: DVec3,
         aperture_paper: f64,
     ) -> Option<ViewportDimPick> {
-        let frame = self.viewport_frame_at_paper_point(paper)?;
-        let model_point = frame.paper_to_model(paper);
-        let aperture_model =
-            (aperture_paper.max(0.0) * frame.paper_to_model_length_factor()).max(1e-9);
+        for frame in self.viewport_frames_at_paper_point(paper) {
+            let model_point = frame.paper_to_model(paper);
+            let aperture_model =
+                (aperture_paper.max(0.0) * frame.paper_to_model_length_factor()).max(1e-9);
 
-        let top = self.nearest_model_wire_handle(frame.viewport, model_point, aperture_model)?;
-        let (entity, block_path) = self.resolve_measurable_entity(top, model_point)?;
+            let Some(top) =
+                self.nearest_model_wire_handle(frame.viewport, model_point, aperture_model)
+            else {
+                continue;
+            };
+            let Some((entity, block_path)) =
+                self.resolve_measurable_entity(top, model_point, Some(frame.viewport))
+            else {
+                continue;
+            };
 
-        let mut paper_entity = entity;
-        crate::scene::view::dispatch::apply_transform(
-            &mut paper_entity,
-            &EntityTransform::Affine(viewport_model_to_paper_transform(&frame)),
-        );
-        let entity_handle = paper_entity.as_entity().handle();
-        Some(ViewportDimPick {
-            frame,
-            entity_handle: if entity_handle.is_valid() { entity_handle } else { top },
-            block_path,
-            paper_entity,
-            paper_point: paper,
-            model_point,
-        })
+            let mut paper_entity = entity;
+            crate::scene::view::dispatch::apply_transform(
+                &mut paper_entity,
+                &EntityTransform::Affine(viewport_model_to_paper_transform(&frame)),
+            );
+            let entity_handle = paper_entity.as_entity().handle();
+            return Some(ViewportDimPick {
+                frame,
+                entity_handle: if entity_handle.is_valid() {
+                    entity_handle
+                } else {
+                    top
+                },
+                block_path,
+                paper_entity,
+                paper_point: paper,
+                model_point,
+            });
+        }
+        None
     }
 
