View-dependent display refinement: finer curved faces and edges as you zoom in (#1320) (#1425)

* View-dependent display refinement: bodies seen up close are drawn from finer display plans (#1320)

Each frame the viewport works out every shown body's screen-space error (the display
quality's chord tolerance in pixels where the body is nearest the eye) and picks a
refinement step past the chosen display quality, each halving the chord, with hysteresis.
Steps are planned off the UI thread and swapped in when ready, within a triangle budget
and a cache size. Faces (GPU plan or CPU mesh), edges, silhouettes and highlights of a
refined body all come from the refined plan, so a highlight stays on the drawn triangles.
Planned bodies are chunked by content on the GPU, so a refinement re-evaluates only its
own chunk. Display only: the reference mesh, the B-rep and exports never change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Display refinement: only collect finished plans while the view stands still, time the choice, keep the web planning one step a frame

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Format

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Keep the Display quality note as it was (the longer one pushed the Colour vision list off the Graphics page)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/doc/src/display_quality.rs

@@ -77,6 +77,57 @@ impl DisplayQuality {
     }
 }
 
+/// Most refinement steps view-dependent display refinement takes past the chosen level (#1320):
+/// each halves the chord tolerance, so the finest is 1/16 of the chosen level's.
+pub const MAX_REFINE: u8 = 4;
+
+/// Screen-space error (physical pixels) above which a body is refined: a facet's sag shows.
+pub const REFINE_PX: f64 = 1.0;
+/// Error a refinement aims for (below [`REFINE_PX`], so a body just refined is not refined again
+/// at the next small zoom step).
+pub const TARGET_PX: f64 = 0.75;
+/// Error below which a refined body steps back toward the chosen level. A refinement lands
+/// between half [`TARGET_PX`] and [`TARGET_PX`], and a step back between this and twice this, so
+/// zooming out or in by up to 2× from either never flips the level back.
+pub const COARSEN_PX: f64 = 0.175;
+
+impl DisplayQuality {
+    /// Chord tolerance (mm) of a body of this size refined `k` steps past this level: halved
+    /// per step.
+    pub fn refined_chord(self, size: f64, k: u8) -> f64 {
+        self.chord(size) / f64::from(1u32 << k.min(MAX_REFINE))
+    }
+
+    /// Facet angle (radians) `k` steps past this level: an arc's sag goes with the square of its
+    /// angle, so the angle shrinks by √2 per step, keeping pace with the chord.
+    pub fn refined_angle(self, k: u8) -> f64 {
+        self.angle() / std::f64::consts::SQRT_2.powi(i32::from(k.min(MAX_REFINE)))
+    }
+}
+
+/// The refinement step a body should be drawn at (0: the chosen level), from `err_px`: the
+/// screen-space error of the chosen level, its chord tolerance in physical pixels where the body
+/// is nearest the eye (0 for a body off screen or too small to matter). `current` is the step
+/// drawn now; steps halve the error. A body is refined once its error passes [`REFINE_PX`], to the
+/// first step under [`TARGET_PX`], and steps back only once the error falls under [`COARSEN_PX`],
+/// to the coarsest step that stays under twice that: the band between keeps an orbit or a small
+/// zoom from flipping levels. Never past `max` (at most [`MAX_REFINE`]).
+pub fn refinement_for(err_px: f64, current: u8, max: u8) -> u8 {
+    let max = max.min(MAX_REFINE);
+    let current = current.min(max);
+    if !(err_px.is_finite() && err_px > 0.0) {
+        return 0;
+    }
+    let err = |k: u8| err_px / f64::from(1u32 << k);
+    if err(current) > REFINE_PX {
+        (current..=max).find(|&k| err(k) <= TARGET_PX).unwrap_or(max)
+    } else if err(current) < COARSEN_PX {
+        (0..=current).find(|&k| err(k) <= 2.0 * COARSEN_PX).unwrap_or(current)
+    } else {
+        current
+    }
+}
+
 static CURRENT: AtomicU8 = AtomicU8::new(1);
 
 /// The level the viewport draws at (process-wide; set from the preferences).
@@ -140,4 +191,58 @@ mod tests {
             plan.vertex_count()
         });
     }
+
+    #[test]
+    fn refinement_follows_the_screen_space_error_with_hysteresis() {
+        // Off screen or tiny: the chosen level.
+        assert_eq!(refinement_for(0.0, 3, MAX_REFINE), 0);
+        assert_eq!(refinement_for(f64::NAN, 2, MAX_REFINE), 0);
+        // Fine already: stays.
+        assert_eq!(refinement_for(0.6, 0, MAX_REFINE), 0);
+        assert_eq!(refinement_for(1.0, 0, MAX_REFINE), 0);
+        // Past a pixel: the first step under the target (each halves the error).
+        assert_eq!(refinement_for(1.2, 0, MAX_REFINE), 1);
+        assert_eq!(refinement_for(5.0, 0, MAX_REFINE), 3);
+        assert_eq!(refinement_for(5.0, 1, MAX_REFINE), 3);
+        // Capped.
+        assert_eq!(refinement_for(1e6, 0, MAX_REFINE), MAX_REFINE);
+        assert_eq!(refinement_for(1e6, 0, 2), 2);
+        assert_eq!(refinement_for(1e6, 9, 99), MAX_REFINE);
+        // Zooming back out: kept until the error is well under a pixel, then the coarsest step
+        // that stays under half a pixel.
+        assert_eq!(refinement_for(5.0, 3, MAX_REFINE), 3);
+        assert_eq!(refinement_for(2.5, 3, MAX_REFINE), 3, "0.31 px at step 3: in the band");
+        assert_eq!(refinement_for(1.2, 3, MAX_REFINE), 2, "0.15 px at step 3: down to 0.3 px");
+        assert_eq!(refinement_for(0.05, 3, MAX_REFINE), 0);
+        // No flicker: any error, once settled, gives the same step again, and a zoom by up to
+        // 2x either way from where a level was chosen keeps it.
+        let mut e = 0.01;
+        while e < 1e4 {
+            for start in 0..=MAX_REFINE {
+                let k = refinement_for(e, start, MAX_REFINE);
+                assert_eq!(refinement_for(e, k, MAX_REFINE), k, "error {e}, from {start}");
+                // Settled by zooming in (from the chosen level) or out (from the finest).
+                if start == 0 || start == MAX_REFINE {
+                    for f in [0.5, 0.7, 1.0, 1.4, 2.0] {
+                        let back = refinement_for(e * f, k, MAX_REFINE);
+                        let zoomed_in = f >= 1.0;
+                        assert!(
+                            back == k || (zoomed_in && back > k && start == 0) || (!zoomed_in && back < k && start == MAX_REFINE),
+                            "error {e} x{f} from {start}: {k} -> {back}"
+                        );
+                    }
+                }
+            }
+            e *= 1.13;
+        }
+    }
+
+    #[test]
+    fn refined_steps_halve_the_chord() {
+        let q = DisplayQuality::Normal;
+        assert_eq!(q.refined_chord(200.0, 0), q.chord(200.0));
+        assert_eq!(q.refined_chord(200.0, 2), q.chord(200.0) / 4.0);
+        assert_eq!(q.refined_chord(200.0, 99), q.chord(200.0) / 16.0);
+        assert!((q.refined_angle(2) - q.angle() / 2.0).abs() < 1e-12);
+    }
 }

