render: translucent faces in both renderers; datum planes get a light fill (#302)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/render/src/glyphs.rs

@@ -51,7 +51,7 @@ use std::collections::{BTreeMap, BTreeSet, HashSet};
 use fieldcraft_model::{BoundaryCondition, ContactSurfaces, ElementFace, Load, Model, Procedure, ResolvedSet, Target};
 
 use crate::display::{centre, polygon_normal};
-use crate::{Aabb3, Rgb, Scene, SceneLabel, SceneLine, ScenePoint, Vec3};
+use crate::{Aabb3, Rgb, Scene, SceneFill, SceneLabel, SceneLine, ScenePoint, SceneTri, Vec3};
 
 /// The symbol unit drawn when the caller gives none: a fraction of the model's bounding box
 /// diagonal.
@@ -894,8 +894,8 @@ impl Glyphs {
 const DATUM_MARGIN: f64 = 0.05;
 /// A datum point's cross: each arm's length, as a fraction of the bounding box diagonal.
 const DATUM_POINT: f64 = 0.02;
-/// Hatch lines across a datum plane's rectangle (its light fill).
-const DATUM_HATCH: usize = 4;
+/// A datum plane's fill: how opaque its translucent rectangle is.
+pub const DATUM_FILL_ALPHA: f64 = 0.12;
 
 /// Corner `k` (0..8, one bit per axis) of box `b`.
 fn box_corner(b: &Aabb3, k: usize) -> Vec3 {
@@ -958,7 +958,7 @@ fn axis_ends(bounds: &Aabb3, p: Vec3, a: Vec3) -> (Vec3, Vec3) {
     (p + a * (t0 - margin), p + a * (t1 + margin))
 }
 
-/// Draw the model's datum planes (an outlined, lightly hatched rectangle: the model's extent
+/// Draw the model's datum planes (an outlined rectangle with a light translucent fill: the model's extent
 /// within the plane, `bounds`, plus a margin), axes (a line along the model's extent) and points
 /// (a small cross), each with its name; returns the box around them. Only the datums `shown`
 /// takes are drawn, and `highlight` (the one selected in the browser) whether shown or not, in
@@ -976,7 +976,7 @@ pub fn datums(
     for (name, datum) in model.datums().iter().filter(|(n, _)| shown(n) || highlight == Some(n.as_str())) {
         let picked = highlight == Some(name.as_str());
         let color = if picked { colors.highlight } else { colors.datum };
-        let (outline, thin) = if picked { (HIGHLIGHT_WIDTH, OUTLINE_WIDTH) } else { (OUTLINE_WIDTH, COUPLING_WIDTH) };
+        let outline = if picked { HIGHLIGHT_WIDTH } else { OUTLINE_WIDTH };
         // The one selected in the browser shows through the part.
         (d.color, d.on_top) = (color, picked);
         match *datum {
@@ -991,11 +991,9 @@ pub fn datums(
                     let b = ring.get((k + 1) % 4).copied().unwrap_or(*a);
                     d.line(*a, b);
                 }
-                d.width = thin;
-                for k in 1..DATUM_HATCH {
-                    let t = -1.0 + 2.0 * k as f64 / DATUM_HATCH as f64;
-                    d.line(corner(t, -1.0), corner(t, 1.0));
-                    d.line(corner(-1.0, t), corner(1.0, t));
+                let [a, b, c2, e] = ring;
+                for p in [[a, b, c2], [a, c2, e]] {
+                    d.scene.fills.push(SceneFill { tri: SceneTri { p, normal: n, color }, alpha: DATUM_FILL_ALPHA });
                 }
                 d.scene.labels.push(SceneLabel { p: corner(1.0, 1.0), text: name.clone(), color });
             }
@@ -1712,9 +1710,11 @@ mod tests {
         let bounds = cube(0.0, 10.0);
         let mut scene = Scene::default();
         let drawn = datums(&m, &bounds, &|_| true, None, &COLORS, &mut scene);
-        // A rectangle with its hatch (4 + 2 × 3 lines), the axis line and the point's three arms,
-        // each crossing at the point.
-        assert_eq!(scene.lines.len(), 14);
+        // A rectangle (4 lines, filled by two translucent triangles), the axis line and the
+        // point's three arms, each crossing at the point.
+        assert_eq!(scene.lines.len(), 8);
+        assert_eq!(scene.fills.len(), 2);
+        assert!(scene.fills.iter().all(|f| f.alpha == DATUM_FILL_ALPHA && f.tri.normal == Vec3::X));
         let p = Vec3::new(1.0, 2.0, 3.0);
         assert!(scene.lines.iter().rev().take(3).all(|l| ((l.a + l.b) * 0.5).dist(p) < 1e-12));
         let mut names: Vec<&str> = scene.labels.iter().map(|l| l.text.as_str()).collect();
@@ -1734,7 +1734,7 @@ mod tests {
         assert_eq!((only.lines.len(), only.labels.len()), (1, 1));
         let mut picked = Scene::default();
         datums(&m, &bounds, &|_| false, Some("Mid"), &COLORS, &mut picked);
-        assert_eq!((picked.lines.len(), picked.on_top.len()), (0, 10));
+        assert_eq!((picked.lines.len(), picked.on_top.len(), picked.fills.len()), (0, 4, 2));
         assert!(picked.on_top.iter().all(|l| l.color == COLORS.highlight));
     }
 

