feat(render): double-single anchors for both hatch pipelines

The hatch anchor (world_origin / pattern origin) was cast to a single
f32, quantizing UTM-scale fills to ~0.5 m and reintroducing jitter the
wire path no longer had. Both the batched and per-hatch pipelines now
carry the anchor as a high/low f32 pair (HatchInstance.world_origin_low,
HatchUniformData.origin_low) and reconstruct the eye-relative position in
the shader as (origin_high - eye_high) + (local + origin_low - eye_low).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/scene/pipeline/hatch_batched_gpu.rs

@@ -51,26 +51,38 @@ pub struct HatchInstance {
     pub color: [f32; 4],            //   0
     pub color2: [f32; 4],           //  16  (gradient end)
     pub aabb: [f32; 4],             //  32  (local-space xmin,ymin,xmax,ymax)
-    pub world_origin: [f32; 2],     //  48  (anchor added back in VS)
-    pub angle_offset: f32,          //  56
-    pub scale: f32,                 //  60
-    pub grad_cos: f32,              //  64
-    pub grad_sin: f32,              //  68
-    pub grad_min: f32,              //  72
-    pub grad_range: f32,            //  76
-    pub mode: u32,                  //  80  (0=pattern, 1=solid, 2=gradient)
-    pub visible: u32,               //  84  (CPU sets to 0 to skip)
-    pub boundary_offset: u32,       //  88  (first boundary vert index)
-    pub boundary_count: u32,        //  92
-    pub family_offset: u32,         //  96
-    pub family_count: u32,          // 100
+    pub world_origin: [f32; 2],     //  48  (anchor high half, added back in VS)
+    pub world_origin_low: [f32; 2], //  56  (anchor low residual — double-single)
+    pub angle_offset: f32,          //  64
+    pub scale: f32,                 //  68
+    pub grad_cos: f32,              //  72
+    pub grad_sin: f32,              //  76
+    pub grad_min: f32,              //  80
+    pub grad_range: f32,            //  84
+    pub mode: u32,                  //  88  (0=pattern, 1=solid, 2=gradient)
+    pub visible: u32,               //  92  (CPU sets to 0 to skip)
+    pub boundary_offset: u32,       //  96  (first boundary vert index)
+    pub boundary_count: u32,        // 100
+    pub family_offset: u32,         // 104
+    pub family_count: u32,          // 108
     /// Signed draw-order depth (-1,1); 0.0 = neutral. Applied as a clip-z
     /// bias in the vertex shader so this fill orders against other types.
-    pub draw_depth: f32,            // 104
-    pub _pad0: u32,                 // 108  (pad to 112 = 16-byte stride)
+    pub draw_depth: f32,            // 112
+    pub _pad0: u32,                 // 116  (pad to 128 = 16-byte stride)
+    pub _pad1: u32,                 // 120
+    pub _pad2: u32,                 // 124
 }
 
-const _: () = assert!(std::mem::size_of::<HatchInstance>() == 112);
+const _: () = assert!(std::mem::size_of::<HatchInstance>() == 128);
+
+/// Split a hatch's f64 world-origin anchor into double-single (high, low) f32
+/// pairs so the GPU keeps sub-unit precision at UTM-scale coordinates.
+#[inline]
+fn split_origin_ds(o: [f64; 2]) -> ([f32; 2], [f32; 2]) {
+    let hx = o[0] as f32;
+    let hy = o[1] as f32;
+    ([hx, hy], [(o[0] - hx as f64) as f32, (o[1] - hy as f64) as f32])
+}
 
 /// Mirrors the per-family struct used by the existing per-hatch shader,
 /// but the dash slice lives in a separate concatenated DashBuffer (the
@@ -261,11 +273,13 @@ impl HatchBatchedGpu {
             if !min_x.is_finite() {
                 // Empty / all-NaN — skip but keep the slot so indices
                 // stay in lockstep with the input list (visibility=0).
+                let (wo_hi, wo_lo) = split_origin_ds(h.world_origin);
                 instances.push(HatchInstance {
                     color: h.color,
                     color2,
                     aabb: [0.0, 0.0, 0.0, 0.0],
-                    world_origin: h.world_origin.map(|v| v as f32),
+                    world_origin: wo_hi,
+                    world_origin_low: wo_lo,
                     angle_offset: h.angle_offset,
                     scale: h.scale.max(1e-6),
                     grad_cos,
@@ -280,6 +294,8 @@ impl HatchBatchedGpu {
                     family_count,
                     draw_depth: h.draw_depth,
                     _pad0: 0,
+                    _pad1: 0,
+                    _pad2: 0,
                 });
                 continue;
             }
@@ -300,11 +316,13 @@ impl HatchBatchedGpu {
             };
             let pad = (diag * 0.8 + max_spacing * 2.0 * h.scale).max(1.0);
 