crates/doc/src/eval.rs

@@ -126,6 +126,25 @@ impl ModelBody {
             .clone()
     }
 
+    /// The display plan and its mesh `k` steps finer than `q` (view-dependent refinement, #1320;
+    /// see [`crate::display_quality::refinement_for`]). Step 0 is the cached plan at `q`; finer
+    /// steps are made on each call and kept by the viewport, which evicts them when it runs over
+    /// its budget. `None` when the body has no plan at that step.
+    pub fn refined_display(&self, q: crate::display_quality::DisplayQuality, k: u8) -> Option<(Arc<TessPlan>, Arc<Mesh>)> {
+        if k == 0 {
+            return Some((self.display_plan_at(q)?, self.plan_mesh_at(q)?));
+        }
+        let plan = match self.body.display_plan_angular(q.refined_chord(self.body.size(), k), q.refined_angle(k)) {
+            Ok(plan) => plan,
+            Err(e) => {
+                log::debug!("{}: no display plan refined {k} steps ({e})", self.name);
+                return None;
+            }
+        };
+        let mesh = self.body.display_mesh_of(&plan).ok()?;
+        Some((Arc::new(plan), Arc::new(mesh)))
+    }
+
     /// The display mesh's chord tolerance: 1/1000 of the body's size.
     fn display_tol(&self) -> f64 {
         (self.body.size() * 1e-3).max(1e-3)

crates/ui-egui/src/control.rs

@@ -245,6 +245,7 @@ fn dispatch(app: &mut SolveApp, ctx: &egui::Context, method: &str, p: &Value) ->
             "file_picker_busy": app.file_picker_busy(),
             "file_dialog_open": app.file_picker_busy(),
             "build_ms": app.viewport.build_ms,
+            "display_lod": app.viewport.lod.lock().map(|l| serde_json::to_value(&l.stats).unwrap_or_default()).unwrap_or_default(),
             "preview": {"active": app.preview.active, "busy": app.preview.busy, "error": app.preview.error, "ms": app.preview.ms, "replaced": app.preview.replaced},
             "home": app.home.open,
             "recent_thumbnails_ready": app.home.ready_recents(),

