App icon: a fringe cube with flow streamlines replaces the meshed bracket

The mark is now a hex-meshed cube in isometric view whose faces carry a
banded colour fringe (blue to red, hottest at the front corner), with three
tapered streamlines wrapping around it. The small variant keeps the fringe
cube and one bolder streamline. Icon Composer gets four groups (floor, flow
behind, cube with mesh, flow in front), its maximum. The title-bar mark is
redrawn as the fringe cube.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/ui-egui/src/appbar.rs

@@ -61,30 +61,40 @@ fn button_style(t: &Tokens) -> IconButtonStyle {
     }
 }
 
-/// FieldCraft's mark: the application icon's bracket (`assets/app-icon/icon-small.svg`) drawn in
-/// code from the SVG's own coordinates, without its tile, each face in the middle of its gradient.
+/// FieldCraft's mark: the application icon's fringe cube (`assets/app-icon/icon-small.svg`, after
+/// `xtask/src/app_icon.rs`) drawn in code without its tile or streamline: each visible face split
+/// into small cells coloured by the fringe band at the cell's centre, shaded per face.
 fn mark(p: &egui::Painter, c: egui::Pos2) {
-    // The bracket spans y 141–883 in the SVG; draw it 22 points tall around its centre.
-    let s = 22.0 / 742.0;
-    let at = |x: f32, y: f32| c + vec2((x - 512.0) * s, (y - 512.0) * s);
-    let face = |col: Color32, pts: &[(f32, f32)]| {
-        p.add(egui::Shape::convex_polygon(pts.iter().map(|&(x, y)| at(x, y)).collect(), col, Stroke::NONE));
-    };
-    let (top, front, end) = (Color32::from_rgb(0x69, 0xd4, 0xc2), Color32::from_rgb(0x19, 0xa0, 0x8d), Color32::from_rgb(0x0d, 0x66, 0x5d));
-    face(top, &[(512.0, 141.0), (833.3, 326.5), (730.48, 385.86), (409.19, 200.36)]);
-    face(front, &[(409.19, 200.36), (730.48, 385.86), (730.48, 638.14), (409.19, 452.64)]);
-    face(top, &[(409.19, 452.64), (730.48, 638.14), (512.0, 764.28), (190.7, 578.78)]);
-    face(front, &[(190.7, 578.78), (512.0, 764.28), (512.0, 883.0), (190.7, 697.5)]);
-    face(end, &[(730.48, 385.86), (833.3, 326.5), (833.3, 697.5), (730.48, 638.14)]);
-    face(end, &[(730.48, 638.14), (833.3, 697.5), (512.0, 883.0), (512.0, 764.28)]);
-    // The bolt hole.
-    let hole: Vec<egui::Pos2> = (0..24)
-        .map(|k| {
-            let a = k as f32 / 24.0 * std::f32::consts::TAU;
-            at(460.6 + 86.0 * a.cos(), 608.5 + 49.5 * a.sin())
-        })
-        .collect();
-    p.add(egui::Shape::convex_polygon(hole, Color32::from_rgb(0x08, 0x46, 0x3f), Stroke::NONE));
+    // The cube is 2 units tall on screen (half side 0.5, isometric); draw it 22 points tall.
+    const S: f32 = 11.0;
+    const N: usize = 8;
+    const FRINGE: [(u8, u8, u8); 7] =
+        [(0x2c, 0x4f, 0xbf), (0x2f, 0x86, 0xe0), (0x22, 0xbc, 0xd8), (0x3f, 0xc5, 0x84), (0xf0, 0xd1, 0x3a), (0xf2, 0x8a, 0x2b), (0xde, 0x3b, 0x2f)];
+    const RADII: [f32; 6] = [1.28, 1.07, 0.87, 0.67, 0.47, 0.27];
+    let at = |q: [f32; 3]| c + vec2((q[0] - q[1]) * 0.866 * S, ((q[0] + q[1]) * 0.5 - q[2]) * S);
+    // Each face: origin corner, two edge vectors, and how much it is darkened (0 to 1).
+    let faces: [([f32; 3], [f32; 3], [f32; 3], f32); 3] = [
+        ([-0.5, -0.5, 0.5], [1.0, 0.0, 0.0], [0.0, 1.0, 0.0], 0.0),
+        ([-0.5, 0.5, -0.5], [1.0, 0.0, 0.0], [0.0, 0.0, 1.0], 0.12),
+        ([0.5, -0.5, -0.5], [0.0, 1.0, 0.0], [0.0, 0.0, 1.0], 0.3),
+    ];
+    let point = |o: [f32; 3], u: [f32; 3], v: [f32; 3], a: f32, b: f32| [0, 1, 2].map(|k| o[k] + u[k] * a + v[k] * b);
+    for (o, u, v, shade) in faces {
+        for i in 0..N {
+            for j in 0..N {
+                let (a0, a1, b0, b1) = (i as f32 / N as f32, (i + 1) as f32 / N as f32, j as f32 / N as f32, (j + 1) as f32 / N as f32);
+                let m = point(o, u, v, (a0 + a1) / 2.0, (b0 + b1) / 2.0);
+                let r = m.iter().map(|x| (0.5 - x) * (0.5 - x)).sum::<f32>().sqrt();
+                let band = RADII.iter().filter(|&&rad| r < rad).count();
+                let (cr, cg, cb) = FRINGE.get(band).copied().unwrap_or(FRINGE[0]);
+                let k = 1.0 - shade;
+                let col = Color32::from_rgb((cr as f32 * k) as u8, (cg as f32 * k) as u8, (cb as f32 * k) as u8);
+                let quad = vec![at(point(o, u, v, a0, b0)), at(point(o, u, v, a1, b0)), at(point(o, u, v, a1, b1)), at(point(o, u, v, a0, b1))];
+                // A hairline in the cell's own colour closes the seams between neighbouring cells.
+                p.add(egui::Shape::convex_polygon(quad, col, Stroke::new(0.5, col)));
+            }
+        }
+    }
 }
 
 /// Where the title bar's parts go.