fix(hatch): finite GPU sentinel for boundary loop separators (#386, #416)

Hatch/wipeout boundary sub-loops were separated by NaN vertices that the
fill shaders detected with `p.x == p.x`. Intel Mesa folds that NaN
self-compare to `true` under fast math, so separators read as real
vertices, the ray-cast crossing count broke, and fills bled outside
their boundary — only on Intel iGPUs. GPU uploads now rewrite the
separators to a finite 1e30 sentinel (GPU_BOUNDARY_SEP) and the shaders
test `abs(x) < 1e29`, which every driver evaluates identically. The
CPU-side NaN representation is unchanged.

Co-Authored-By: Claude Fable 5 <[email protected]>

src/scene/model/hatch_model.rs

@@ -133,6 +133,13 @@ pub enum HatchPattern {
     },
 }
 
+/// GPU-side separator between disconnected boundary sub-loops. A finite
+/// magnitude sentinel instead of NaN: shaders detect it with a plain
+/// `abs(x) < 1e29` compare, which every driver evaluates identically,
+/// unlike NaN self-comparison (#386, #416). Boundary verts are local
+/// (anchor-relative), so real coordinates never approach it.
+pub const GPU_BOUNDARY_SEP: f32 = 1.0e30;
+
 /// A hatched region defined by a closed polygon boundary.
 #[derive(Clone, Debug)]
 pub struct HatchModel {
@@ -152,6 +159,10 @@ pub struct HatchModel {
     /// World-XY coordinates of the boundary polygon vertices, stored as
     /// f32 offsets from `world_origin`. NaN-NaN sentinels separate
     /// disconnected paths and must be preserved un-shifted by consumers.
+    /// GPU uploads rewrite them to [`GPU_BOUNDARY_SEP`] — some drivers
+    /// (Intel Mesa) fold the shader-side `x == x` NaN test to `true`
+    /// under fast math, which turned separators into real vertices and
+    /// bled fills outside their boundary (#386, #416).
     pub boundary: Arc<Vec<[f32; 2]>>,
     /// Exact absolute-WCS boundary in f64, set only by the draw commands so a
     /// typed boundary vertex is persisted without the f32 quantization the

src/scene/pipeline/hatch_gpu.rs

@@ -216,7 +216,16 @@ impl HatchGpu {
         for h in hatches {
             let boundary_offset = boundary.len() as u32;
             for &[x, y] in h.boundary.iter() {
-                boundary.push([x, y, 0.0, 0.0]);
+                // NaN sub-loop separators become the finite sentinel on the
+                // GPU: Intel drivers fold the shader NaN self-compare to
+                // `true`, turning separators into vertices and bleeding the
+                // fill outside its boundary (#386, #416).
+                if x.is_finite() && y.is_finite() {
+                    boundary.push([x, y, 0.0, 0.0]);
+                } else {
+                    let s = crate::scene::model::hatch_model::GPU_BOUNDARY_SEP;
+                    boundary.push([s, s, 0.0, 0.0]);
+                }
             }
             let boundary_count = boundary.len() as u32 - boundary_offset;
 

src/scene/pipeline/hatch_web_gpu.rs

@@ -203,13 +203,15 @@ impl HatchWebGpu {
 
         // ── Pack the data texture: boundary | families | dashes ───────────
         let mut texels: Vec<[f32; 4]> = Vec::new();
-        // Boundary section (texels 0..vcount). NaN separators preserved (they
-        // survive the RGBA32F fetch byte-for-byte).
+        // Boundary section (texels 0..vcount). NaN separators become the
+        // finite GPU sentinel — the shader NaN self-compare is folded to
+        // `true` on some drivers (#386, #416).
         for &[x, y] in model.boundary.iter() {
             if x.is_finite() && y.is_finite() {
                 texels.push([x + drift[0], y + drift[1], 0.0, 0.0]);
             } else {
-                texels.push([f32::NAN, f32::NAN, 0.0, 0.0]);
+                let s = crate::scene::model::hatch_model::GPU_BOUNDARY_SEP;
+                texels.push([s, s, 0.0, 0.0]);
             }
         }
         let vcount = texels.len() as u32;

src/scene/pipeline/wipeout_gpu.rs

@@ -310,12 +310,14 @@ impl WipeoutGpu {
             .take(MAX_HATCH_BOUNDARY_VERTS)
             .enumerate()
         {
-            // Preserve NaN separators — they signal path breaks to the
-            // in_polygon ray-cast, must not be shifted into finite numbers.
+            // Path-break separators go up as the finite GPU sentinel, not
+            // NaN — the shader NaN self-compare is folded to `true` on some
+            // drivers (#386, #416) — and must not be shifted by `drift`.
             if x.is_finite() && y.is_finite() {
                 boundary_data.verts[i] = [x + drift[0], y + drift[1], 0.0, 0.0];
             } else {
-                boundary_data.verts[i] = [f32::NAN, f32::NAN, 0.0, 0.0];
+                let s = crate::scene::model::hatch_model::GPU_BOUNDARY_SEP;
+                boundary_data.verts[i] = [s, s, 0.0, 0.0];
             }
         }
 

src/shaders/hatch.wgsl

@@ -142,7 +142,11 @@ struct VOut {
 // ── Point-in-polygon (ray casting) over a sub-range of BoundaryBuffer ────
 
 fn valid_vertex(p: vec2<f32>) -> bool {
-    return p.x == p.x && p.y == p.y;
+    // Sub-loop separators are a huge finite sentinel (1e30, see
+    // GPU_BOUNDARY_SEP), not NaN: `x == x` NaN detection gets folded to
+    // `true` by some drivers (Intel Mesa fast math), which made separators
+    // read as real vertices and bleed fills past their boundary (#386, #416).
+    return abs(p.x) < 1.0e29 && abs(p.y) < 1.0e29;
 }
 
 fn edge_crosses(p: vec2<f32>, a: vec2<f32>, c: vec2<f32>) -> bool {

src/shaders/hatch_web.wgsl

@@ -116,7 +116,11 @@ struct VOut {
 // ── Point-in-polygon (ray casting) ────────────────────────────────────────
 
 fn valid_vertex(p: vec2<f32>) -> bool {
-    return p.x == p.x && p.y == p.y;
+    // Sub-loop separators are a huge finite sentinel (1e30, see
+    // GPU_BOUNDARY_SEP), not NaN: `x == x` NaN detection gets folded to
+    // `true` by some drivers (Intel Mesa fast math), which made separators
+    // read as real vertices and bleed fills past their boundary (#386, #416).
+    return abs(p.x) < 1.0e29 && abs(p.y) < 1.0e29;
 }
 
 fn edge_crosses(p: vec2<f32>, a: vec2<f32>, c: vec2<f32>) -> bool {

src/shaders/wipeout.wgsl

@@ -118,7 +118,11 @@ struct VOut {
 // ── Point-in-polygon (ray casting) ────────────────────────────────────────
 
 fn valid_vertex(p: vec2<f32>) -> bool {
-    return p.x == p.x && p.y == p.y;
+    // Sub-loop separators are a huge finite sentinel (1e30, see
+    // GPU_BOUNDARY_SEP), not NaN: `x == x` NaN detection gets folded to
+    // `true` by some drivers (Intel Mesa fast math), which made separators
+    // read as real vertices and bleed fills past their boundary (#386, #416).
+    return abs(p.x) < 1.0e29 && abs(p.y) < 1.0e29;
 }
 
 fn edge_crosses(p: vec2<f32>, a: vec2<f32>, c: vec2<f32>) -> bool {