fix(blocks): one base point convention for creation, redefine, insert, explode, XCLIP and REFEDIT

world = insert_point + Rot * Scale * (P - base_point). Attribute placement, EXPLODE, XCLIP storage, REFEDIT's working copy, the UCS reframe of attributes and the dynamic-block visibility grip ignored the base point and were offset by Rot*Scale*base. They now share insert_transform_with_base. Creation stores a ZERO base and redefine resets it; a no-op redefine moves nothing; the Block Definition dialog prefills the base from the definition. XCLIP filters stored by older builds keep their old displacement.

src/app/command_driver/entity_commit.rs

@@ -1049,6 +1049,15 @@ impl OpenCADStudio {
                 }
             }
         } else {
+            // Attributes are placed through the same base-point convention as
+            // the render path, so hand the command the definition's base point.
+            let block_base = crate::scene::render_graph::block_base_point(
+                &self.tabs[i].scene.document,
+                &block_name,
+            );
+            if let Some(cmd) = self.tabs[i].active_cmd.as_mut() {
+                cmd.attreq_set_block_base(block_base);
+            }
             let completed = self.tabs[i]
                 .active_cmd
                 .as_mut()

src/app/command_driver/tests.rs

@@ -1479,3 +1479,278 @@ mod bedit_characterization_tests {
         );
     }
 }
+
+#[cfg(test)]
+mod block_base_point_tests {
+    use super::super::*;
+    use crate::command::{CadCommand, CmdResult};
+    use crate::modules::insert::insert_block::InsertBlockCommand;
+    use codec::entities::AttributeDefinition;
+    use codec::types::Vector3;
+    use codec::EntityType;
+
+    /// base-point fix (A), host side: `AttreqNeeded` is where the host resolves the
+    /// block's ATTDEFs, so it also injects the definition's base point. Without
+    /// that the committed attribute lands on the codec transform's position.
+    #[test]
+    fn locked_attreq_injects_the_block_base_point_for_attribute_placement() {
+        let mut app = OpenCADStudio::new_for_test();
+        let _ = app.automation_op(r#"{"op":"new"}"#);
+        let i = app.active_tab;
+        // Block "ATTREQ_BP": base (10,5,0); LINE (10,5)-(30,5); ATTDEF TAG1 at
+        // (12,8). Rendering convention (see scene::block_base_point_tests): the
+        // ATTDEF lands at (100,100) + Rot(30)*Scale(2)*((12,8) - (10,5))
+        //   = (100,100) + 2*(1.732051 - 1.5, 1 + 2.598076) = (100.464102, 107.196152).
+        app.tabs[i].scene.define_block_raw(
+            "ATTREQ_BP",
+            Vector3::new(10.0, 5.0, 0.0),
+            vec![
+                EntityType::Line(codec::entities::Line::from_points(
+                    Vector3::new(10.0, 5.0, 0.0),
+                    Vector3::new(30.0, 5.0, 0.0),
+                )),
+                EntityType::AttributeDefinition(AttributeDefinition {
+                    insertion_point: Vector3::new(12.0, 8.0, 0.0),
+                    height: 1.0,
+                    ..AttributeDefinition::new(
+                        "TAG1".to_string(),
+                        "Tag one".to_string(),
+                        String::new(),
+                    )
+                }),
+            ],
+        );
+
+        let mut cmd = InsertBlockCommand::classic_for_block(
+            "ATTREQ_BP".to_string(),
+            Vec::new(),
+            String::new(),
+        );
+        let _ = cmd.on_point(glam::DVec3::new(100.0, 100.0, 0.0));
+        assert!(matches!(cmd.on_text_input("2"), Some(CmdResult::NeedPoint)));
+        assert!(matches!(cmd.on_text_input("2"), Some(CmdResult::NeedPoint)));
+        assert!(matches!(
+            cmd.on_text_input("30"),
+            Some(CmdResult::AttreqNeeded { .. })
+        ));
+        app.tabs[i].active_cmd = Some(Box::new(cmd));
+
+        let _ = app.apply_cmd_result(CmdResult::AttreqNeeded {
+            block_name: "ATTREQ_BP".to_string(),
+        });
+        let result = app.tabs[i]
+            .active_cmd
+            .as_mut()
+            .expect("the INSERT command waits for the attribute value")
+            .on_text_input("VALUE1")