crates/ui-egui/src/display_lod.rs

@@ -0,0 +1,503 @@
+//! View-dependent display refinement (#1320): bodies seen up close are drawn from finer display
+//! plans, so curved faces, their edges and silhouettes stay smooth as the view zooms in.
+//!
+//! Each frame the viewport works out every shown body's screen-space error: the chord tolerance
+//! of the chosen display quality (Preferences ▸ Graphics) in physical pixels, where the body is
+//! nearest the eye. [`refinement_for`] turns that into a refinement step (each halves the chord),
+//! with hysteresis. A step not made yet is planned off the UI thread and swapped in when it is
+//! ready; until then the body keeps the step it has. Only bodies on screen and big enough get
+//! refined, within a triangle budget (the bodies that need it most first) and a cache size.
+//!
+//! The model scene draws a refined body's faces from the refined plan (on the GPU, or its CPU
+//! mesh), and its edges and highlights from that plan's mesh, so a highlight keeps lying on the
+//! drawn triangles (#1064). Display only: the reference mesh the model reads, exports and the
+//! B-rep never change.
+
+use std::collections::HashMap;
+use std::sync::Arc;
+
+use solidcraft_engine::doc::ModelBody;
+use solidcraft_engine::doc::display_quality::{DisplayQuality, MAX_REFINE, refinement_for};
+use solidcraft_engine::geom::tess::TessPlan;
+use solidcraft_engine::geom::{Aabb3, Mesh, Vec3};
+
+/// Most triangles all refined bodies may draw together (on top of the rest of the model).
+pub(crate) const TRIANGLE_BUDGET: usize = 6_000_000;
+/// Most bytes the refined plans and meshes kept may take; the least recently drawn go first.
+pub(crate) const CACHE_BYTES: usize = 768 << 20;
+/// A body smaller than this on screen (its box's longer side, physical pixels) is not refined.
+pub(crate) const MIN_PX: f64 = 32.0;
+/// Refinements planned at once, off the UI thread.
+#[cfg(not(target_arch = "wasm32"))]
+const MAX_JOBS: usize = 2;
+
+/// A refined plan and its CPU mesh (none: the body has no plan at that step).
+pub(crate) type Refined = Option<(Arc<TessPlan>, Arc<Mesh>)>;
+
+/// Whether the viewport refines at all: `SOLIDCRAFT_DISPLAY_REFINE=0` turns it off (to compare
+/// pictures and frame times with and without it).
+pub(crate) fn enabled() -> bool {
+    static ON: std::sync::OnceLock<bool> = std::sync::OnceLock::new();
+    *ON.get_or_init(|| std::env::var("SOLIDCRAFT_DISPLAY_REFINE").map_or(true, |v| v != "0"))
+}
+
+/// A body's identity here: the address of its reference mesh, which is shared by every copy of
+/// the same body across timeline snapshots and held by the cache so the address stays its own.
+pub(crate) fn body_id(b: &ModelBody) -> (usize, Arc<Mesh>) {
+    let m = b.mesh();
+    (Arc::as_ptr(&m) as usize, m)
+}
+
+/// How the camera maps millimetres to pixels.
+#[derive(Clone, Copy, Debug)]
+pub(crate) struct Lens {
+    pub eye: Vec3,
+    /// Unit direction from the target toward the eye.
+    pub back: Vec3,
+    /// Orthographic: pixels per millimetre everywhere. Perspective: pixels per millimetre at a
+    /// depth of 1 mm (divided by the depth).
+    pub scale: f64,
+    pub perspective: bool,
+    /// Nearest depth considered (mm), so a body around the eye does not ask for infinity.
+    pub near: f64,
+}
+
+impl Lens {
+    /// The view of `cam` in a viewport `height_px` physical pixels high.
+    pub fn new(cam: &solidcraft_engine::render::Camera, height_px: f64) -> Lens {
+        let perspective = cam.fov > 0.0;
+        let scale = if perspective { height_px / (2.0 * (cam.fov * 0.5).tan()) } else { height_px / (2.0 * cam.half_height()) };
+        Lens { eye: cam.eye(), back: cam.back(), scale, perspective, near: (cam.distance * 1e-2).max(1e-3) }
+    }
+
+    /// Whether any of a box lies in front of the eye.
+    pub fn in_front(&self, b: &Aabb3) -> bool {
+        let toward = -self.back;
+        let far_corner = Vec3::new(
+            if toward.x > 0.0 { b.max.x } else { b.min.x },
+            if toward.y > 0.0 { b.max.y } else { b.min.y },
+            if toward.z > 0.0 { b.max.z } else { b.min.z },
+        );
+        !self.perspective || (far_corner - self.eye).dot(toward) > 0.0
+    }
+
+    /// Pixels per millimetre where a box comes nearest the eye.
+    pub fn px_per_mm(&self, b: &Aabb3) -> f64 {
+        if !self.perspective {
+            return self.scale;
+        }
+        // The box's nearest depth along the view: its support point toward the eye.
+        let toward = -self.back;
+        let near_corner = Vec3::new(
+            if toward.x > 0.0 { b.min.x } else { b.max.x },
+            if toward.y > 0.0 { b.min.y } else { b.max.y },
+            if toward.z > 0.0 { b.min.z } else { b.max.z },
+        );
+        let depth = (near_corner - self.eye).dot(toward);
+        self.scale / depth.max(self.near)
+    }
+}
+
+/// The screen-space error (physical pixels) of drawing a body of `size` at `q` and its box's
+/// projected extent (`screen_px`, physical pixels; `None` when the box is not on screen).
+pub(crate) fn error_px(q: DisplayQuality, size: f64, bounds: &Aabb3, screen_px: Option<f64>, lens: &Lens) -> f64 {
+    match screen_px {
+        Some(px) if px >= MIN_PX => q.chord(size) * lens.px_per_mm(bounds),
+        _ => 0.0,
+    }
+}
+
+/// One shown body as the refinement sees it this frame.
+pub(crate) struct Candidate<'a> {
+    pub body: &'a ModelBody,
+    /// The chosen level's screen-space error ([`error_px`]).
+    pub err_px: f64,
+    /// Triangles at the chosen level (an estimate of the step-0 count).
+    pub triangles: usize,
+}
+
+struct Entry {
+    /// Keeps the body's id address its own while the entry lives.
+    _holder: Arc<Mesh>,
+    refined: Refined,
+    bytes: usize,
+    used: u64,
+}
+
+#[cfg(not(target_arch = "wasm32"))]
+struct Job {
+    key: (usize, DisplayQuality, u8),
+    holder: Arc<Mesh>,
+    rx: std::sync::mpsc::Receiver<Refined>,
+}
+
+/// What the refinement holds now (`ui.inspect` → `display_lod`).
+#[derive(Clone, Debug, Default, serde::Serialize)]
+pub(crate) struct Stats {
+    /// Bodies drawn finer than the chosen level.
+    pub refined_bodies: usize,
+    /// Triangles those bodies draw.
+    pub refined_triangles: usize,
+    /// Refined plans and meshes kept, and their bytes.
+    pub cached: usize,
+    pub cache_bytes: usize,
+    /// Refinements being planned.
+    pub planning: usize,
+    /// Refinements planned so far, and the time they took (ms, off the UI thread).
+    pub planned: usize,
+    pub planning_ms: f64,
+    /// Time the last choice of steps took on the UI thread (ms).
+    pub choose_ms: f64,
+}
+
+/// The refinement state of a viewport.
+#[derive(Default)]
+pub(crate) struct Lod {
+    /// The step each body is drawn at, by [`body_id`] (absent: 0, the chosen level).
+    shown: HashMap<usize, u8>,
+    cache: HashMap<(usize, DisplayQuality, u8), Entry>,
+    #[cfg(not(target_arch = "wasm32"))]
+    jobs: Vec<Job>,
+    /// The web plans one refinement per frame: more are waiting.
+    #[cfg(target_arch = "wasm32")]
+    more: bool,
+    /// Bumped whenever a body's drawn step changes (part of the scene's key).
+    pub generation: u64,
+    clock: u64,
