Model renders (screenshot with source model, ui.render) show the section the window shows: the CPU renderer drew the cut face on the far walls at their own depth in a plain grey hatch, so the inside of the body, its edges and the grid showed through it and the cut barely read as a section, it had no outline, perspective views cut along a curve, and an open Section Analysis dialog's preview (or a sketch's Slice) was ignored. The cut face now writes the section plane's depth where it is the visible surface (the CPU counterpart of the viewport's stencil cap), takes the viewport's cap tint and hatch, the cut outline is drawn (section_segments moved from the UI to engine::view and shared), world positions and lines are clipped perspective-correctly, and ui.render in the app uses the viewport's section plane

crates/engine/src/view.rs

@@ -35,6 +35,8 @@ pub mod colors {
     pub const AXIS_Z: Rgb = Rgb(60, 100, 230);
     pub const BG_TOP: Rgb = Rgb(240, 243, 247);
     pub const BG_BOTTOM: Rgb = Rgb(203, 210, 221);
+    /// What the section cap is tinted toward.
+    pub const SECTION_CAP: Rgb = solvecraft_render::SECTION_CAP;
 }
 
 /// Bounds of everything visible.
@@ -119,10 +121,23 @@ pub fn sketch_point_visible(sk: &Sketch, i: usize) -> bool {
     }
 }
 
-/// The scene for a headless render.
+/// The scene for a headless render, cut by the session's section.
 pub fn scene(s: &Session, cam: &Camera) -> Scene {
+    scene_with_section(s, cam, s.section)
+}
+
+/// The scene for a headless render, cut by `section` (origin, normal) instead of the session's:
+/// the app passes the cut its viewport shows (an open Section Analysis dialog's preview, a
+/// sketch's Slice), so model renders match the window.
+pub fn scene_with_section(s: &Session, cam: &Camera, section: Option<(Vec3, Vec3)>) -> Scene {
     let st = s.world_state();
-    let mut sc = Scene { background: Some((colors::BG_TOP, colors::BG_BOTTOM)), clip: s.section.map(|(o, n)| (n, n.dot(o))), ..Default::default() };
+    let section = section.filter(|(o, n)| o.is_finite() && n.is_finite() && n.len() > 1e-9);
+    let mut sc = Scene {
+        background: Some((colors::BG_TOP, colors::BG_BOTTOM)),
+        clip: section.map(|(o, n)| (n, n.dot(o))),
+        cap: Some(colors::SECTION_CAP),
+        ..Default::default()
+    };
     let b = bounds(s);
     sc.radius = (b.diagonal() * 0.5).max(cam.half_height()).max(10.0) + b.center().dist(cam.target);
     // Grid on the ground plane (XY with Z up, XZ with Y up).
@@ -172,6 +187,12 @@ pub fn scene(s: &Session, cam: &Camera) -> Scene {
             }
             sc.lines.push(SceneLine { points: e.clone(), color: colors::EDGE, width: 1.2, on_top: false });
         }
+        // The outline of the body where the section cuts it.
+        if let Some((o, n)) = section {
+            for (a, b) in section_segments(&mesh, o, n) {
+                sc.lines.push(SceneLine { points: vec![a, b], color: colors::EDGE, width: 1.6, on_top: false });
+            }
+        }
     }
     for ss in &st.sketches {
         let active = s.active_sketch == Some(ss.feature);
@@ -185,6 +206,28 @@ pub fn scene(s: &Session, cam: &Camera) -> Scene {
     sc
 }
 
+/// Where the plane through `o` with normal `n` cuts a mesh: one segment per crossed triangle,
+/// nudged a hair to the kept side (so the clip leaves them drawn).
+pub fn section_segments(m: &solvecraft_geom::Mesh, o: Vec3, n: Vec3) -> Vec<(Vec3, Vec3)> {
+    let eps = 1e-4 * (1.0 + m.bounds().diagonal());
+    let mut out = Vec::new();
+    for t in &m.triangles {
+        let Some(p) = m.tri(t) else { continue };
+        let d = p.map(|q| n.dot(q - o));
+        let mut pts = Vec::with_capacity(2);
+        for (i, j) in [(0, 1), (1, 2), (2, 0)] {
+            if (d[i] < 0.0) != (d[j] < 0.0) {
+                let s = d[i] / (d[i] - d[j]);
+                pts.push(p[i] + (p[j] - p[i]) * s - n * eps);
+            }
+        }
+        if let [a, b] = pts[..] {
+            out.push((a, b));
+        }
+    }
+    out
+}
+
 /// Is a sketch drawn? The active one is; others unless hidden. Finished sketches are hidden
 /// once a feature uses them, like Fusion's default, unless shown one by one.
 pub fn sketch_shown(s: &Session, id: u64) -> bool {
@@ -315,4 +358,31 @@ mod tests {
         let changed = whole.chunks(4).zip(cut.chunks(4)).filter(|(a, b)| a != b).count();
         assert!(changed > 500, "{changed} pixels changed");
     }
+
+    /// The section's cut face hides what lies inside the body behind it (it is drawn at the
+    /// plane's depth, not on the far wall like a pane of glass), shows in the cap colour, and
+    /// the cut's outline is drawn.
+    #[test]
+    fn snapshot_section_caps_hide_the_inside() {
+        let mut s = Session::default();
+        s.execute("solid.box", &json!({"length": 40, "width": 30, "height": 20})).unwrap();
+        // Cut away the front half (y < 15), so the iso camera looks into the cut.
+        s.execute("inspect.section", &json!({"plane": {"origin": [0, 15, 0], "normal": [0, -1, 0]}})).unwrap();
+        for fov in [0.0, 0.6] {
+            let mut cam = home_camera(&s);
+            cam.fov = fov;
+            let mut sc = scene(&s, &cam);
+            assert!(!section_segments(&s.world_state().bodies[0].mesh(), Vec3::new(0.0, 15.0, 0.0), -Vec3::Y).is_empty());
+            // A marker inside the body, behind the cut face.
+            let red = Rgb(255, 0, 0);
+            sc.lines.push(SceneLine { points: vec![Vec3::new(5.0, 25.0, 10.0), Vec3::new(35.0, 25.0, 10.0)], color: red, width: 3.0, on_top: false });
+            let img = solvecraft_render::render(&sc, &cam, 300, 220);
+            let px = || img.rgba.chunks(4);
+            let marker = px().filter(|p| p[0] > 200 && p[1] < 60 && p[2] < 60).count();
+            assert_eq!(marker, 0, "fov {fov}: the inside shows through the cut face");
+            // The cap: warm (red over blue), unlike the grey-blue body and background.
+            let cap = px().filter(|p| p[0] as i32 - p[2] as i32 > 20).count();
+            assert!(cap > 500, "fov {fov}: cap pixels {cap}");
+        }
+    }
 }