fix(render): mesh triangles that share an edge leave no seam

The mesh sampler took each pixel's colour at its corner, and only when the
corner was strictly inside a triangle. A corner on an edge two triangles
share belonged to neither, so a Type 4-7 shading drew a white seam along
every shared edge that ran through the sample grid, and dropped pixels
along the mesh's own sides (a 30 x 30 square cut along its diagonal:
812 of 900 pixels; MuPDF 900).

Such a pixel now goes to the triangle that covers its centre, edges
included (Triangle::covers), and still takes the colour at its corner, so
every pixel that was drawn keeps its colour. Vendored hayro-interpret
patch 41.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RUNQSPVXx3dDBabRZHAnjz

crates/render/src/raster.rs

@@ -5001,6 +5001,87 @@ trailer << /Root 1 0 R >>
         assert!(p != [255, 255, 255, 255] && p[3] == 255, "the triangles are drawn: {p:?}");
     }
 
+    /// Two triangles of a mesh that share an edge cover it together. The sampler tested each
+    /// pixel's corner, and only strictly inside a triangle, so the diagonal two triangles share
+    /// was left white wherever it ran through the samples, and so was a pixel row or column
+    /// along the mesh's own sides. Vendored hayro-interpret patch 41.
+    #[test]
+    fn mesh_triangles_sharing_an_edge_leave_no_seam() {
+        // A red square from 10 to 40 on a 50 × 50 page, cut along its diagonal; in device space
+        // the diagonal runs through pixel centres and corners alike.
+        let vertex = |flag: u8, x: u8, y: u8| [flag, x, y, 255, 0, 0];
+        let (lo, hi) = (10, 40);
+        let mut triangles = Vec::new();
+        for [a, b, c] in [[(lo, lo), (hi, lo), (hi, hi)], [(lo, lo), (hi, hi), (lo, hi)]] {
+            for (x, y) in [a, b, c] {
+                triangles.extend(vertex(0, x, y));
+            }
+        }
+        // Each triangle as its own three vertices, and the second one as a strip (flag 1).
+        let mut strip = Vec::new();
+        for (flag, (x, y)) in [(0, (lo, lo)), (0, (hi, lo)), (0, (hi, hi)), (2, (lo, hi))] {
+            strip.extend(vertex(flag, x, y));
+        }
+        for (label, data) in [("separate triangles", triangles), ("a strip", strip)] {
+            let content = b"/Sh0 sh";
+            let mut pdf = b"%PDF-1.7\n1 0 obj << /Type /Catalog /Pages 2 0 R >> endobj\n2 0 obj << /Type /Pages /Kids [3 0 R] /Count 1 >> endobj\n3 0 obj << /Type /Page /Parent 2 0 R /MediaBox [0 0 50 50] /Contents 4 0 R /Resources << /Shading << /Sh0 5 0 R >> >> >> endobj\n".to_vec();
+            pdf.extend_from_slice(format!("4 0 obj << /Length {} >> stream\n", content.len()).as_bytes());
+            pdf.extend_from_slice(content);
+            pdf.extend_from_slice(format!("\nendstream endobj\n5 0 obj << /ShadingType 4 /ColorSpace /DeviceRGB /BitsPerCoordinate 8 /BitsPerComponent 8 /BitsPerFlag 8 /Decode [0 255 0 255 0 1 0 1 0 1] /Length {} >> stream\n", data.len()).as_bytes());
+            pdf.extend_from_slice(&data);
+            pdf.extend_from_slice(b"\nendstream endobj\ntrailer << /Root 1 0 R >>\n%%EOF\n");
+            let mut r = PageRenderer::new(Arc::new(pdf), RenderConfig::default());
+            let page = r.render(RenderRequest { page: 0, kind: RequestKind::Pixels, tile: None, scale: 1.0, tag: 0, ..Default::default() });
+            assert!(page.error.is_none(), "{label}: {:?}", page.error);
+            assert_eq!((page.width, page.height), (50, 50));
+            let (mut white_inside, mut red_outside) = (Vec::new(), Vec::new());
+            for y in 0..50_usize {
+                for x in 0..50_usize {
+                    let p = &page.rgba[(y * 50 + x) * 4..][..4];
+                    let inside = (10..40).contains(&x) && (10..40).contains(&y);
+                    if inside && p != [255, 0, 0, 255] {
+                        white_inside.push((x, y));
+                    } else if !inside && p != [255, 255, 255, 255] {
+                        red_outside.push((x, y));
+                    }
+                }
+            }
+            assert!(white_inside.is_empty(), "{label}: unshaded pixels inside the square: {white_inside:?}");
+            assert!(red_outside.is_empty(), "{label}: shaded pixels outside the square: {red_outside:?}");
+        }
+
+        // One triangle alone, the square's upper left half: a pixel whose corner lies on its
+        // diagonal is taken by its centre, so one wholly across the diagonal (x + y ≥ 50 in
+        // device space) is not shaded, and one wholly inside is.
+        let mut data = Vec::new();
+        for (x, y) in [(lo, lo), (hi, hi), (lo, hi)] {
+            data.extend(vertex(0, x, y));
+        }
+        let content = b"/Sh0 sh";
+        let mut pdf = b"%PDF-1.7\n1 0 obj << /Type /Catalog /Pages 2 0 R >> endobj\n2 0 obj << /Type /Pages /Kids [3 0 R] /Count 1 >> endobj\n3 0 obj << /Type /Page /Parent 2 0 R /MediaBox [0 0 50 50] /Contents 4 0 R /Resources << /Shading << /Sh0 5 0 R >> >> >> endobj\n".to_vec();
+        pdf.extend_from_slice(format!("4 0 obj << /Length {} >> stream\n", content.len()).as_bytes());
+        pdf.extend_from_slice(content);
+        pdf.extend_from_slice(format!("\nendstream endobj\n5 0 obj << /ShadingType 4 /ColorSpace /DeviceRGB /BitsPerCoordinate 8 /BitsPerComponent 8 /BitsPerFlag 8 /Decode [0 255 0 255 0 1 0 1 0 1] /Length {} >> stream\n", data.len()).as_bytes());
+        pdf.extend_from_slice(&data);
+        pdf.extend_from_slice(b"\nendstream endobj\ntrailer << /Root 1 0 R >>\n%%EOF\n");
+        let mut r = PageRenderer::new(Arc::new(pdf), RenderConfig::default());
+        let page = r.render(RenderRequest { page: 0, kind: RequestKind::Pixels, tile: None, scale: 1.0, tag: 0, ..Default::default() });
+        assert!(page.error.is_none(), "{:?}", page.error);
+        let (mut spilled, mut missing) = (Vec::new(), Vec::new());
+        for y in 10..40_usize {
+            for x in 10..40_usize {
+                let p = &page.rgba[(y * 50 + x) * 4..][..4];
+                if x + y >= 50 && p == [255, 0, 0, 255] {
+                    spilled.push((x, y));
+                } else if x + y + 2 <= 50 && p != [255, 0, 0, 255] {
+                    missing.push((x, y));
+                }
+            }
+        }
+        assert!(spilled.is_empty(), "shaded pixels wholly across the diagonal: {spilled:?}");
+        assert!(missing.is_empty(), "unshaded pixels wholly inside the triangle: {missing:?}");
+    }
+
     /// Review of the mesh-shading patch: every patch was cut into 722 triangles before anything
     /// limited them (about 150 KB per patch), and a lattice mesh with `/VerticesPerRow 0` pushed
     /// empty rows for ever. Vendored hayro-interpret patch: patches are triangulated one at a