+    pub stats: Stats,
+}
+
+impl Lod {
+    /// The refined plan and mesh `b` is drawn from, when it is drawn finer than the chosen level.
+    pub fn refined(&self, b: &ModelBody, q: DisplayQuality) -> Refined {
+        let (id, _) = body_id(b);
+        let k = *self.shown.get(&id)?;
+        self.cache.get(&(id, q, k))?.refined.clone()
+    }
+
+    /// The step `b` is drawn at.
+    #[cfg(test)]
+    pub fn step(&self, b: &ModelBody) -> u8 {
+        self.shown.get(&body_id(b).0).copied().unwrap_or(0)
+    }
+
+    /// Choose each candidate's step, start planning the steps not made yet, and swap in those
+    /// that are ready. `live` are the ids of every body in the model now (hidden ones too): the
+    /// cache drops the others. Returns whether any body's drawn step changed.
+    pub fn update(&mut self, q: DisplayQuality, candidates: Vec<Candidate<'_>>, live: &std::collections::HashSet<usize>) -> bool {
+        self.clock += 1;
+        self.collect();
+        let mut changed = false;
+        // The bodies that need it most first, within the triangle budget.
+        let mut order: Vec<usize> = (0..candidates.len()).collect();
+        order.sort_by(|&a, &b| candidates[b].err_px.total_cmp(&candidates[a].err_px));
+        let mut remaining = TRIANGLE_BUDGET;
+        let mut wanted: Vec<(usize, u8)> = Vec::new();
+        let mut requests: Vec<(usize, u8)> = Vec::new();
+        let mut seen = std::collections::HashSet::new();
+        for i in order {
+            let c = &candidates[i];
+            let (id, _) = body_id(c.body);
+            if !seen.insert(id) {
+                continue;
+            }
+            let current = self.shown.get(&id).copied().unwrap_or(0);
+            let mut k = refinement_for(c.err_px, current, MAX_REFINE);
+            let estimate = |k: u8| self.cache.get(&(id, q, k)).and_then(|e| e.refined.as_ref()).map_or(c.triangles << k, |(_, m)| m.triangles.len());
+            while k > 0 && estimate(k) > remaining {
+                k -= 1;
+            }
+            if k > 0 {
+                remaining -= estimate(k);
+            }
+            wanted.push((i, k));
+        }
+        for (i, k) in wanted {
+            let (id, holder) = body_id(candidates[i].body);
+            let current = self.shown.get(&id).copied().unwrap_or(0);
+            if k == current {
+                continue;
+            }
+            if k == 0 {
+                self.shown.remove(&id);
+                changed = true;
+                continue;
+            }
+            match self.cache.get(&(id, q, k)) {
+                Some(e) if e.refined.is_some() => {
+                    self.shown.insert(id, k);
+                    changed = true;
+                }
+                // The planner refused this step: keep what is drawn.
+                Some(_) => {}
+                None => {
+                    let _ = holder;
+                    requests.push((i, k));
+                }
+            }
+        }
+        self.start(q, &candidates, requests);
+        // Keep the steps drawn now from being evicted.
+        for (id, k) in &self.shown {
+            if let Some(e) = self.cache.get_mut(&(*id, q, *k)) {
+                e.used = self.clock;
+            }
+        }
+        self.shown.retain(|id, _| live.contains(id));
+        self.evict(live);
+        self.refresh_stats(q);
+        if changed {
+            self.generation += 1;
+        }
+        changed
+    }
+
+    /// Whether refinements are still being planned (the viewport repaints to collect them).
+    pub fn busy(&self) -> bool {
+        #[cfg(not(target_arch = "wasm32"))]
+        {
+            !self.jobs.is_empty()
+        }
+        #[cfg(target_arch = "wasm32")]
+        {
+            self.more
+        }
+    }
+
+    fn insert(&mut self, key: (usize, DisplayQuality, u8), holder: Arc<Mesh>, refined: Refined) {
+        let bytes = refined.as_ref().map_or(0, |(plan, mesh)| mesh_bytes(mesh) + plan.vertex_count() * 48 + mesh.triangles.len() * 12);
+        self.cache.insert(key, Entry { _holder: holder, refined, bytes, used: self.clock });
+    }
+
+    /// Collect finished refinements; whether any finished. They are swapped in by the next
+    /// choice of steps ([`Lod::update`]).
+    #[cfg(not(target_arch = "wasm32"))]
+    pub fn collect(&mut self) -> bool {
+        let mut done = Vec::new();
+        self.jobs.retain(|j| match j.rx.try_recv() {
+            Ok(r) => {
+                done.push((j.key, j.holder.clone(), r));
+                false
+            }
+            Err(std::sync::mpsc::TryRecvError::Empty) => true,
+            Err(std::sync::mpsc::TryRecvError::Disconnected) => {
+                done.push((j.key, j.holder.clone(), None));
+                false
+            }
+        });
+        let any = !done.is_empty();
+        for (key, holder, r) in done {
+            self.insert(key, holder, r);
+        }
+        any
+    }
+
+    #[cfg(target_arch = "wasm32")]
+    pub fn collect(&mut self) -> bool {
+        self.more
+    }
+
+    /// Plan the requested steps: off the UI thread, a few at a time, the most needed first (the
+    /// web has no threads: one per frame, right away).
+    fn start(&mut self, q: DisplayQuality, candidates: &[Candidate<'_>], mut requests: Vec<(usize, u8)>) {
+        requests.sort_by(|a, b| candidates[b.0].err_px.total_cmp(&candidates[a.0].err_px));
+        #[cfg(not(target_arch = "wasm32"))]
+        for (i, k) in requests {
+            if self.jobs.len() >= MAX_JOBS {
+                break;
+            }
+            let body = candidates[i].body.clone();
+            let (id, holder) = body_id(&body);
+            let key = (id, q, k);
+            if self.jobs.iter().any(|j| j.key.0 == id) {
+                continue;
+            }
+            let (tx, rx) = std::sync::mpsc::channel();
+            let spawned = std::thread::Builder::new().name("display-refine".into()).spawn(move || {
+                let t0 = std::time::Instant::now();
+                let r = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| body.refined_display(q, k))).ok().flatten();
+                log::debug!("refined {} {k} steps in {:.1} ms", body.name, t0.elapsed().as_secs_f64() * 1e3);
+                PLANNING_US.fetch_add(t0.elapsed().as_micros() as u64, std::sync::atomic::Ordering::Relaxed);
+                let _ = tx.send(r);
+            });
+            match spawned {
+                Ok(_) => self.jobs.push(Job { key, holder, rx }),
+                Err(e) => {
+                    log::warn!("display refinement: {e}");
+                    break;
+                }
+            }
+        }
+        #[cfg(target_arch = "wasm32")]
+        {
+            self.more = requests.len() > 1;
+        }
+        #[cfg(target_arch = "wasm32")]
+        if let Some((i, k)) = requests.into_iter().next() {
+            let body = candidates[i].body;
+            let (id, holder) = body_id(body);
+            let r = body.refined_display(q, k);
+            self.insert((id, q, k), holder, r);
+        }
+    }
+
+    /// Drop what belongs to bodies no longer in the model, then the least recently drawn steps
+    /// until the cache fits its budget.
