feat(render): double-single corners for the image pipeline

Raster-image quad corners derive from a large f64 insertion point plus
small u/v spans; the single-f32 cast quantized them at UTM scale. The
ImageModel now keeps a corners_low residual, ImageVertex carries pos_low
(location 2), and image.wgsl reconstructs the eye-relative position from
the high/low pair.

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

src/scene/model/image_model.rs

@@ -23,6 +23,9 @@ pub struct ImageModel {
     ///   [2] origin + U*W + V*H (top-right)
     ///   [3] origin + V*H (top-left)
     pub corners: [[f32; 3]; 4],
+    /// Low residual paired with `corners` (double-single) so the GPU keeps
+    /// sub-unit precision at UTM-scale insertion points.
+    pub corners_low: [[f32; 3]; 4],
     /// Optional world-space XY rect [x0, y0, x1, y1] for paper-space
     /// viewport clipping. Mirrors `WireModel.vp_scissor` /
     /// `HatchModel.vp_scissor`.
@@ -44,21 +47,33 @@ impl ImageModel {
         let h = img.size.y;
         // Model-space geometry is drawn in (WCS - world_offset) so large UTM-
         // scale coordinates stay within f32 precision; offset the image too.
-        let ox = (img.insertion_point.x - world_offset[0]) as f32;
+        // Corners come from a large insertion point plus small u/v spans.
-        let oy = (img.insertion_point.y - world_offset[1]) as f32;
+        // Split each into double-single (high, low) f32 so the GPU keeps
-        let oz = (img.insertion_point.z - world_offset[2]) as f32;
+        // sub-unit precision at UTM scale and after a cross-drawing paste.
+        let oxv = img.insertion_point.x - world_offset[0];
+        let oyv = img.insertion_point.y - world_offset[1];
+        let ozv = img.insertion_point.z - world_offset[2];
         let ux = (img.u_vector.x * w) as f32;
         let uy = (img.u_vector.y * w) as f32;
         let uz = (img.u_vector.z * w) as f32;
         let vx = (img.v_vector.x * h) as f32;
         let vy = (img.v_vector.y * h) as f32;
         let vz = (img.v_vector.z * h) as f32;
+        // High/low split of the anchor; the u/v spans are small and added to
+        // the high half (their own residual is below f32 noise at this scale).
+        let ox = oxv as f32;
+        let oy = oyv as f32;
+        let oz = ozv as f32;
+        let oxl = (oxv - ox as f64) as f32;
+        let oyl = (oyv - oy as f64) as f32;
+        let ozl = (ozv - oz as f64) as f32;
         let corners = [
             [ox, oy, oz],
             [ox + ux, oy + uy, oz + uz],
             [ox + ux + vx, oy + uy + vy, oz + uz + vz],
             [ox + vx, oy + vy, oz + vz],
         ];
+        let corners_low = [[oxl, oyl, ozl]; 4];
         let opacity = 1.0 - img.fade as f32 / 100.0;
 
         let (pixels, width, height) = load_pixels(&img.file_path)?;
@@ -69,6 +84,7 @@ impl ImageModel {
             height,
             opacity,
             corners,
+            corners_low,
             vp_scissor: None,
             draw_depth: 0.0,
         })

src/scene/pipeline/image_gpu.rs

@@ -16,25 +16,32 @@ use iced::wgpu::util::DeviceExt;
 pub struct ImageVertex {
     pub pos: [f32; 3],
     pub uv: [f32; 2],
+    pub pos_low: [f32; 3],
 }
 
 impl ImageVertex {
     pub fn layout<'a>() -> wgpu::VertexBufferLayout<'a> {
+        const ATTRS: &[wgpu::VertexAttribute] = &[
+            wgpu::VertexAttribute {
+                offset: std::mem::offset_of!(ImageVertex, pos) as u64,
+                shader_location: 0,
+                format: wgpu::VertexFormat::Float32x3,
+            },
+            wgpu::VertexAttribute {
+                offset: std::mem::offset_of!(ImageVertex, uv) as u64,
+                shader_location: 1,
+                format: wgpu::VertexFormat::Float32x2,
+            },
+            wgpu::VertexAttribute {
+                offset: std::mem::offset_of!(ImageVertex, pos_low) as u64,
+                shader_location: 2,
+                format: wgpu::VertexFormat::Float32x3,
+            },
+        ];
         wgpu::VertexBufferLayout {
             array_stride: std::mem::size_of::<ImageVertex>() as u64,
             step_mode: wgpu::VertexStepMode::Vertex,
-            attributes: &[
+            attributes: ATTRS,
-                wgpu::VertexAttribute {
-                    offset: 0,
-                    shader_location: 0,
-                    format: wgpu::VertexFormat::Float32x3,
-                },
-                wgpu::VertexAttribute {
-                    offset: 12,
-                    shader_location: 1,
-                    format: wgpu::VertexFormat::Float32x2,
-                },
-            ],
         }
     }
 }
@@ -154,31 +161,14 @@ impl ImageGpu {
         // corners: [p0=BL, p1=BR, p2=TR, p3=TL]
         // UV: BL=(0,1), BR=(1,1), TR=(1,0), TL=(0,0)
         let [p0, p1, p2, p3] = model.corners;
+        let [l0, l1, l2, l3] = model.corners_low;
         let verts = [
-            ImageVertex {
+            ImageVertex { pos: p0, uv: [0.0, 1.0], pos_low: l0 },
-                pos: p0,
+            ImageVertex { pos: p1, uv: [1.0, 1.0], pos_low: l1 },
-                uv: [0.0, 1.0],
+            ImageVertex { pos: p2, uv: [1.0, 0.0], pos_low: l2 },
-            },
+            ImageVertex { pos: p0, uv: [0.0, 1.0], pos_low: l0 },
-            ImageVertex {
+            ImageVertex { pos: p2, uv: [1.0, 0.0], pos_low: l2 },
-                pos: p1,
+            ImageVertex { pos: p3, uv: [0.0, 0.0], pos_low: l3 },
-                uv: [1.0, 1.0],
-            },
-            ImageVertex {
-                pos: p2,
-                uv: [1.0, 0.0],
-            },
-            ImageVertex {
-                pos: p0,
-                uv: [0.0, 1.0],
-            },
-            ImageVertex {
-                pos: p2,
-                uv: [1.0, 0.0],
-            },
-            ImageVertex {
-                pos: p3,
-                uv: [0.0, 0.0],
-            },
         ];
         let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
             label: Some("image.vbuf"),

src/shaders/image.wgsl

@@ -40,8 +40,9 @@ const DRAW_ORDER_BIAS: f32 = 0.001;
 
 // ── Vertex stage ──────────────────────────────────────────────────────────────
 struct VertIn {
-    @location(0) pos:  vec3<f32>,
+    @location(0) pos:     vec3<f32>,
-    @location(1) uv:   vec2<f32>,
+    @location(1) uv:      vec2<f32>,
+    @location(2) pos_low: vec3<f32>,
 };
 
 struct VertOut {
@@ -52,7 +53,8 @@ struct VertOut {
 @vertex
 fn vs_main(in: VertIn) -> VertOut {
     var out: VertOut;
-    out.clip_pos = u.view_rot * vec4<f32>((in.pos - u.eye_high) - u.eye_low, 1.0);
+    let rel = (in.pos - u.eye_high) + (in.pos_low - u.eye_low);
+    out.clip_pos = u.view_rot * vec4<f32>(rel, 1.0);
     out.clip_pos.z = out.clip_pos.z - img_params.draw_depth * DRAW_ORDER_BIAS * out.clip_pos.w;
     out.uv = in.uv;
     return out;