     /// Nearest resident model wire of `viewport` to `model_point`, within
@@ -119,7 +115,23 @@ impl Scene {
         model_point: DVec3,
         aperture: f64,
     ) -> Option<Handle> {
-        let wires = self.model_wires_for_viewport_arc(viewport, 0.0);
+        let camera = self.camera_for_viewport(viewport)?;
+        let bounds = iced::Rectangle {
+            x: 0.0,
+            y: 0.0,
+            width: 1000.0,
+            height: 1000.0,
+        };
+        let radius_px =
+            (aperture / (2.0 * camera.ortho_size() as f64) * bounds.height as f64) as f32;
+        let wires = self.interaction_pick_candidates_near(
+            self.model_wires_for_viewport_arc(viewport, 0.0),
+            model_point,
+            camera.view_proj_rte(bounds),
+            camera.eye(),
+            bounds,
+            radius_px.max(1.0),
+        );
         let mut best: Option<(f64, Handle)> = None;
         for wire in wires.iter() {
             if !wire.display_visible {
@@ -133,8 +145,17 @@ impl Scene {
                 .as_ref()
                 .map(|inst| DVec3::from_array(inst.translation))
                 .unwrap_or(DVec3::ZERO);
-            let distance = wire_polyline_distance(wire, offset, model_point);
-            if let Some(distance) = distance {
+            let distance = wire_polyline_nearest(wire, offset, model_point);
+            if let Some((distance, nearest)) = distance {
+                let Some(frame) = self.viewport_frame(viewport) else {
+                    continue;
+                };
+                if !self.viewport_displays_paper_point(
+                    viewport,
+                    frame.model_to_paper(nearest).truncate(),
+                ) {
+                    continue;
+                }
                 if distance <= aperture && best.as_ref().is_none_or(|(d, _)| distance < *d) {
                     best = Some((distance, handle));
                 }
@@ -146,10 +167,11 @@ impl Scene {
     /// Turn a picked handle into a measurable planar entity in model (WCS)
     /// coordinates, descending through block instances and baking the
     /// instance transform into the returned clone.
-    fn resolve_measurable_entity(
+    pub(crate) fn resolve_measurable_entity(
         &self,
         handle: Handle,
         model_point: DVec3,
+        viewport: Option<Handle>,
     ) -> Option<(EntityType, Vec<Handle>)> {
         let entity = self.document.get_entity(handle)?;
         match entity {
@@ -161,6 +183,7 @@ impl Scene {
                     model_point,
                     &mut path,
                     0,
+                    viewport,
                 )?;
                 Some((found.0, found.1))
             }
@@ -180,6 +203,7 @@ impl Scene {
         model_point: DVec3,
         path: &mut Vec<Handle>,
         depth: usize,
+        viewport: Option<Handle>,
     ) -> Option<(EntityType, Vec<Handle>)> {
         const MAX_DEPTH: usize = 8;
         if depth > MAX_DEPTH {
@@ -199,6 +223,14 @@ impl Scene {
             let Some(child_entity) = self.document.get_entity(child) else {
                 continue;
             };
+            let common = child_entity.common();
+            let layer = self.document.layers.get(&common.layer);
+            let frozen = viewport.and_then(|h| self.document.get_entity(h)).is_some_and(|entity| {
+                matches!(entity, EntityType::Viewport(vp) if layer.is_some_and(|l| vp.frozen_layers.contains(&l.handle)))
+            });
+            if common.invisible || layer.is_some_and(|l| l.flags.off || l.flags.frozen) || frozen {
+                continue;
+            }
             if matches!(child_entity, EntityType::Insert(_)) {
                 let mut nested = Vec::new();
                 if let Some((entity, mut nested_path)) = self.descend_block_instance(
@@ -207,6 +239,7 @@ impl Scene {
                     model_point,
                     &mut nested,
                     depth + 1,
+                    viewport,
                 ) {
                     if let Some(distance) = planar_pick_distance(&entity, model_point) {
                         if best.as_ref().is_none_or(|(d, _, _)| distance < *d) {
@@ -218,6 +251,22 @@ impl Scene {
                 }
                 continue;
             }
+            // A nonuniform block transform turns circular geometry into an
+            // ellipse. The entity transform API retains the circle type, so
+            // decline this object pick instead of manufacturing a radius.
+            let curved = matches!(child_entity, EntityType::Circle(_) | EntityType::Arc(_))
+                || matches!(child_entity, EntityType::LwPolyline(p) if p.vertices.iter().any(|v| v.bulge != 0.0));
+            let m = &combined.matrix.m;
+            let x = DVec3::new(m[0][0], m[1][0], m[2][0]);
+            let y = DVec3::new(m[0][1], m[1][1], m[2][1]);
+            let magnitude = x.length_squared().max(y.length_squared());
+            if curved
+                && (magnitude < 1e-24
+                    || (x.length_squared() - y.length_squared()).abs() > magnitude * 1e-10
+                    || x.dot(y).abs() > magnitude * 1e-10)
+            {
+                continue;
+            }
             let mut placed = child_entity.clone();
             crate::scene::view::dispatch::apply_transform(
                 &mut placed,
@@ -259,11 +308,11 @@ pub fn viewport_model_to_paper_transform(frame: &ViewportFrame) -> Transform {
 