+    fn evict(&mut self, live: &std::collections::HashSet<usize>) {
+        self.cache.retain(|(id, _, _), _| live.contains(id));
+        let mut total: usize = self.cache.values().map(|e| e.bytes).sum();
+        if total <= CACHE_BYTES {
+            return;
+        }
+        let mut by_age: Vec<((usize, DisplayQuality, u8), u64, usize)> = self.cache.iter().map(|(k, e)| (*k, e.used, e.bytes)).collect();
+        by_age.sort_by_key(|(_, used, _)| *used);
+        for (key, used, bytes) in by_age {
+            if total <= CACHE_BYTES || used == self.clock {
+                break;
+            }
+            self.cache.remove(&key);
+            total -= bytes;
+        }
+    }
+
+    fn refresh_stats(&mut self, q: DisplayQuality) {
+        let drawn: Vec<&Entry> = self.shown.iter().filter_map(|(id, k)| self.cache.get(&(*id, q, *k))).collect();
+        self.stats.refined_bodies = drawn.len();
+        self.stats.refined_triangles = drawn.iter().filter_map(|e| e.refined.as_ref()).map(|(_, m)| m.triangles.len()).sum();
+        self.stats.cached = self.cache.len();
+        self.stats.cache_bytes = self.cache.values().map(|e| e.bytes).sum();
+        #[cfg(not(target_arch = "wasm32"))]
+        {
+            self.stats.planning = self.jobs.len();
+        }
+        self.stats.planning_ms = PLANNING_US.load(std::sync::atomic::Ordering::Relaxed) as f64 / 1e3;
+        self.stats.planned = self.cache.len();
+    }
+
+    /// Plan a step right away and draw `b` at it (tests).
+    #[cfg(test)]
+    pub fn force(&mut self, b: &ModelBody, q: DisplayQuality, k: u8) -> bool {
+        let (id, holder) = body_id(b);
+        let r = b.refined_display(q, k);
+        let ok = r.is_some();
+        self.insert((id, q, k), holder, r);
+        if ok {
+            self.shown.insert(id, k);
+            self.generation += 1;
+        }
+        ok
+    }
+}
+
+/// Microseconds spent planning refinements (all threads).
+static PLANNING_US: std::sync::atomic::AtomicU64 = std::sync::atomic::AtomicU64::new(0);
+
+fn mesh_bytes(m: &Mesh) -> usize {
+    m.positions.len() * 24
+        + m.normals.len() * 24
+        + m.triangles.len() * 12
+        + m.tri_face.len() * 4
+        + m.edges.iter().map(|e| e.len() * 24).sum::<usize>()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use solidcraft_engine::render::Camera;
+
+    fn cylinder() -> ModelBody {
+        let body = solidcraft_engine::kernel::cylinder(Vec3::ZERO, Vec3::Z, 10.0, 20.0).unwrap();
+        ModelBody::new("Body1".into(), body, 1)
+    }
+
+    /// Zooming in raises the error with the pixels per millimetre, perspective or not, and a
+    /// box nearer the eye counts more.
+    #[test]
+    fn the_error_follows_the_zoom() {
+        let b = Aabb3 { min: Vec3::new(-10.0, -10.0, 0.0), max: Vec3::new(10.0, 10.0, 20.0) };
+        let q = DisplayQuality::Normal;
+        for fov in [0.0, 0.6] {
+            let mut cam = Camera { distance: 200.0, fov, ..Camera::default() };
+            let far = error_px(q, 30.0, &b, Some(300.0), &Lens::new(&cam, 1000.0));
+            cam.distance = 50.0;
+            let near = error_px(q, 30.0, &b, Some(1200.0), &Lens::new(&cam, 1000.0));
+            assert!(near > far * 3.0, "fov {fov}: {far} -> {near}");
+            assert_eq!(error_px(q, 30.0, &b, None, &Lens::new(&cam, 1000.0)), 0.0, "off screen");
+            assert_eq!(error_px(q, 30.0, &b, Some(MIN_PX / 2.0), &Lens::new(&cam, 1000.0)), 0.0, "too small");
+        }
+        // Orthographic: chord × pixels per mm.
+        let cam = Camera { distance: 100.0, fov: 0.0, ..Camera::default() };
+        let e = error_px(q, 30.0, &b, Some(500.0), &Lens::new(&cam, 800.0));
+        assert!((e - q.chord(30.0) * 800.0 / (2.0 * cam.half_height())).abs() < 1e-12);
+    }
+
+    /// A close-up refines the body (planned off the thread, then swapped in), a step finer has
+    /// more triangles and stays within its chord, and zooming back out returns to the chosen level.
+    #[test]
+    fn a_close_up_refines_and_zooming_out_returns() {
+        let b = cylinder();
+        let q = DisplayQuality::Normal;
+        let live: std::collections::HashSet<usize> = [body_id(&b).0].into();
+        let base = b.plan_mesh_at(q).unwrap().triangles.len();
+        let mut lod = Lod::default();
+        let cand = |err: f64| vec![Candidate { body: &b, err_px: err, triangles: base }];
+        assert!(!lod.update(q, cand(0.5), &live));
+        assert_eq!(lod.step(&b), 0);
+        // 6 px of error: three steps.
+        let mut swapped = false;
+        for _ in 0..2000 {
+            swapped |= lod.update(q, cand(6.0), &live);
+            if lod.step(&b) == 3 {
+                break;
+            }
+            std::thread::sleep(std::time::Duration::from_millis(5));
+        }
+        assert!(swapped && lod.step(&b) == 3, "refined to step {}", lod.step(&b));
+        let (_, mesh) = lod.refined(&b, q).unwrap();
+        assert!(mesh.triangles.len() > base * 2, "{} vs {base}", mesh.triangles.len());
+        let r = |v: Vec3| (v.x * v.x + v.y * v.y).sqrt();
+        let sag = mesh
+            .triangles
+            .iter()
+            .map(|t| t.map(|i| mesh.positions[i as usize]))
+            .filter(|t| t.iter().all(|p| (r(*p) - 10.0).abs() < 1e-9))
+            .map(|[a, b, c]| 10.0 - r((a + b + c) * (1.0 / 3.0)))
+            .fold(0.0, f64::max);
+        assert!(sag <= q.refined_chord(b.body.size(), 3) * 1.01, "sag {sag}");
+        // The reference mesh never changes.
+        assert!(Arc::ptr_eq(&body_id(&b).1, &b.mesh()));
+        // Zoomed out a little: kept; far out: the chosen level.
+        assert!(!lod.update(q, cand(3.0), &live));
+        assert_eq!(lod.step(&b), 3);
+        assert!(lod.update(q, cand(0.1), &live));
+        assert_eq!(lod.step(&b), 0);
+        assert!(lod.refined(&b, q).is_none());
+        // Back in: swapped from the cache at once.
+        assert!(lod.update(q, cand(6.0), &live));
+        assert_eq!(lod.step(&b), 3);
+        // The body leaves the model: dropped.
+        lod.update(q, Vec::new(), &Default::default());
+        assert_eq!(lod.stats.cached, 0);
+        assert_eq!(lod.step(&b), 0);
+    }
+
+    /// The budget refines the bodies that need it most and leaves the rest at the chosen level.
+    #[test]
+    fn the_budget_goes_to_the_largest_errors() {
+        let (a, b) = (cylinder(), cylinder());
+        let q = DisplayQuality::Normal;
+        let live: std::collections::HashSet<usize> = [body_id(&a).0, body_id(&b).0].into();
+        let mut lod = Lod::default();
+        // Each would want four steps at 3M triangles a step-0 count: only the first fits.
+        let cands = vec![
+            Candidate { body: &a, err_px: 2.0, triangles: TRIANGLE_BUDGET / 3 },
+            Candidate { body: &b, err_px: 12.0, triangles: TRIANGLE_BUDGET / 3 },
+        ];
+        // Distinct bodies (two ModelBody::new calls), so distinct ids.
+        assert_ne!(body_id(&a).0, body_id(&b).0);
+        lod.update(q, cands, &live);
+        assert_eq!(lod.stats.planning, 1, "one refinement fits the budget");
+        #[cfg(not(target_arch = "wasm32"))]
+        assert_eq!(lod.jobs[0].key.0, body_id(&b).0, "the larger error first");
+    }
+}