+            .expect("the value finishes the INSERT");
+        let _ = app.apply_cmd_result(result);
+
+        let attribute = app.tabs[i]
+            .scene
+            .document
+            .entities()
+            .find_map(|entity| match entity {
+                EntityType::Insert(ins) if ins.block_name == "ATTREQ_BP" => {
+                    ins.attributes.first().cloned()
+                }
+                _ => None,
+            })
+            .expect("the committed INSERT's attribute");
+        assert_eq!(attribute.tag, "TAG1");
+        let point = attribute.insertion_point;
+        assert!(
+            (point.x - 100.464102).abs() < 1e-6
+                && (point.y - 107.196152).abs() < 1e-6
+                && point.z.abs() < 1e-9,
+            "attribute at ({:.6},{:.6},{:.6}), expected (100.464102,107.196152,0)",
+            point.x,
+            point.y,
+            point.z
+        );
+    }
+
+    /// base-point fix (D), host side: picking an existing block in the Block
+    /// Definition dialog prefills the base point from that definition.
+    #[test]
+    fn locked_block_def_name_select_prefills_the_definition_base_point() {
+        let mut app = OpenCADStudio::new_for_test();
+        let _ = app.automation_op(r#"{"op":"new"}"#);
+        let i = app.active_tab;
+        app.tabs[i].scene.define_block_raw(
+            "DEF_BASE",
+            Vector3::new(10.0, 5.0, 0.0),
+            vec![EntityType::Line(codec::entities::Line::from_points(
+                Vector3::new(10.0, 5.0, 0.0),
+                Vector3::new(30.0, 5.0, 0.0),
+            ))],
+        );
+        app.block_definition = Some(
+            crate::ui::window::block_definition::BlockDefinitionState::new(
+                vec!["DEF_BASE".to_string()],
+                Vec::new(),
+                0,
+            ),
+        );
+        assert_eq!(
+            app.block_definition
+                .as_ref()
+                .map(|s| s.base_point_y.as_str()),
+            Some("0.0000"),
+            "a new block still starts at 0.0000"
+        );
+
+        let _ = app.update(Message::BlockDefNameSelect("DEF_BASE".to_string()));
+
+        let state = app.block_definition.as_ref().expect("the dialog is open");
+        assert_eq!(state.name, "DEF_BASE");
+        assert_eq!(state.base_point_x, "10.0000");
+        assert_eq!(state.base_point_y, "5.0000");
+        assert_eq!(state.base_point_z, "0.0000");
+        assert_eq!(state.parse_base_point(), glam::DVec3::new(10.0, 5.0, 0.0));
+    }
+
+    /// A block with base (10,5,0) and LINE (10,5)-(30,5), referenced by an
+    /// INSERT at (100,100) with 30° rotation and uniform scale 2. Under the
+    /// convention the LINE renders at (100,100)-(134.641016,120).
+    fn block_and_instance_with_base(app: &mut OpenCADStudio, name: &str) -> codec::Handle {
+        let _ = app.automation_op(r#"{"op":"new"}"#);
+        let i = app.active_tab;
+        app.tabs[i].scene.define_block_raw(
+            name,
+            Vector3::new(10.0, 5.0, 0.0),
+            vec![EntityType::Line(codec::entities::Line::from_points(
+                Vector3::new(10.0, 5.0, 0.0),
+                Vector3::new(30.0, 5.0, 0.0),
+            ))],
+        );
+        let mut insert =
+            codec::entities::Insert::new(name.to_string(), Vector3::new(100.0, 100.0, 0.0));
+        insert.set_x_scale(2.0);
+        insert.set_y_scale(2.0);
+        insert.set_z_scale(2.0);
+        insert.rotation = std::f64::consts::PI / 6.0;
+        app.tabs[i].scene.add_entity(EntityType::Insert(insert))
+    }
+
+    /// base-point follow-up fix: the REFEDIT working copy is placed through the
+    /// base-point-aware INSERT transform, so it sits exactly on the rendered
+    /// reference, and saving folds it back to the same block-local geometry.
+    #[test]
+    fn locked_refedit_working_copy_sits_on_the_rendered_reference() {
+        let mut app = OpenCADStudio::new_for_test();
+        let handle = block_and_instance_with_base(&mut app, "REFEDIT_BP");
+        let i = app.active_tab;
+
+        let _ = app.dispatch_command(&format!("REFEDIT_BEGIN:{}", handle.value()));
+        assert!(
+            app.tabs[i].refedit_session.is_some(),