crates/render/src/lib.rs

@@ -18,7 +18,7 @@ mod vec;
 pub use camera::{Camera, FIT_MARGIN, FULL_VIEW, Mat4, Orientation, StandardView};
 pub use display::{DisplayEdge, DisplayLine, DisplayMesh, DisplayNode, DisplayTri, SurfaceCurve, SurfaceTri};
 pub use raster::{
-    Canvas, LABEL_OFFSET, MAX_DIM, MAX_PIXELS, MAX_PRIMITIVES, RenderError, Rgb, Scene, SceneLabel, SceneLine, ScenePoint, SceneTri, fit_size, png,
-    render, render_layers, render_layers_png, render_png, shade,
+    Canvas, LABEL_OFFSET, MAX_DIM, MAX_PIXELS, MAX_PRIMITIVES, RenderError, Rgb, Scene, SceneFill, SceneLabel, SceneLine, ScenePoint, SceneTri,
+    fit_size, png, render, render_layers, render_layers_png, render_png, shade,
 };
 pub use vec::{Aabb3, Vec3};

crates/render/src/raster.rs

@@ -26,6 +26,14 @@ pub struct SceneTri {
     pub color: Rgb,
 }
 
+/// A translucent triangle (a datum plane's fill): drawn after the opaque faces, depth-tested
+/// against them without hiding what is behind, blended at `alpha` (0–1).
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct SceneFill {
+    pub tri: SceneTri,
+    pub alpha: f64,
+}
+
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub struct SceneLine {
     pub a: Vec3,
@@ -55,6 +63,8 @@ pub struct SceneLabel {
 #[derive(Clone, Debug, Default, PartialEq)]
 pub struct Scene {
     pub tris: Vec<SceneTri>,
+    /// Translucent triangles ([`SceneFill`]).
+    pub fills: Vec<SceneFill>,
     pub lines: Vec<SceneLine>,
     pub points: Vec<ScenePoint>,
     pub labels: Vec<SceneLabel>,
@@ -71,6 +81,7 @@ impl Scene {
     /// Add `other`'s primitives (not its background or radius) to this scene.
     pub fn append(&mut self, other: Scene) {
         self.tris.extend(other.tris);
+        self.fills.extend(other.fills);
         self.lines.extend(other.lines);
         self.points.extend(other.points);
         self.labels.extend(other.labels);
@@ -221,7 +232,7 @@ const DEPTH_BIAS: f64 = 2e-4;
 