crates/ui-egui/src/gpu.rs

@@ -295,6 +295,12 @@ fn joined<'a>(blocks: impl Iterator<Item = &'a Option<Batch>>, own: Option<Batch
     (!parts.is_empty()).then_some(Batch { parts, chunk })
 }
 
+/// Planned bodies end a GPU chunk one time in this many (by their plan's address), and a chunk
+/// holds at most [`MAX_CHUNK_BODIES`]: a scene of a thousand bodies takes a few dozen dispatches
+/// once, and a changed body re-evaluates a few dozen bodies, not all of them.
+const CHUNK_SPLIT: u64 = 32;
+const MAX_CHUNK_BODIES: usize = 128;
+
 #[derive(Default)]
 struct Batches {
     /// The scene blocks on the GPU, by key.
@@ -381,18 +387,25 @@ impl Batches {
         encoder: &mut wgpu::CommandEncoder,
         bodies: &[PlannedBody],
     ) -> Vec<PlannedDraw> {
+        // Chunks end where a body's own plan says (or when full), so a body whose plan changes
+        // (an edit, or a refinement as the view zooms in, #1320) re-evaluates only its own
+        // chunk, not the whole scene.
         let mut chunks: Vec<Vec<&PlannedBody>> = Vec::new();
         let mut filled = 0usize;
+        let mut open = false;
         for b in bodies {
             let n = b.plan.vertex_count();
             match chunks.last_mut() {
-                Some(c) if filled + n <= solidcraft_gpu::MAX_BATCH => c.push(b),
+                Some(c) if open && filled + n <= solidcraft_gpu::MAX_BATCH => c.push(b),
                 _ => {
                     chunks.push(vec![b]);
                     filled = 0;
                 }
             }
             filled += n;
+            let mut h = std::collections::hash_map::DefaultHasher::new();
+            std::hash::Hash::hash(&(Arc::as_ptr(&b.plan) as usize), &mut h);
+            open = !std::hash::Hasher::finish(&h).is_multiple_of(CHUNK_SPLIT) && chunks.last().is_some_and(|c| c.len() < MAX_CHUNK_BODIES);
         }
         let key = |c: &[&PlannedBody]| c.iter().map(|b| Arc::as_ptr(&b.plan) as usize).collect::<Vec<usize>>();
         self.resident.retain(|k, _| chunks.iter().any(|c| key(c) == *k));

crates/ui-egui/src/lib.rs

@@ -90,6 +90,7 @@ mod file_picker;
 pub use file_picker::FilePickerFuture;
 #[cfg(test)]
 mod blend_arrow_tests;
+pub(crate) mod display_lod;
 #[cfg(test)]
 mod file_load_ui_tests;
 #[cfg(test)]

crates/ui-egui/src/viewport.rs

@@ -44,6 +44,11 @@ pub struct ViewportState {
     pick_bounds: std::sync::Mutex<Option<BodyBounds>>,
     /// The model scene's blocks of bodies by key, from the last scene built.
     pub(crate) blocks: std::sync::Mutex<std::collections::HashMap<u64, CachedBlock>>,
+    /// View-dependent display refinement (#1320): which bodies are drawn finer than the chosen
+    /// display quality, and their refined plans.
+    pub(crate) lod: std::sync::Mutex<crate::display_lod::Lod>,
+    /// The view and model the refinement was last chosen for.
+    lod_view: u64,
     /// Each body mesh's box (the mesh held, so its address stays its own), from the last state.
     mesh_bounds: std::sync::Mutex<std::collections::HashMap<usize, (std::sync::Arc<solidcraft_engine::geom::Mesh>, solidcraft_engine::geom::Aabb3)>>,
     /// The last click in the view was the middle button's (for the middle double-click).
@@ -274,6 +279,7 @@ fn scene_key(app: &SolveApp) -> u64 {
     app.ui.visual_style.hash(&mut h);
     app.ui.ground_shadow.hash(&mut h);
     solidcraft_engine::doc::display_quality::current().hash(&mut h);
+    lod_generation(app).hash(&mut h);
     // The section plane (its cut outlines are part of the model scene).
     section_plane(app).map(|(o, n)| [o.x, o.y, o.z, n.x, n.y, n.z].map(f64::to_bits)).hash(&mut h);
     app.cam.up.hash(&mut h);
@@ -372,7 +378,8 @@ const SS_SETTLE: f64 = 0.25;
 /// The display plan the model scene draws `b` from on the GPU, or `None` when its faces are built
 /// here from the reference mesh. Opaque shaded bodies (`opaque`: not faded, and no see-through
 /// body or face) are drawn from their plans where the device evaluates them; faded and
-/// see-through ones (sorted and blended per triangle) are built here.
+/// see-through ones (sorted and blended per triangle) are built here. A body seen up close is
+/// drawn from its refined plan ([`crate::display_lod`]).
 fn drawn_plan(
     app: &SolveApp,
     b: &solidcraft_engine::doc::ModelBody,
@@ -380,11 +387,22 @@ fn drawn_plan(
 ) -> Option<std::sync::Arc<solidcraft_engine::geom::tess::TessPlan>> {
     let shaded = app.ui.visual_style != 2;
     (shaded && opaque && app.viewport.gpu.is_some_and(|g| g.tess))
-        .then(|| b.display_plan())
+        .then(|| refined(app, b).map(|(plan, _)| plan).or_else(|| b.display_plan()))
         .flatten()
         .filter(|plan| plan.vertex_count() <= solidcraft_gpu::MAX_BATCH)
 }
 
+/// The refined plan and mesh `b` is drawn from, when the view has it drawn finer than the chosen
+/// display quality (#1320).
+fn refined(app: &SolveApp, b: &solidcraft_engine::doc::ModelBody) -> crate::display_lod::Refined {
+    app.viewport.lod.lock().ok()?.refined(b, solidcraft_engine::doc::display_quality::current())
+}
+
+/// Changes whenever a body's refinement changes (the scene and highlight keys).
+fn lod_generation(app: &SolveApp) -> u64 {
+    app.viewport.lod.lock().map_or(0, |l| l.generation)
+}
+
 /// The mesh whose triangles the model scene shows for `b`'s faces: the plan's (evaluated on the
 /// CPU, the same triangles the GPU draws) or [`drawn_mesh`]. A face highlighted over the model
 /// must use the same triangles: another triangulation of the same face fights it at equal depth
@@ -394,6 +412,10 @@ fn shown_mesh(
     b: &solidcraft_engine::doc::ModelBody,
     active_bodies: &Option<Vec<String>>,
 ) -> std::sync::Arc<solidcraft_engine::geom::Mesh> {
+    if let Some((_, mesh)) = refined(app, b) {
+        // Refined: faces drawn from the refined plan on the GPU or built from its mesh here.
+        return mesh;
+    }
     let doc = &app.session.doc;
     let opaque =
         || !body_faded(app, b, active_bodies) && doc.model_body_opacity(b) >= 0.999 && doc.face_colors(b).iter().all(|(_, l)| l.opacity >= 0.999);
@@ -401,18 +423,84 @@ fn shown_mesh(
         .then(|| drawn_plan(app, b, true))
         .flatten()
         .and_then(|_| b.plan_mesh())
-        .unwrap_or_else(|| drawn_mesh(b))
+        .unwrap_or_else(|| drawn_mesh(app, b))
 }
 