+            "setup: REFEDIT_BEGIN opens a session"
+        );
+
+        // The working copy is the LINE the session moved into model space.
+        let temp: Vec<codec::Handle> = app.tabs[i]
+            .refedit_session
+            .as_ref()
+            .expect("the session")
+            .temp_handles
+            .clone();
+        let line = temp
+            .iter()
+            .find_map(|h| match app.tabs[i].scene.document.get_entity(*h) {
+                Some(EntityType::Line(line)) => Some(line.clone()),
+                _ => None,
+            })
+            .expect("the working-copy LINE");
+        // (100,100) + Rot(30)*2*((10,5)-(10,5)) and ...*((30,5)-(10,5)).
+        assert!(
+            (line.start.x - 100.0).abs() < 1e-6
+                && (line.start.y - 100.0).abs() < 1e-9
+                && (line.end.x - 134.641016).abs() < 1e-6
+                && (line.end.y - 120.0).abs() < 1e-6,
+            "working copy at ({:.6},{:.6})-({:.6},{:.6}), expected (100,100)-(134.641016,120)",
+            line.start.x,
+            line.start.y,
+            line.end.x,
+            line.end.y
+        );
+
+        // Saving without editing restores the definition verbatim: the
+        // inverse is the exact inverse of the forward transform.
+        let _ = app.dispatch_command("REFCLOSE_SAVE");
+        assert!(app.tabs[i].refedit_session.is_none());
+        let stored = app.tabs[i]
+            .scene
+            .document
+            .block_records
+            .get("REFEDIT_BP")
+            .expect("the block record")
+            .entity_handles
+            .iter()
+            .find_map(|h| match app.tabs[i].scene.document.get_entity(*h) {
+                Some(EntityType::Line(line)) => Some(line.clone()),
+                _ => None,
+            })
+            .expect("the stored LINE");
+        assert!(
+            (stored.start.x - 10.0).abs() < 1e-9
+                && (stored.start.y - 5.0).abs() < 1e-9
+                && (stored.end.x - 30.0).abs() < 1e-9
+                && (stored.end.y - 5.0).abs() < 1e-9,
+            "stored LINE at ({:.9},{:.9})-({:.9},{:.9}), expected (10,5)-(30,5)",
+            stored.start.x,
+            stored.start.y,
+            stored.end.x,
+            stored.end.y
+        );
+        assert_eq!(
+            crate::scene::render_graph::block_base_point(&app.tabs[i].scene.document, "REFEDIT_BP"),
+            Vector3::new(10.0, 5.0, 0.0),
+            "the base point survives the round trip"
+        );
+    }
+
+    /// base-point follow-up fix: the dynamic-block visibility grip is a
+    /// block-definition coordinate, so it follows the base-point convention
+    /// too: (100,100) + Rot(30)*2*((12,3) - (10,5)) = (105.464102, 98.535898).
+    #[test]
+    fn locked_visibility_grip_follows_the_base_point_convention() {
+        let mut app = OpenCADStudio::new_for_test();
+        let handle = block_and_instance_with_base(&mut app, "VIS_BP");
+        let i = app.active_tab;
+
+        let def_block = app.tabs[i]
+            .scene
+            .document
+            .block_records
+            .get("VIS_BP")
+            .expect("the block record")
+            .handle;
+        let mut param = codec::objects::BlockVisibilityParameter::default();
+        param.owner = def_block;
+        param.def_point = Vector3::new(12.0, 3.0, 0.0);
+        app.tabs[i]
+            .scene
+            .document
+            .block_visibility_params
+            .insert(codec::Handle::new(0x70), param);
+        app.tabs[i].selected_handle = Some(handle);
+
+        app.refresh_visibility_grip([0.0, 0.0, 0.0]);
+
+        let grip = app.tabs[i]
+            .selected_grips
+            .last()
+            .expect("the visibility lookup grip");
+        assert!(
+            (grip.world.x - 105.464102).abs() < 1e-6 && (grip.world.y - 98.535898).abs() < 1e-6,
+            "grip at ({:.6},{:.6}), expected (105.464102,98.535898)",
+            grip.world.x,
+            grip.world.y
+        );
+        assert!(
+            app.tabs[i].visibility_grip.is_some(),
+            "the selection is a dynamic-block reference"
+        );
+    }
+}