 /// Distance from `point` to a wire's polyline, in XY. `None` when the wire has
 /// no usable segment.
-fn wire_polyline_distance(
+fn wire_polyline_nearest(
     wire: &crate::scene::WireModel,
     offset: DVec3,
     point: DVec3,
-) -> Option<f64> {
+) -> Option<(f64, DVec3)> {
     let count = wire.points.len();
     if count == 0 {
         return None;
@@ -278,6 +327,7 @@ fn wire_polyline_distance(
         ) + offset
     };
     let mut best = f64::INFINITY;
+    let mut best_point = DVec3::ZERO;
     let mut previous: Option<DVec3> = None;
     for i in 0..count {
         let current = at(i);
@@ -285,77 +335,41 @@ fn wire_polyline_distance(
             previous = None;
             continue;
         }
-        match previous {
-            Some(start) => best = best.min(segment_distance_xy(start, current, point)),
-            None => best = best.min((current.truncate() - point.truncate()).length()),
+        let nearest = previous.map_or(current, |start| {
+            let delta = current - start;
+            let len2 = delta.truncate().length_squared();
+            let t = if len2 > 1e-24 {
+                ((point - start).truncate().dot(delta.truncate()) / len2).clamp(0.0, 1.0)
+            } else {
+                0.0
+            };
+            start + delta * t
+        });
+        let distance = (nearest - point).truncate().length();
+        if distance < best {
+            best = distance;
+            best_point = nearest;
         }
         previous = Some(current);
     }
-    best.is_finite().then_some(best)
-}
-
-fn segment_distance_xy(a: DVec3, b: DVec3, p: DVec3) -> f64 {
-    let ab = b.truncate() - a.truncate();
-    let ap = p.truncate() - a.truncate();
-    let len2 = ab.length_squared();
-    if len2 < 1e-24 {
-        return ap.length();
-    }
-    let t = (ap.dot(ab) / len2).clamp(0.0, 1.0);
-    (ap - ab * t).length()
+    best.is_finite().then_some((best, best_point))
 }
 
 /// Distance from `point` to the planar geometry object-pick dimensioning
 /// supports. `None` for entity types that cannot be dimensioned by picking.
 pub fn planar_pick_distance(entity: &EntityType, point: DVec3) -> Option<f64> {
-    let p = point.truncate();
-    match entity {
-        EntityType::Line(line) => Some(segment_distance_xy(
-            DVec3::new(line.start.x, line.start.y, line.start.z),
-            DVec3::new(line.end.x, line.end.y, line.end.z),
-            point,
-        )),
-        EntityType::Circle(circle) => {
-            let c = DVec2::new(circle.center.x, circle.center.y);
-            Some(((p - c).length() - circle.radius).abs())
-        }
-        EntityType::Arc(arc) => {
-            let c = DVec2::new(arc.center.x, arc.center.y);
-            Some(((p - c).length() - arc.radius).abs())
-        }
-        EntityType::LwPolyline(polyline) => polyline_distance(
-            polyline
-                .vertices
-                .iter()
-                .map(|v| DVec3::new(v.location.x, v.location.y, polyline.elevation)),
-            polyline.is_closed,
-            point,
-        ),
-        EntityType::Polyline2D(polyline) => polyline_distance(
-            polyline
-                .vertices
-                .iter()
-                .map(|v| DVec3::new(v.location.x, v.location.y, polyline.elevation)),
-            polyline.is_closed(),
-            point,
-        ),
-        _ => None,
-    }
-}
-
-fn polyline_distance(
-    points: impl IntoIterator<Item = DVec3>,
-    closed: bool,
-    point: DVec3,
-) -> Option<f64> {
-    let points: Vec<DVec3> = points.into_iter().collect();
-    if points.len() < 2 {
+    let planar = crate::entities::curve::entity_curve(entity)?;
+    let normal = DVec3::from_array(planar.plane.normal()?);
+    if normal.cross(DVec3::Z).length_squared() > 1e-12 {
         return None;
     }
-    let count = if closed { points.len() } else { points.len() - 1 };
-    (0..count)
-        .map(|i| segment_distance_xy(points[i], points[(i + 1) % points.len()], point))
-        .reduce(f64::min)
+    let uv = planar.plane.project(point.to_array())?;
+    let nearest = cadkernel::geom2d::closest_point(&planar.curve, uv);
+    Some(
+        (DVec3::from_array(planar.plane.point_at(nearest.point)) - point)
+            .truncate()
+            .length(),
+    )
 }
 