 /// The mesh the model scene builds `b` from on the CPU (its edges always, its faces when they
-/// are not drawn from the plan on the GPU): the reference mesh at Normal display quality, else
+/// are not drawn from the plan on the GPU): a refined plan's mesh when the view has one for it
-/// the plan's mesh at the chosen quality (finer or coarser faces and edges alike), falling back
+/// (#1320), the reference mesh at Normal display quality, else the plan's mesh at the chosen
-/// to the reference mesh when the body has no plan.
+/// quality (finer or coarser faces and edges alike), falling back to the reference mesh when the
-fn drawn_mesh(b: &solidcraft_engine::doc::ModelBody) -> std::sync::Arc<solidcraft_engine::geom::Mesh> {
+/// body has no plan.
+fn drawn_mesh(app: &SolveApp, b: &solidcraft_engine::doc::ModelBody) -> std::sync::Arc<solidcraft_engine::geom::Mesh> {
     use solidcraft_engine::doc::display_quality::{DisplayQuality, current};
+    if let Some((_, mesh)) = refined(app, b) {
+        return mesh;
+    }
     if current() == DisplayQuality::Normal { b.mesh() } else { b.plan_mesh().unwrap_or_else(|| b.mesh()) }
 }
 
+/// Choose which bodies to draw finer than the chosen display quality for this view (#1320):
+/// each shown body's screen-space error from its box on screen, refinements planned off the UI
+/// thread and swapped in when ready. Keeps repainting while refinements are being planned.
+fn update_refinement(app: &mut SolveApp, ctx: &egui::Context, proj: &Proj) {
+    if app.viewport.gpu.is_none() || !crate::display_lod::enabled() {
+        return; // The CPU picture draws the reference mesh.
+    }
+    let q = solidcraft_engine::doc::display_quality::current();
+    // Nothing to choose again while the view, the model and the planning stand still.
+    let mut h = std::collections::hash_map::DefaultHasher::new();
+    scene_key(app).hash(&mut h);
+    crate::sketch_display::projection_key(proj, app.viewport.pixels_per_point()).hash(&mut h);
+    let view = h.finish();
+    // While refinements are being planned, the view standing still: only collect finished ones.
+    if view == app.viewport.lod_view {
+        let (busy, finished) = app.viewport.lod.lock().map_or((false, false), |mut l| {
+            let finished = l.collect();
+            (l.busy(), finished)
+        });
+        if busy {
+            ctx.request_repaint_after(std::time::Duration::from_millis(30));
+        }
+        if !finished {
+            return;
+        }
+    }
+    let t0 = crate::now_ms();
+    app.viewport.lod_view = view;
+    let st = app.session.world_state();
+    let ppp = f64::from(app.viewport.pixels_per_point());
+    let lens = crate::display_lod::Lens::new(&proj.cam, f64::from(proj.rect.height()) * ppp);
+    let bounds = body_bounds(app, &st);
+    let mut live = std::collections::HashSet::with_capacity(st.bodies.len());
+    let mut candidates = Vec::new();
+    for (b, (name, bx)) in st.bodies.iter().zip(bounds.iter()) {
+        let id = crate::display_lod::body_id(b).0;
+        live.insert(id);
+        if b.name != *name || b.body.is_mesh() {
+            continue;
+        }
+        let shown = app.session.body_visible(&b.name) && !app.preview.replaced.contains(&b.name);
+        let on_screen = match screen_box(proj, bx, 0.0).filter(|_| shown) {
+            Some(r) => r.intersects(proj.rect).then(|| f64::from(r.width().max(r.height())) * ppp),
+            // A corner behind the eye: the view is inside or right by the box.
+            None => (shown && proj.cam.fov > 0.0 && lens.in_front(bx)).then_some(f64::INFINITY),
+        };
+        let err_px = crate::display_lod::error_px(q, b.body.size(), bx, on_screen, &lens);
+        candidates.push(crate::display_lod::Candidate { body: b, err_px, triangles: b.mesh().triangles.len() });
+    }
+    let busy = match app.viewport.lod.lock() {
+        Ok(mut lod) => {
+            lod.update(q, candidates, &live);
+            lod.stats.choose_ms = crate::now_ms() - t0;
+            lod.busy()
+        }
+        Err(_) => false,
+    };
+    if busy {
+        ctx.request_repaint_after(std::time::Duration::from_millis(30));
+    }
+}
+
 /// A model scene block kept by the viewport: its bytes and the meshes they were built from
 /// (held, so no other mesh can take their addresses while the block is kept).
 pub(crate) struct CachedBlock {
@@ -561,7 +649,7 @@ fn build_scene(app: &SolveApp) -> GpuScene {
                 .collect();
             sc.planned.push(crate::gpu::PlannedBody { plan, face_colors });
         }
-        let body = BodyDraw { mesh: drawn_mesh(b), col, faded, opacity, face_looks, on_gpu };
+        let body = BodyDraw { mesh: drawn_mesh(app, b), col, faded, opacity, face_looks, on_gpu };
         let key = body.key(shaded, edges, hidden_edges, edge_col, hidden_col);
         key.hash(&mut run_key);
         run.push(body);
@@ -1683,6 +1771,7 @@ fn highlight_key(app: &SolveApp) -> u64 {
     app.viewport.hover_feature.hash(&mut h);
     app.viewport.hover_bodies.hash(&mut h);
     // Which mesh lights a face follows how the model draws its body (see `shown_mesh`).
+    lod_generation(app).hash(&mut h);
     app.session.active_component.hash(&mut h);
     app.session.active_occurrence_path.hash(&mut h);
     app.ui.visual_style.hash(&mut h);
@@ -1783,7 +1872,7 @@ fn build_highlight(app: &SolveApp) -> GpuScene {
                 // A live preview stands in for the body: its old faces would fight with it.
                 if let Some(b) = st.body(body).filter(|_| app.session.body_visible(body) && !app.preview.replaced.contains(body)) {
                     face_tris(&mut sc, &shown(b), *index, c4(t.sel_face), false);
-                    hidden_face_outline(&mut sc, &drawn_mesh(b), *index, t.sel_edge);
+                    hidden_face_outline(&mut sc, &drawn_mesh(app, b), *index, t.sel_edge);
                 }
             }
             Sel::Body { name } => {
@@ -1799,7 +1888,7 @@ fn build_highlight(app: &SolveApp) -> GpuScene {
             Sel::Edge { body, index, point } => {
                 if let Some(b) = st.body(body)
                     && app.session.body_visible(body)
-                    && let Some(e) = edge_of(&drawn_mesh(b), *index, *point)
+                    && let Some(e) = edge_of(&drawn_mesh(app, b), *index, *point)
                 {
                     edge_lines(&mut sc, e, t.sel_edge, t.sel_edge_rim, 3.0);
                 }
@@ -1863,7 +1952,7 @@ fn build_highlight(app: &SolveApp) -> GpuScene {
         }
         let k = t.hover_face_lift;
         let c = body_rgb(app, &b.name);
-        let m = drawn_mesh(b);
+        let m = drawn_mesh(app, b);
         let lit = shown(b);
         for &f in &faces {
             if !sel.iter().any(|x| {
@@ -1898,7 +1987,7 @@ fn build_highlight(app: &SolveApp) -> GpuScene {
             if let Some(b) = st.body(body) {
                 let k = t.hover_face_lift;
                 let c = body_rgb(app, body);
-                let m = drawn_mesh(b);
+                let m = drawn_mesh(app, b);
                 let patch = m.surface_patch(*index);
                 let lit = shown(b);
                 for &f in &patch {
@@ -1916,7 +2005,7 @@ fn build_highlight(app: &SolveApp) -> GpuScene {
         Some(Hit::Edge { body, index, mid }) => {
             if let Some(b) = st.body(body) {
                 // Lit over the drawn edges (finer away from Normal display quality).
-                let m = drawn_mesh(b);
+                let m = drawn_mesh(app, b);
                 for i in m.circular_rim(*index) {
                     let selected = sel.iter().any(|x| matches!(x, Sel::Edge { body: b, index: j, .. } if b == body && *j == i));
                     if !selected && let Some(e) = if i == *index { edge_of(&m, i, *mid) } else { m.edges.get(i) } {
@@ -2439,6 +2528,7 @@ pub fn show(app: &mut SolveApp, ui: &mut egui::Ui) {
     painter.add(Shape::mesh(mesh));
 