vendor/hayro-interpret/src/encode.rs

@@ -321,9 +321,17 @@ impl TriangleSampler {
                     return false;
                 }
                 self.visits_left -= 1;
-                let point = Point::new(x as f64, y as f64);
-                if t.contains_point(point) {
-                    self.map.insert((x, y), t.interpolate(point));
+                let corner = Point::new(x as f64, y as f64);
+                if t.contains_point(corner) {
+                    self.map.insert((x, y), t.interpolate(corner));
+                } else if t.covers(Point::new(x as f64 + 0.5, y as f64 + 0.5)) {
+                    // PdfCraft patch (36): a pixel whose corner no triangle holds strictly (it
+                    // lies on an edge two triangles share, or on the mesh's side) still belongs
+                    // to the triangle that covers its centre, edges included. Testing only the
+                    // corner left a white seam along every shared edge. Such a pixel takes the
+                    // colour at its corner, as every other pixel does (where MuPDF's colours
+                    // agree), unless a triangle that holds the corner gives it one.
+                    self.map.entry((x, y)).or_insert_with(|| t.interpolate(corner));
                 }
             }
         }

vendor/hayro-interpret/src/shading.rs

@@ -375,6 +375,24 @@ impl Triangle {
         self.kurbo_tri.winding(pos) != 0
     }
 
+    /// PdfCraft patch (36): whether the point is inside the triangle or on one of its edges.
+    /// Two triangles of a mesh share an edge; a sample on it belongs to both (their colours
+    /// agree there), where `contains_point` can give it to neither and leave a seam. A
+    /// degenerate triangle covers nothing.
+    pub fn covers(&self, pos: Point) -> bool {
+        // Relative to the barycentric coordinates' scale (1), well above f64 rounding.
+        const EDGE: f64 = 1e-9;
+        let (a, b, c) = (self.p0.point, self.p1.point, self.p2.point);
+        let (v0, v1, v2) = (b - a, c - a, pos - a);
+        let denom = self.d00 * self.d11 - self.d01 * self.d01;
+        let (d20, d21) = (v2.dot(v0), v2.dot(v1));
+        let v = (self.d11 * d20 - self.d01 * d21) / denom;
+        let w = (self.d00 * d21 - self.d01 * d20) / denom;
+        // A degenerate triangle divides by zero: NaN fails every comparison, and infinite
+        // coordinates can't sum to at most one.
+        v >= -EDGE && w >= -EDGE && v + w <= 1.0 + EDGE
+    }
+
     /// Return the bounding box of the triangle.
     pub fn bounding_box(&self) -> kurbo::Rect {
         self.kurbo_tri.bounding_box()