 #[cfg(test)]

src/scene/viewport_ref.rs

@@ -1,20 +1,4 @@
-//! Shared interfaces for paper-space snapping through layout viewports and
-//! for viewport-aware dimension measurement.
-//!
-//! Three PRs build on this module concurrently:
-//!  * PR1 (viewport snap) constructs [`ViewportFrame`] from the live viewport
-//!    camera and fills [`SnapSourceRef`] / [`AcceptedSnap`] when a snap is
-//!    accepted in paper space.
-//!  * PR2 (dimension measurement) consumes [`AcceptedSnap`] and uses
-//!    [`MeasurementScale`] to split the user's DIMLFAC from viewport
-//!    compensation so the compensation is applied exactly once.
-//!  * PR3 (associations) turns [`SnapSourceRef`] into persistent
-//!    `AssocDimensionReference` chains (dimension -> viewport -> block path ->
-//!    entity -> feature) and resolves them back.
-//!
-//! Anything here is a contract between those branches. Extend by adding
-//! fields/methods; do not rename or remove what exists without updating the
-//! sibling branches.
+//! Coordinates and feature identity shared by viewport acquisition and dimensions.
 
 use acadrust::types::Handle;
 use glam::{DMat3, DVec2, DVec3};
@@ -27,8 +11,7 @@ use crate::snap::SnapType;
 /// scale (paper units per model unit) and in-plane rotation (`twist`,
 /// radians, counter-clockwise, applied to model geometry before scaling).
 ///
-/// PR1 owns construction (`Scene::viewport_frame`); everyone else only reads
-/// it. All arithmetic is `f64`.
+/// Constructed by `Scene::viewport_frame`. All arithmetic is `f64`.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub struct ViewportFrame {
     /// Viewport entity handle.
@@ -105,6 +88,8 @@ pub struct SnapSourceRef {
     pub source: DimensionAssociationSource,
     pub block_path: Vec<Handle>,
     pub snap_type: SnapType,
+    /// The other entity/instance path for a true intersection.
+    pub intersection: Option<Box<SnapSourceRef>>,
 }
 
 /// A snap accepted by a point-input command, with enough information for
@@ -149,31 +134,29 @@ impl AcceptedSnap {
         self.viewport.is_some()
     }
 
-    /// Build an accepted snap from a [`crate::snap::SnapResult`].
-    ///
-    /// * `frame` `Some`: the snap ran against MODEL geometry through that
-    ///   viewport, so `snap.world` is a model point and the paper point is its
-    ///   projection onto the sheet.
-    /// * `frame` `None`: the snap ran in the current space directly, so paper
-    ///   and model point coincide.
-    ///
-    /// PR1 fills `source` from `SnapResult::source` (the owning entity handle
-    /// the snap engine attributed the feature to); `block_path` stays empty
-    /// until PR3 resolves block instance chains.
+    /// `snap.world` is always in the command's current space. A projected
+    /// viewport hit also retains its original model coordinate explicitly.
     pub fn from_snap(snap: &crate::snap::SnapResult, frame: Option<ViewportFrame>) -> Self {
-        let (paper_point, model_point) = match &frame {
-            Some(frame) => (frame.model_to_paper(snap.world), snap.world),
-            None => (snap.world, snap.world),
+        let frame = frame.filter(|frame| snap.viewport == Some(frame.viewport));
+        let source_ref = |source| SnapSourceRef {
+            source,
+            block_path: Vec::new(),
+            snap_type: snap.snap_type,
+            intersection: None,
         };
         Self {
-            paper_point,
-            model_point,
+            paper_point: snap.world,
+            model_point: frame.map_or(snap.world, |frame| {
+                snap.model_point
+                    .unwrap_or_else(|| frame.paper_to_model(snap.world))
+            }),
             viewport: frame.map(|f| f.viewport),
             frame,
             source: snap.source.map(|source| SnapSourceRef {
-                source,
-                block_path: Vec::new(),
-                snap_type: snap.snap_type,
+                intersection: snap
+                    .secondary_source
+                    .map(|source| Box::new(source_ref(source))),
+                ..source_ref(source)
             }),
         }
     }
@@ -222,6 +205,53 @@ pub struct MeasurementScale {
 }
 