 /// [`render`] with overlays drawn over the scene (the setup's symbols, a selection highlight):
 /// their faces over the scene's faces they coincide with (a highlight repeats the faces it
-/// marks), then every layer's lines and points depth-tested, then every layer's
+/// marks), then every layer's translucent faces blended over them, then every layer's lines and points depth-tested, then every layer's
 /// [`Scene::on_top`] lines and labels over everything. The overlays' background and radius are
 /// not used.
 pub fn render_layers(scene: &Scene, overlays: &[&Scene], cam: &Camera, w: usize, h: usize) -> Result<Canvas, RenderError> {
@@ -229,7 +240,13 @@ pub fn render_layers(scene: &Scene, overlays: &[&Scene], cam: &Camera, w: usize,
         return Err(RenderError::Size { w, h });
     }
     let count = |s: &Scene| {
-        s.tris.len().saturating_add(s.lines.len()).saturating_add(s.points.len()).saturating_add(s.labels.len()).saturating_add(s.on_top.len())
+        s.tris
+            .len()
+            .saturating_add(s.fills.len())
+            .saturating_add(s.lines.len())
+            .saturating_add(s.points.len())
+            .saturating_add(s.labels.len())
+            .saturating_add(s.on_top.len())
     };
     let primitives = overlays.iter().fold(count(scene), |n, o| n.saturating_add(count(o)));
     if primitives > MAX_PRIMITIVES {
@@ -277,6 +294,18 @@ pub fn render_layers(scene: &Scene, overlays: &[&Scene], cam: &Camera, w: usize,
         });
     }
     let layers = || std::iter::once(scene).chain(overlays.iter().copied());
+    // Translucent faces over the opaque ones, without writing depth: what is behind shows.
+    for f in layers().flat_map(|s| &s.fills) {
+        let t = &f.tri;
+        let (Some(a), Some(b), Some(cc)) = (project(t.p[0]), project(t.p[1]), project(t.p[2])) else { continue };
+        let alpha = if f.alpha.is_finite() { f.alpha.clamp(0.0, 1.0) } else { 0.0 };
+        let col = shade(t.normal, view_dir, t.color.f());
+        raster_tri(a, b, cc, sw, sh, |x, y, z| {
+            if c.depth.get(y * sw + x).is_some_and(|d| z < *d) {
+                c.put(x, y, col, alpha);
+            }
+        });
+    }
     let line = |c: &mut Canvas, l: &SceneLine, depth_test: bool| {
         let (Some(a), Some(b)) = (biased(l.a, 1.0), biased(l.b, 1.0)) else { return };
         let len = ((b[0] - a[0]).powi(2) + (b[1] - a[1]).powi(2)).sqrt();
@@ -486,6 +515,28 @@ mod tests {
         assert!(p[2] > 200 && p[0] < 60, "{p:?}");
     }
 
+    /// A translucent face tints what is behind it and hides nothing; one behind an opaque face
+    /// does not show (#302).
+    #[test]
+    fn translucent_faces_blend_over_what_is_in_front_only() {
+        let mut cam = Camera { distance: 50.0, ..Default::default() };
+        cam.set_view(StandardView::Front);
+        let px = |c: &Canvas| c.rgba[(32 * c.w + 32) * 4..(32 * c.w + 32) * 4 + 3].to_vec();
+        let red = red_triangle();
+        let plain = px(&render(&red, &cam, 64, 64).unwrap());
+        let fill = |y: f64| SceneFill {
+            tri: SceneTri { p: red.tris[0].p.map(|p| Vec3::new(p.x, y, p.z)), color: Rgb(0, 0, 255), ..red.tris[0] },
+            alpha: 0.25,
+        };
+        let mut front = red.clone();
+        front.fills.push(fill(-1.0));
+        let tinted = px(&render(&front, &cam, 64, 64).unwrap());
+        assert!(tinted[2] > plain[2] + 20 && tinted[0] > 120, "red still shows, tinted blue: {tinted:?} over {plain:?}");
+        let mut back = red.clone();
+        back.fills.push(fill(1.0));
+        assert_eq!(px(&render(&back, &cam, 64, 64).unwrap()), plain);
+    }
+
     /// In perspective, close to the part (the near plane at its smallest), a line a little
     /// behind a face stays hidden and one on it shows (#282).
     #[test]