#983: supersample Art Optimized edges in exports

Asset Export, File › Export, Save for Web and Object › Rasterize drew
each shape with its own smoothed edge, so where two shapes met edge to
edge (Divide, Shape Builder, abutting art) both half-covered the pixels
on the line and the background showed through as a thin ghost border.

With Art Optimized (the default) these now supersample, as the option's
name in Illustrator says: the art is drawn with hard edges at four times
the size (three or two times for large images, within a 40-megapixel
budget and the renderer's 65,535-pixel side) and each block averaged
into one pixel. Pixels shared by two shapes are fully owned by one or
the other at the finer size, so no seam is left, and the outer edges
are still smooth. The canvas keeps shape-by-shape smoothing for speed;
None and Type Optimized are unchanged.

RenderOptions::supersample turns it on (exports and Rasterize set it).
A render test checks two triangles sharing a diagonal: a seam with
shape-by-shape smoothing, none supersampled, and the factor's limits.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/render/src/tests.rs

@@ -385,3 +385,41 @@ fn images_take_their_nearest_pixel_without_smoothing() {
     assert!(greys(true) >= 4, "smooth: a ramp from black to white");
     assert_eq!(greys(false), 0, "nearest: black, then white");
 }
+
+/// Art Optimized exports are supersampled (#983): two red shapes meeting along a diagonal leave no
+/// seam of the white background between them, where shape-by-shape smoothing shows one; the outer
+/// edges stay smooth.
+#[test]
+fn supersampled_art_leaves_no_seam_between_shapes_that_meet() {
+    let red = || Paint::solid(Color::rgb(1.0, 0.0, 0.0));
+    let mut d = Document::new(100.0, 100.0);
+    let layer = d.layers[0].id;
+    // Two triangles sharing the diagonal of the square (10, 10)–(90, 90).
+    for pts in [[(10.0, 10.0), (90.0, 10.0), (90.0, 90.0)], [(10.0, 10.0), (90.0, 90.0), (10.0, 90.0)]] {
+        let mut b = vectorcraft_geom::BezPath::new();
+        b.move_to(pts[0]);
+        b.line_to(pts[1]);
+        b.line_to(pts[2]);
+        b.close_path();
+        let id = d.alloc_id();
+        d.insert(Some(layer), 0, Node::path(id, vectorcraft_geom::PathData::from_bezpath(&b), Appearance::basic(red(), Paint::None, 0.0))).unwrap();
+    }
+    let draw = |supersample: bool| {
+        let opts = RenderOptions { background: Some([255, 255, 255, 255]), precise: true, supersample, ..Default::default() };
+        Renderer::new().render(&d, 100, 100, Affine::IDENTITY, &opts)
+    };
+    // The palest pixel on the shared diagonal, away from the corners.
+    let palest = |r: &Rendered| (20..80).map(|i| r.pixel(i, i)[1].max(r.pixel(i + 1, i)[1])).max().unwrap();
+    assert!(palest(&draw(false)) > 20, "shape-by-shape smoothing shows the seam");
+    assert_eq!(palest(&draw(true)), 0, "supersampled: no seam");
+    // The outer edge is still smoothed: a pixel half covered is between white and red.
+    let ss = draw(true);
+    let edge = ss.pixel(50, 9);
+    assert!(edge == [255, 255, 255, 255] || (edge[1] > 0 && edge[1] < 255) || ss.pixel(50, 10)[1] < 255, "{edge:?}");
+    // The factor fits the size and the renderer's limits.
+    assert_eq!(supersample_factor(100, 100), Some(4));
+    assert_eq!(supersample_factor(2000, 2000), Some(3));
+    assert_eq!(supersample_factor(3000, 3000), Some(2));
+    assert_eq!(supersample_factor(5000, 5000), None);
+    assert_eq!(supersample_factor(40_000, 10), None);
+}