 impl MeasurementScale {
+    /// OCS bookkeeping keeps user scaling separate from the changing viewport
+    /// scale. Standard DIMLFAC still carries the full factor for other readers.
+    pub const APP_ID: &'static str = "OCS_VIEWPORT_MEASUREMENT";
+
+    pub fn read(data: &acadrust::xdata::ExtendedData) -> Option<Self> {
+        use acadrust::xdata::XDataValue;
+        let record = data.get_record(Self::APP_ID)?;
+        let [XDataValue::Integer16(1), XDataValue::Real(user), XDataValue::Real(compensation)] =
+            record.values.as_slice()
+        else {
+            return None;
+        };
+        if !user.is_finite() || *user <= 0.0 || !compensation.is_finite() || *compensation <= 0.0 {
+            return None;
+        }
+        Some(Self {
+            user_lfac: *user,
+            viewport_compensation: *compensation,
+        })
+    }
+
+    pub fn write_to_entity(self, entity: &mut acadrust::EntityType) {
+        use acadrust::xdata::{ExtendedData, ExtendedDataRecord, XDataValue};
+        crate::entities::dim_override::set_on_entity(
+            entity,
+            crate::entities::dim_override::DIMLFAC,
+            Some(XDataValue::Real(-self.paper_factor())),
+        );
+        let common = entity.common_mut();
+        let mut data = ExtendedData::new();
+        for record in common
+            .extended_data
+            .records()
+            .iter()
+            .filter(|r| r.application_name != Self::APP_ID)
+        {
+            data.add_record(record.clone());
+        }
+        let mut record = ExtendedDataRecord::new(Self::APP_ID);
+        record.add_value(XDataValue::Integer16(1));
+        record.add_value(XDataValue::Real(self.user_lfac));
+        record.add_value(XDataValue::Real(self.viewport_compensation));
+        data.add_record(record);
+        // Parsed records are authoritative after changing measurement data.
+        common.extended_data = data;
+    }
+
     pub const IDENTITY: Self = Self {
         user_lfac: 1.0,
         viewport_compensation: 1.0,
@@ -272,8 +302,7 @@ impl MeasurementScale {
     ///
     /// Rendering an *existing* dimension only knows whether the entity lives
     /// in a paper-space layout; the viewport compensation is already baked
-    /// into the persisted magnitude (see
-    /// [`MeasurementScale::viewport_dimlfac_override`]). This is the same rule
+    /// into the persisted magnitude. This is the same rule
     /// as [`MeasurementScale::from_dimlfac`] with `viewport.is_some()`
     /// replaced by `paper_space`.
     pub fn user_lfac_for_space(dimlfac: f64, paper_space: bool) -> f64 {
@@ -287,45 +316,6 @@ impl MeasurementScale {
             1.0
         }
     }
-
-    /// The persisted representation of viewport compensation, PR2's single
-    /// source of truth.
-    ///
-    /// A paper-space dimension that measures model geometry through a viewport
-    /// keeps its definition points in **paper** coordinates and carries a
-    /// **negative DIMLFAC override** whose magnitude is
-    /// `user_lfac * viewport_compensation`. Because a viewport frame is a
-    /// similarity (uniform scale + in-plane twist), the raw paper measurement
-    /// times that magnitude is exactly the model measurement times the user's
-    /// linear factor — so the compensation is applied exactly once, at
-    /// formatting time, and round-trips through DXF/DWG unchanged (AutoCAD
-    /// applies |DIMLFAC| to paper-space dimensions by the same rule).
-    ///
-    /// Returns `None` when no override is needed:
-    /// * the compensation is 1:1, or
-    /// * the drawing/style DIMLFAC is already negative with exactly this
-    ///   magnitude (the drawing-wide DIMLFAC is set for this purpose).
-    ///
-    /// `style_dimlfac` is the effective DIMLFAC the dimension would inherit
-    /// *without* an override.
-    pub fn viewport_dimlfac_override(style_dimlfac: f64, compensation: f64) -> Option<f64> {
-        if !compensation.is_finite() || compensation.abs() < 1e-12 {
-            return None;
-        }
-        if (compensation - 1.0).abs() < 1e-12 {
-            return None;
-        }
-        if style_dimlfac < 0.0
-            && ((-style_dimlfac) - compensation).abs() <= 1e-9 * compensation.abs().max(1.0)
-        {
-            return None;
-        }
-        // A positive style DIMLFAC is the user's own multiplier and must be
-        // preserved; a negative one of a *different* magnitude was meant for
-        // another viewport scale, so the user multiplier there is 1.0.
-        let user = if style_dimlfac > 0.0 { style_dimlfac } else { 1.0 };
-        Some(-(user * compensation))
-    }
 }
 