+            let (wo_hi, wo_lo) = split_origin_ds(h.world_origin);
             instances.push(HatchInstance {
                 color: h.color,
                 color2,
                 aabb: [min_x - pad, min_y - pad, max_x + pad, max_y + pad],
-                world_origin: h.world_origin.map(|v| v as f32),
+                world_origin: wo_hi,
+                world_origin_low: wo_lo,
                 angle_offset: h.angle_offset,
                 scale: h.scale.max(1e-6),
                 grad_cos,
@@ -319,6 +337,8 @@ impl HatchBatchedGpu {
                 family_count,
                 draw_depth: h.draw_depth,
                 _pad0: 0,
+                _pad1: 0,
+                _pad2: 0,
             });
         }
 

src/scene/pipeline/hatch_gpu.rs

@@ -58,13 +58,15 @@ pub struct HatchUniformData {
     pub grad_sin: f32,       // 52: gradient direction sin
     pub grad_min: f32,       // 56: gradient proj_min
     pub grad_range: f32,     // 60: gradient proj_range
-    pub origin: [f32; 2],    // 64: hatch-local origin (boundary AABB centre).
+    pub origin: [f32; 2],    // 64: hatch-local origin (boundary AABB centre),
+    //                       //     high half of the double-single anchor.
     //                       //     The pattern fragment shader subtracts this
     //                       //     from xz before the perp/perp_step quotient
     //                       //     so f32 doesn't catastrophically cancel when
     //                       //     world coords are large and pattern spacing
     //                       //     is small (e.g. UTM drawing + sub-mm hatch).
-    pub _pad: [f32; 2],      // 72: 16-byte alignment
+    pub origin_low: [f32; 2], // 72: anchor low residual (double-single) so the
+    //                        //     vertex clip position stays precise at UTM scale.
 } // total 80 bytes
 
 /// Boundary polygon (binding 1).  Matches WGSL `Boundary`.
@@ -288,7 +290,10 @@ impl HatchGpu {
             grad_min,
             grad_range,
             origin: [origin[0] as f32, origin[1] as f32],
-            _pad: [0.0; 2],
+            origin_low: [
+                (origin[0] - origin[0] as f32 as f64) as f32,
+                (origin[1] - origin[1] as f32 as f64) as f32,
+            ],
         };
 
         // ── BoundaryData (in snapped-local space) ────────────────────────

src/shaders/hatch.wgsl

@@ -53,7 +53,7 @@ struct HatchUniforms {
     //                        //     `xz - origin` so the f32 modulo doesn't
     //                        //     catastrophically cancel when world coords
     //                        //     are large and pattern spacing is small.
-    _pad:         vec2<f32>,  // 72: 16-byte alignment
+    origin_low:   vec2<f32>,  // 72: anchor low residual (double-single)
 }
 @group(1) @binding(0) var<uniform> h: HatchUniforms;
 
@@ -104,8 +104,15 @@ struct VOut {
     // jitter is invisible. The local-space `xz` we export to the
     // fragment shader stays at full f32 precision so the in_polygon /
     // pattern math doesn't catastrophically cancel.
-    let world = vec3<f32>(v.pos.x + h.origin.x, v.pos.y + h.origin.y, v.pos.z);
-    o.clip = u.view_rot * vec4<f32>((world - u.eye_high) - u.eye_low, 1.0);
+    // Double-single relative-to-eye: anchor high cancels eye_high (Sterbenz);
+    // boundary-local v.pos + anchor low + (−eye_low) carry the residual.
+    let hi = vec3<f32>(h.origin.x - u.eye_high.x,
+                       h.origin.y - u.eye_high.y,
+                       -u.eye_high.z);
+    let lo = vec3<f32>(v.pos.x + h.origin_low.x - u.eye_low.x,
+                       v.pos.y + h.origin_low.y - u.eye_low.y,
+                       v.pos.z - u.eye_low.z);
+    o.clip = u.view_rot * vec4<f32>(hi + lo, 1.0);
     o.xz   = vec2<f32>(v.pos.x, v.pos.y);
     return o;
 }

src/shaders/hatch_batched.wgsl

@@ -41,7 +41,8 @@ struct HatchInstance {
     color:           vec4<f32>,
     color2:          vec4<f32>,
     aabb:            vec4<f32>,   // (xmin, ymin, xmax, ymax) — local space
-    world_origin:    vec2<f32>,
+    world_origin:    vec2<f32>,     // anchor high half
+    world_origin_low: vec2<f32>,    // anchor low residual (double-single)
     angle_offset:    f32,
     scale:           f32,
     grad_cos:        f32,
@@ -56,6 +57,8 @@ struct HatchInstance {
     family_count:    u32,
     draw_depth:      f32,          // signed (-1,1) draw-order bias; 0 = neutral
     _pad0:           u32,
+    _pad1:           u32,
+    _pad2:           u32,
 }
 
 // Draw-order depth bias (see wire.wgsl). Higher draw_depth → smaller z →
@@ -132,10 +135,17 @@ fn corner_xy(c: u32, aabb: vec4<f32>) -> vec2<f32> {
     }
 
     let local = corner_xy(v.corner, inst.aabb);
-    let world = vec3<f32>(local.x + inst.world_origin.x,
-                          local.y + inst.world_origin.y,
-                          0.0);
-    o.clip = u.view_rot * vec4<f32>((world - u.eye_high) - u.eye_low, 1.0);
+    // Double-single relative-to-eye: the anchor high half cancels exactly
+    // against eye_high (Sterbenz); local + anchor low + (−eye_low) carry the
+    // residual. `local` is small (boundary-relative), so adding it in the low
+    // term keeps full precision at UTM-scale anchors.
+    let hi = vec3<f32>(inst.world_origin.x - u.eye_high.x,
+                       inst.world_origin.y - u.eye_high.y,
+                       -u.eye_high.z);
+    let lo = vec3<f32>(local.x + inst.world_origin_low.x - u.eye_low.x,
+                       local.y + inst.world_origin_low.y - u.eye_low.y,
+                       -u.eye_low.z);
+    o.clip = u.view_rot * vec4<f32>(hi + lo, 1.0);
     o.clip.z = o.clip.z - inst.draw_depth * DRAW_ORDER_BIAS * o.clip.w;
     o.xz = local;
     o.instance_index = v.instance_index;