src/app/commands/inquiry.rs

@@ -809,12 +809,19 @@ impl OpenCADStudio {
                 };
 
                 // Build the INSERT's full placement transform (OCS + rotation +
-                // scale, including non-uniform / mirrored) and its inverse, so
+                // scale, including non-uniform / mirrored, minus the block base
-                // edits round-trip back to block-local coordinates on SAVE.
+                // point) and its inverse, so edits round-trip back to
+                // block-local coordinates on SAVE. The base point is part of
+                // the placement: without it the working copy sits Rot*Scale*base
+                // away from the reference the user picked.
                 let sx = insert.x_scale();
                 let sy = insert.y_scale();
                 let sz = insert.z_scale();
-                let forward = insert.get_transform();
+                let base = crate::scene::render_graph::block_base_point(
+                    &self.tabs[i].scene.document,
+                    &insert.block_name,
+                );
+                let forward = crate::scene::render_graph::insert_transform_with_base(&insert, base);
                 let inverse = {
                     use codec::types::{Matrix3, Matrix4, Transform};
                     let ocs_t =
@@ -826,8 +833,11 @@ impl OpenCADStudio {
                     );
                     let r_inv = Matrix4::rotation_z(-insert.rotation);
                     let s_inv = Matrix4::scaling(1.0 / sx, 1.0 / sy, 1.0 / sz);
-                    // inverse(OCS·T·R·S) = S⁻¹·R⁻¹·T⁻¹·OCSᵀ
+                    // inverse(T(-base)·OCS·T·R·S) = T(+base)·S⁻¹·R⁻¹·T⁻¹·OCSᵀ
-                    Transform::from_matrix(s_inv * r_inv * t_inv * ocs_t)
+                    Transform::from_matrix(
+                        Matrix4::translation(base.x, base.y, base.z)
+                            * (s_inv * r_inv * t_inv * ocs_t),
+                    )
                 };
 
                 // Find the block record.

src/app/commands/sheet_set.rs

@@ -2269,8 +2269,8 @@ impl OpenCADStudio {
         let host = self.tabs[i].scene.document.header.insertion_units;
         let k = crate::app::properties::insert_unit_scale(host, units).unwrap_or(1.0);
         let mut insert = codec::entities::Insert::new(name.to_string(), at).with_scale(k, k, k);
-        let transform = insert.get_transform();
         let doc = &self.tabs[i].scene.document;
+        let transform = crate::scene::render_graph::insert_transform(doc, &insert);
         let attdefs: Vec<codec::entities::AttributeDefinition> = doc
             .block_records
             .get(name)

src/app/commands/xclip.rs

@@ -6,13 +6,14 @@ use crate::scene::pick::xclip as clip;
 