 #[cfg(test)]
@@ -392,29 +382,4 @@ mod tests {
             );
         }
     }
-
-    #[test]
-    fn override_encodes_compensation_once() {
-        // 1:10 viewport, no user factor -> -10, and a 10-unit paper span then
-        // formats as 100.
-        let ovr = MeasurementScale::viewport_dimlfac_override(1.0, 10.0).unwrap();
-        assert!((ovr + 10.0).abs() < 1e-12);
-        let lfac = MeasurementScale::user_lfac_for_space(ovr, true);
-        assert!((10.0 * lfac - 100.0).abs() < 1e-9);
-        // Same dimension read back in model space must not scale.
-        assert_eq!(MeasurementScale::user_lfac_for_space(ovr, false), 1.0);
-    }
-
-    #[test]
-    fn override_preserves_user_multiplier_and_skips_redundant_cases() {
-        assert_eq!(MeasurementScale::viewport_dimlfac_override(1.0, 1.0), None);
-        // Drawing-wide negative DIMLFAC already carries this compensation.
-        assert_eq!(MeasurementScale::viewport_dimlfac_override(-10.0, 10.0), None);
-        // Positive user factor is folded in.
-        let ovr = MeasurementScale::viewport_dimlfac_override(25.4, 10.0).unwrap();
-        assert!((ovr + 254.0).abs() < 1e-9);
-        // Negative but for a different scale -> user multiplier is 1.0.
-        let ovr = MeasurementScale::viewport_dimlfac_override(-2.0, 10.0).unwrap();
-        assert!((ovr + 10.0).abs() < 1e-12);
-    }
 }

src/snap.rs

@@ -97,6 +97,10 @@ pub struct SnapResult {
     /// INSERT handle; the block path is resolved later by
     /// [`crate::scene::viewport_ref::SnapSourceRef::block_path`]. (PR1)
     pub source: Option<DimensionAssociationSource>,
+    /// Second real object at an intersection, when both objects are known.
+    pub secondary_source: Option<DimensionAssociationSource>,
+    /// Original model point when `world` has been projected onto a sheet.
+    pub model_point: Option<glam::DVec3>,
 }
 
 impl SnapResult {
@@ -984,6 +988,8 @@ impl Snapper {
             extension_dir: None,
                         viewport: None,
                         source: None,
+                        secondary_source: None,
+                        model_point: None,
         })
     }
 
@@ -1119,6 +1125,8 @@ impl Snapper {
                         extension_dir: None,
                         viewport: None,
                         source: None,
+                        secondary_source: None,
+                        model_point: None,
                     });
                     best_rank = snap_tier(SnapType::Grid);
                     best_sub = snap_priority(SnapType::Grid);
@@ -1216,7 +1224,7 @@ impl Snapper {
 