     // ---- model ----
+    update_refinement(app, ui.ctx(), &proj);
     let key = scene_key(app);
     let t0 = crate::now_ms();
     if let Some(_g) = app.viewport.gpu {
@@ -4623,6 +4713,65 @@ mod tests {
         assert!(app.viewport.hover.is_some());
     }
 
+    /// After a close-up refines a body (#1320), the model draws it from the refined plan (on the
+    /// GPU) or its mesh (on the CPU), its edges come from that mesh too, and a hovered face lights
+    /// exactly triangles the model draws, never another triangulation (#1064).
+    #[test]
+    fn a_refined_body_highlights_on_its_drawn_triangles() {
+        use solidcraft_engine::doc::display_quality::current;
+        let tris_of = |bytes: &[u8]| -> Vec<[u32; 9]> {
+            bytes
+                .chunks(3 * crate::gpu::TRI_SIZE)
+                .map(|t| {
+                    let mut v: Vec<[u32; 3]> = t
+                        .chunks(crate::gpu::TRI_SIZE)
+                        .map(|c| std::array::from_fn(|i| u32::from_le_bytes(c[i * 4..i * 4 + 4].try_into().unwrap())))
+                        .collect();
+                    v.sort_unstable();
+                    std::array::from_fn(|i| v[i / 3][i % 3])
+                })
+                .collect()
+        };
+        let mesh_tris = |m: &solidcraft_engine::geom::Mesh| -> std::collections::HashSet<[u32; 9]> {
+            let mut sc = GpuScene::default();
+            for f in 0..m.tri_face.iter().map(|f| *f as usize + 1).max().unwrap_or(0) {
+                face_tris(&mut sc, m, f, [0; 4], true);
+            }
+            tris_of(&sc.tris).into_iter().collect()
+        };
+        for tess in [true, false] {
+            let mut app = cylinder_app();
+            app.viewport.no_pixels = true;
+            app.viewport.gpu = Some(GpuTarget { format: egui_wgpu::wgpu::TextureFormat::Rgba8Unorm, depth: None, samples: 1, tess });
+            let body = app.session.world_state().bodies[0].clone();
+            let before = build_scene(&app).merged();
+            let key = scene_key(&app);
+            assert!(app.viewport.lod.lock().unwrap().force(&body, current(), 2));
+            assert_ne!(scene_key(&app), key, "a refinement rebuilds the scene");
+            let (plan, mesh) = refined(&app, &body).unwrap();
+            assert!(mesh.triangles.len() > body.plan_mesh().unwrap().triangles.len() * 2);
+            let scene = build_scene(&app).merged();
+            if tess {
+                assert!(std::sync::Arc::ptr_eq(&scene.planned[0].plan, &plan), "faces drawn from the refined plan");
+            } else {
+                assert_eq!(scene.tris.len(), mesh.triangles.len() * 3 * crate::gpu::TRI_SIZE, "faces built from the refined mesh");
+                assert!(scene.tris.len() > before.tris.len());
+            }
+            // Edges (lines) from the refined mesh: finer polylines.
+            assert!(scene.lines.len() > before.lines.len(), "{} vs {}", scene.lines.len(), before.lines.len());
+            let drawn = mesh_tris(&mesh);
+            let mut lit_faces = 0;
+            for index in 0..body.body.face_count() {
+                app.viewport.hover = Some(Hit::Face { body: body.name.clone(), index, point: Vec3::ZERO });
+                let lit = tris_of(&build_highlight(&app).tris);
+                assert!(!lit.is_empty(), "face {index} lights");
+                assert!(lit.iter().all(|t| drawn.contains(t)), "face {index}: every lit triangle is a drawn one");
+                lit_faces += 1;
+            }
+            assert!(lit_faces >= 3, "{lit_faces} faces");
+        }
+    }
+
     fn cylinder_app() -> SolveApp {
         let mut s = solidcraft_engine::Session::default();
         s.execute("solid.cylinder", &json!({"radius": 10, "height": 20})).unwrap();