 impl OpenCADStudio {
     /// A block reference's extents in WCS: its block's objects through the
-    /// insert transform.
+    /// insert transform — the base-point-aware one, because that is where the
+    /// renderer puts the contents the user drew the boundary around.
     fn insert_extents(&self, i: usize, insert: codec::Handle) -> ([f64; 2], [f64; 2]) {
         let document = &self.tabs[i].scene.document;
         let Some(codec::EntityType::Insert(ins)) = document.get_entity(insert) else {
             return ([0.0; 2], [0.0; 2]);
         };
-        let xform = ins.get_transform();
+        let xform = crate::scene::render_graph::insert_transform(document, ins);
         let (mut lo, mut hi) = ([f64::MAX; 2], [f64::MIN; 2]);
         for e in document.entities_in_block(&ins.block_name) {
             let (a, b) = crate::scene::convert::tess::entity_bounds_in(document, e);
@@ -77,7 +78,14 @@ impl OpenCADStudio {
                     else {
                         continue;
                     };
-                    let outline = clip::clip_outline_world(doc, filter, &ins.get_transform());
+                    // The same base-point-aware transform the spatial filter
+                    // was stored with, so the generated polyline lands on the
+                    // boundary the user drew (the two cancel exactly).
+                    let outline = clip::clip_outline_world(
+                        doc,
+                        filter,
+                        &crate::scene::render_graph::insert_transform(doc, ins),
+                    );
                     let mut pl = codec::entities::LwPolyline::from_points(
                         outline
                             .iter()

src/app/update/mod.rs

@@ -5017,9 +5017,16 @@ impl OpenCADStudio {
             }
             Message::BlockDefNameSelect(chosen) => {
                 if let Some(state) = self.block_definition.as_mut() {
-                    state.name = chosen;
+                    state.name = chosen.clone();
                     state.error_message = None;
                     state.confirm_redefine = None;
+                    // Redefining an existing block starts from the base point
+                    // that definition already has (0.0000 for a new block).
+                    let base = crate::scene::render_graph::block_base_point(
+                        &self.tabs[self.active_tab].scene.document,
+                        &chosen,
+                    );
+                    state.set_base_point(glam::DVec3::new(base.x, base.y, base.z));
                 }
                 Task::none()
             }

src/app/visibility.rs

@@ -136,7 +136,10 @@ impl OpenCADStudio {
             return;
         };
 
-        let wp = ins.get_transform().apply(Vector3::new(
+        // `def_point` is a block-definition coordinate (see the codec's
+        // BlockVisibilityParameter), so the grip follows the same base-point
+        // convention as the rendered reference.
+        let wp = crate::scene::render_graph::insert_transform(doc, ins).apply(Vector3::new(
             param.def_point.x,
             param.def_point.y,
             param.def_point.z,

src/command.rs

@@ -3035,6 +3035,11 @@ pub trait CadCommand: Send {
         None
     }
 
+    /// Supply the referenced block's insertion base point so the attributes
+    /// the command now owns are placed through the same base-point convention
+    /// as the render path. Default: no-op (no base point).
+    fn attreq_set_block_base(&mut self, _base: codec::types::Vector3) {}
+
     /// After the host commits an ATTREQ insert: keep the command running
     /// (repeated placement) instead of ending it.
     fn attreq_continue(&self) -> bool {

src/modules/draw/modify/explode.rs

@@ -23,8 +23,8 @@ use codec::entities::{
 };
 use codec::entities::{Polyline, Polyline2D};
 use codec::tables::BlockRecord;
-use codec::types::Vector3;
+use codec::types::{Transform, Vector3};
-use codec::{CadDocument, EntityType, Handle};
+use codec::{CadDocument, Entity, EntityType, Handle};
 