         let mut try_pt = |world: glam::DVec3,
                           snap_type: SnapType,
-                          src: Option<DimensionAssociationSource>| {
+                          src: Option<DimensionAssociationSource>, secondary: Option<DimensionAssociationSource>| {
             let screen = world_to_screen(world, view_rot, eye, bounds);
             if !in_bounds(screen) {
                 return;
@@ -1245,6 +1253,8 @@ impl Snapper {
                     extension_dir: None,
                         viewport: None,
                         source: src,
+                        secondary_source: secondary,
+                        model_point: None,
                 });
             }
         };
@@ -1262,7 +1272,7 @@ impl Snapper {
                 SnapHint::Endpoint => SnapType::Endpoint,
             };
             if self.is_on(snap_type) {
-                try_pt(world, snap_type, src);
+                try_pt(world, snap_type, src, None);
             }
         };
         if let Some(points) = wires.snap_points() {
@@ -1298,8 +1308,7 @@ impl Snapper {
                     try_pt(
                         DVec3::from_array(point),
                         SnapType::Endpoint,
-                        wire_source(wire),
-                    );
+                        wire_source(wire), None);
                 }
                 // Tessellated open curves have no key-vertex set. Their only
                 // endpoints are first/last, so testing candidate wires remains
@@ -1313,14 +1322,13 @@ impl Snapper {
                         continue;
                     }
                     if !wire.points.is_empty() {
-                        try_pt(wp_f64(wire, 0), SnapType::Endpoint, wire_source(wire));
+                        try_pt(wp_f64(wire, 0), SnapType::Endpoint, wire_source(wire), None);
                     }
                     if wire.points.len() > 1 {
                         try_pt(
                             wp_f64(wire, wire.points.len() - 1),
                             SnapType::Endpoint,
-                            wire_source(wire),
-                        );
+                            wire_source(wire), None);
                     }
                 }
             } else {
@@ -1328,7 +1336,7 @@ impl Snapper {
                     let src = wire_source(wire);
                     if !wire.key_vertices.is_empty() {
                         for &point in &wire.key_vertices {
-                            try_pt(DVec3::from_array(point), SnapType::Endpoint, src);
+                            try_pt(DVec3::from_array(point), SnapType::Endpoint, src, None);
                         }
                     } else {
                         let closed = wire
@@ -1337,14 +1345,13 @@ impl Snapper {
                             .any(|(_, hint)| matches!(hint, SnapHint::Quadrant));
                         if !closed {
                             if !wire.points.is_empty() {
-                                try_pt(wp_f64(wire, 0), SnapType::Endpoint, src);
+                                try_pt(wp_f64(wire, 0), SnapType::Endpoint, src, None);
                             }
                             if wire.points.len() > 1 {
                                 try_pt(
                                     wp_f64(wire, wire.points.len() - 1),
                                     SnapType::Endpoint,
-                                    src,
-                                );
+                                    src, None);
                             }
                         }
                     }
@@ -1371,7 +1378,7 @@ impl Snapper {
                     let a = DVec3::from_array(a);
                     let b = DVec3::from_array(b);
                     if a.distance_squared(b) > 1e-12 {
-                        try_pt((a + b) * 0.5, SnapType::Midpoint, wire_source(wire));
+                        try_pt((a + b) * 0.5, SnapType::Midpoint, wire_source(wire), None);
                     }
                 }
             } else {
@@ -1381,7 +1388,7 @@ impl Snapper {
                         let a = DVec3::from_array(segment[0]);
                         let b = DVec3::from_array(segment[1]);
                         if a.distance_squared(b) > 1e-12 {
-                            try_pt((a + b) * 0.5, SnapType::Midpoint, src);
+                            try_pt((a + b) * 0.5, SnapType::Midpoint, src, None);
                         }
                     }
                 }
@@ -1395,8 +1402,7 @@ impl Snapper {
                     try_pt(
                         nearest_on_segment(cursor_world, seg.a, seg.b),
                         SnapType::Nearest,
-                        wires.source_wire(seg.wire).and_then(wire_source),
-                    );
+                        wires.source_wire(seg.wire).and_then(wire_source), None);
                 }
             } else {
                 for wire in in_range_wires.iter() {
@@ -1404,7 +1410,7 @@ impl Snapper {
                     for i in 0..wire.points.len().saturating_sub(1) {
                         let p =
                             nearest_on_segment(cursor_world, wp_f64(wire, i), wp_f64(wire, i + 1));
-                        try_pt(p, SnapType::Nearest, src);
+                        try_pt(p, SnapType::Nearest, src, None);
                     }
                 }
             }
@@ -1420,8 +1426,7 @@ impl Snapper {
                             try_pt(
                                 foot,
                                 SnapType::Perpendicular,
-                                wires.source_wire(seg.wire).and_then(wire_source),
-                            );
+                                wires.source_wire(seg.wire).and_then(wire_source), None);
                         }
                     }
                 } else {
@@ -1429,7 +1434,7 @@ impl Snapper {
                         let src = wire_source(wire);
                         for i in 0..wire.points.len().saturating_sub(1) {