 use crate::command::{CadCommand, CmdResult, WorkingPlane};
 use crate::entities::curve::lwpolyline_world_xy;
@@ -106,7 +106,24 @@ pub fn explode_entity(entity: &EntityType, document: &CadDocument) -> Vec<Entity
             if !explodable {
                 vec![]
             } else {
-                ins.explode_from_document(document)
+                // The codec applies only `Insert::get_transform`, which does
+                // not know the block's base point, so shift the definition's
+                // contents by -base first: get_transform(P - base) is exactly
+                // world = insert_point + Rot*Scale*(P - base), the placement
+                // the render path uses. MINSERT cells and nested references
+                // get the same treatment because the shift happens on the
+                // contents, not on the insert.
+                let base = crate::scene::render_graph::block_base_point(document, &ins.block_name);
+                let to_block = Transform::from_translation(Vector3::new(-base.x, -base.y, -base.z));
+                let entities: Vec<EntityType> = document
+                    .entities_in_block(&ins.block_name)
+                    .map(|entity| {
+                        let mut shifted = entity.clone();
+                        Entity::apply_transform(shifted.as_entity_mut(), &to_block);
+                        shifted
+                    })
+                    .collect();
+                ins.explode(&entities)
                     .into_iter()
                     .map(normalize_insert_entity)
                     .collect()

src/modules/insert/insert_block.rs

@@ -126,6 +126,10 @@ pub struct InsertBlockCommand {
     rotation: f64,
     pending_insert: Option<Insert>,
     plane: WorkingPlane,
+    /// The referenced block's insertion base point. Attributes are placed
+    /// through the same base-point convention the render path uses, so the
+    /// host injects it when it resolves the block's ATTDEFs.
+    block_base: Vector3,
 }
 
 impl InsertBlockCommand {
@@ -152,6 +156,7 @@ impl InsertBlockCommand {
             rotation: 0.0,
             pending_insert: None,
             plane: WorkingPlane::default(),
+            block_base: Vector3::ZERO,
         }
     }
 
@@ -852,9 +857,18 @@ impl CadCommand for InsertBlockCommand {
     fn attreq_take_insert(&mut self) -> Option<codec::EntityType> {
         self.pending_insert.take().map(EntityType::Insert)
     }
+
+    fn attreq_set_block_base(&mut self, base: Vector3) {
+        self.set_block_base(base);
+    }
 }
 
 impl InsertBlockCommand {
+    /// The referenced block's insertion base point, for attribute placement.
+    pub fn set_block_base(&mut self, base: Vector3) {
+        self.block_base = base;
+    }
+
     fn answer(&mut self, ask: Ask, text: Option<&str>) -> CmdResult {
         let default = !matches!(ask, Ask::RepeatScale | Ask::RepeatRotate);
         let Some(yes) = Self::yes_no(text.unwrap_or(""), default) else {
@@ -955,7 +969,8 @@ impl InsertBlockCommand {
             _ => return None,
         };
         let mut insert = self.pending_insert.take()?;
-        let transform = insert.get_transform();
+        let transform =
+            crate::scene::render_graph::insert_transform_with_base(&insert, self.block_base);
         for (attdef_idx, value) in values {
             let Some(attdef) = attdefs.get(attdef_idx) else {
                 continue;

src/scene/entity.rs

@@ -947,6 +947,14 @@ impl Scene {
         &mut self,
         options: CreateBlockOptions,
     ) -> Result<Handle, String> {
+        // Base-point contract: the contents are stored *relative to the picked
+        // base* because `world_to_block` already subtracts it, so the
+        // definition's own `Block.base_point` stays ZERO and the picked base
+        // becomes the block origin (inserting at a point puts that point where
+        // the base was). Callers must not also store the picked base in the
+        // definition — that would subtract it twice. Imported definitions,
+        // whose geometry arrives relative to their own base point, keep that
+        // base point and are never rewritten here.
         let name = options.name.trim();
         if name.is_empty() {
             return Err("Block name cannot be empty.".into());
@@ -988,6 +996,11 @@ impl Scene {
             // Update existing Block entity fields
             if let Some(EntityType::Block(b)) = self.document.get_entity_mut(existing_blk_h) {
                 b.description = options.description.clone();
+                // The new contents are baked relative to the picked base (see
+                // the contract above), so the definition's base point has to
+                // go back to ZERO. Leaving an imported definition's non-zero
+                // base point here would subtract the picked base twice.
+                b.base_point = codec::types::Vector3::ZERO;
             }
             if options.annotative {
                 let annotative_vals = vec![codec::xdata::XDataValue::Integer16(1)];

src/scene/mod.rs

@@ -26,6 +26,8 @@ pub enum BlockScalePolicy {
 
 // Topic submodules split out of this root (each contributes `impl Scene`
 // blocks and/or free functions). Pure text-move from the original mod.rs.
+#[cfg(test)]
+mod block_base_point_tests;
 mod boundary;
 mod camera_ops;
 pub(crate) mod centerline;

src/scene/modify.rs

@@ -506,7 +506,11 @@ impl Scene {
 
         // ATTRIBs live inline on each INSERT in that reference's owner space.
         // Conjugate the block-local reframe through the INSERT transform:
-        // owner' = M · local_from_old · M⁻¹ · owner.
+        // owner' = M · local_from_old · M⁻¹ · owner. M is the block-local →
+        // world map the render path uses (`insert_transform`), because that is
+        // the map the attribute positions on the INSERT were placed through;
+        // the codec's base-ignoring transform would conjugate them through a
+        // displaced frame whenever the definition has a non-zero base point.
         let references: Vec<(Handle, codec::types::Transform)> = self
             .document
             .entities()
@@ -516,7 +520,8 @@ impl Scene {
                         && !reference.attributes.is_empty()
                         && !owned.contains(&reference.common.handle) =>
                 {
-                    let insertion = reference.get_transform();
+                    let insertion =
+                        crate::scene::render_graph::insert_transform(&self.document, reference);
                     let inverse = inverse_affine(&insertion)?;
                     Some((
                         reference.common.handle,

src/scene/pick/xclip.rs

@@ -250,7 +250,12 @@ pub fn set_insert_clip(
     let Some(codec::EntityType::Insert(ins)) = doc.get_entity(insert) else {
         return false;
     };
-    let t = ins.get_transform().matrix.m;
+    // The stored inverse is the inverse of the transform every consumer
+    // composes the boundary back with — the base-point-aware instance
+    // transform the render path uses. Storing the codec's base-ignoring
+    // inverse would displace the ring by exactly -base on the next render.
+    let forward = crate::scene::render_graph::insert_transform(doc, ins);
+    let t = forward.matrix.m;
     let forward = glam::DMat4::from_cols_array_2d(&[
         [t[0][0], t[1][0], t[2][0], t[3][0]],
         [t[0][1], t[1][1], t[2][1], t[3][1]],

src/scene/render_graph.rs

@@ -688,7 +688,14 @@ pub(crate) fn clear_dim_picture_placement(entity: &mut EntityType) {
 }
 
 pub fn insert_transform(document: &CadDocument, insert: &Insert) -> Transform {
-    let base = block_base_point(document, &insert.block_name);
+    insert_transform_with_base(insert, block_base_point(document, &insert.block_name))
+}
+
+/// [`insert_transform`] for a caller that already resolved the block's base
+/// point (the `-INSERT` command builds the INSERT before the drawing is in
+/// scope), so the base-point convention lives in exactly one place:
+/// world = insert_point + Rot(rotation) * Scale * (P - base).
+pub fn insert_transform_with_base(insert: &Insert, base: Vector3) -> Transform {
     Transform::from_translation(Vector3::new(-base.x, -base.y, -base.z))
         .then(&insert.get_transform())
 }

src/ui/window/block_definition.rs

@@ -139,6 +139,15 @@ impl BlockDefinitionState {
         glam::DVec3::new(x, y, z)
     }
 
+    /// Prefill the base point fields from an existing definition, so
+    /// redefining a block starts from the base it already has (0.0000 for a
+    /// new block). Formatted the same way the pick point button writes them.
+    pub fn set_base_point(&mut self, base: glam::DVec3) {
+        self.base_point_x = format!("{:.4}", base.x);
+        self.base_point_y = format!("{:.4}", base.y);
+        self.base_point_z = format!("{:.4}", base.z);
+    }
+
     pub fn is_existing_name(&self) -> bool {
         let trimmed = self.name.trim();
         !trimmed.is_empty()