                             if let Some(foot) = perp_foot(q, wp_f64(wire, i), wp_f64(wire, i + 1)) {
-                                try_pt(foot, SnapType::Perpendicular, src);
+                                try_pt(foot, SnapType::Perpendicular, src, None);
                             }
                         }
                     }
@@ -1450,8 +1455,7 @@ impl Snapper {
                             try_pt(
                                 point,
                                 SnapType::Intersection,
-                                wires.source_wire(segment.wire).and_then(wire_source),
-                            );
+                                wires.source_wire(segment.wire).and_then(wire_source), None);
                         }
                     }
                 }
@@ -1466,7 +1470,7 @@ impl Snapper {
                             wp_f64(wire, index + 1),
                         ) {
                             if (point - origin).length_squared() > 1e-18 {
-                                try_pt(point, SnapType::Intersection, src);
+                                try_pt(point, SnapType::Intersection, src, None);
                             }
                         }
                     }
@@ -1552,7 +1556,7 @@ impl Snapper {
                             // first. PR3 resolves the true pair.
                             let src = wire_source(wire_i);
                             for point in points {
-                                try_pt(point, SnapType::Intersection, src);
+                                try_pt(point, SnapType::Intersection, src, wire_source(wire_j));
                             }
                             resolved_pairs.insert(pair_key(wire_i, wire_j));
                         }
@@ -1586,8 +1590,7 @@ impl Snapper {
                                 try_pt(
                                     pt,
                                     SnapType::Intersection,
-                                    wires.source_wire(a.wire).and_then(wire_source),
-                                );
+                                    wires.source_wire(a.wire).and_then(wire_source), wires.source_wire(b.wire).and_then(wire_source));
                                 if exact_cursor {
                                     break 'intersection_sweep;
                                 }
@@ -1609,7 +1612,7 @@ impl Snapper {
                         if let Some(pts) = exact_curve_intersections(wire_i, wire_j) {
                             let src = wire_source(wire_i);
                             for pt in pts {
-                                try_pt(pt, SnapType::Intersection, src);
+                                try_pt(pt, SnapType::Intersection, src, wire_source(wire_j));
                             }
                             continue;
                         }
@@ -1633,7 +1636,7 @@ impl Snapper {
                                     continue;
                                 }
                                 if let Some(pt) = seg_intersect_3d(a0, a1, b0, b1) {
-                                    try_pt(pt, SnapType::Intersection, wire_source(wire_i));
+                                    try_pt(pt, SnapType::Intersection, wire_source(wire_i), wire_source(wire_j));
                                 }
                             }
                         }
@@ -1659,7 +1662,7 @@ impl Snapper {
                         bounds,
                         self.osnap_radius_px,
                     ) {
-                        try_pt(ext, SnapType::Extension, None);
+                        try_pt(ext, SnapType::Extension, None, None);
                     }
                 }
             }
@@ -1706,7 +1709,7 @@ impl Snapper {
                     // feet (which sit closer to the cursor on their own lines),
                     // or the cursor would snap to a line instead of the crossing.
                     let pt = glam::DVec3::new(a0.x + t1 * d1.x, a0.y + t1 * d1.y, a0.z);
-                    try_pt(pt, SnapType::Intersection, None);
+                    try_pt(pt, SnapType::Intersection, None, None);
                 }
             }
         }
@@ -1759,8 +1762,7 @@ impl Snapper {
                                 try_pt(
                                     segment_a.a + ta as f64 * (segment_a.b - segment_a.a),
                                     SnapType::ApparentIntersection,
-                                    wires.source_wire(segment_a.wire).and_then(wire_source),
-                                );
+                                    wires.source_wire(segment_a.wire).and_then(wire_source), wires.source_wire(segment_b.wire).and_then(wire_source));
                                 if dist2(apparent_screen, cursor_screen) <= f32::EPSILON {
                                     break 'apparent_sweep;
                                 }
@@ -1800,8 +1802,7 @@ impl Snapper {
                                     try_pt(
                                         wa0 + ta as f64 * (wa1 - wa0),
                                         SnapType::ApparentIntersection,
-                                        wire_source(wire_i),
-                                    );
+                                        wire_source(wire_i), wire_source(wire_j));
                                 }
                             }
                         }
@@ -2024,6 +2025,8 @@ impl Snapper {
                             extension_dir: None,
                         viewport: None,
                         source: wire_source(wire),
+                        secondary_source: None,
+                        model_point: None,
                         });
                     }
                 }
@@ -2088,6 +2091,8 @@ impl Snapper {
                         extension_dir: None,
                         viewport: None,
                         source: wire_source(wire),
+                        secondary_source: None,
+                        model_point: None,
                     });
                 }
             };