Stroke rendering artifact in rounded-rect corner at small output sizes

[nick-uc]

Description

When stroking a rounded rectangle path with tiny-skia and rendering it at a small pixel size, a visible streak/artifact appears in the corners. The exact same path renders cleanly at a larger output size.

The bug is in tiny-skia alone — it does not require resvg/usvg. The path is built directly with tiny_skia::PathBuilder and rendered with Pixmap::stroke_path.

Reproduction

Cargo.toml:

[dependencies]
tiny-skia = "0.12.0"

src/main.rs:

// The path corresponds to this SVG, which is what we started from:
//
// <?xml version="1.0" encoding="UTF-8" standalone="no"?>
// <svg xmlns="http://www.w3.org/2000/svg"
//      xmlns:xlink="http://www.w3.org/1999/xlink"
//      height="3.5in" preserveAspectRatio="none"
//      viewBox="-120 -168 240 336" width="2.5in">
//     <rect width="239" height="335" x="-119.5" y="-167.5"
//           rx="12" ry="12" fill="white" stroke="black"></rect>
// </svg>

use tiny_skia::{Color, FillRule, Paint, PathBuilder, Pixmap, Stroke, Transform};

fn render(to_width: u32, to_height: u32) -> Vec<u8> {
    let svg_w = 240.0_f32;
    let svg_h = 336.0_f32;

    let mut target_height = to_height;
    let mut scale = target_height as f32 / svg_h;
    let mut target_width = (svg_w * scale).round() as u32;
    if target_width > to_width {
        scale = to_width as f32 / svg_w;
        target_width = to_width;
        target_height = (svg_h * scale).round() as u32;
    }

    let mut pixmap = Pixmap::new(target_width, target_height).unwrap();
    pixmap.fill(Color::WHITE);

    let transform = Transform::from_scale(scale, scale)
        .post_translate(120.0 * scale, 168.0 * scale);

    let x = -119.5_f32;
    let y = -167.5_f32;
    let w = 239.0_f32;
    let h = 335.0_f32;
    let r = 12.0_f32;

    let mut pb = PathBuilder::new();
    pb.move_to(x + r, y);
    pb.line_to(x + w - r, y);
    pb.quad_to(x + w, y, x + w, y + r);
    pb.line_to(x + w, y + h - r);
    pb.quad_to(x + w, y + h, x + w - r, y + h);
    pb.line_to(x + r, y + h);
    pb.quad_to(x, y + h, x, y + h - r);
    pb.line_to(x, y + r);
    pb.quad_to(x, y, x + r, y);
    pb.close();
    let path = pb.finish().unwrap();

    let mut fill_paint = Paint::default();
    fill_paint.set_color_rgba8(255, 255, 255, 255);
    fill_paint.anti_alias = true;
    pixmap.fill_path(&path, &fill_paint, FillRule::Winding, transform, None);

    let mut stroke_paint = Paint::default();
    stroke_paint.set_color_rgba8(0, 0, 0, 255);
    stroke_paint.anti_alias = true;
    let stroke = Stroke { width: 1.0, ..Stroke::default() };
    pixmap.stroke_path(&path, &stroke_paint, &stroke, transform, None);

    pixmap.encode_png().unwrap()
}

fn main() {
    std::fs::write("ok.png", render(286, 400)).unwrap();
    std::fs::write("glitches.png", render(143, 200)).unwrap();
}

Expected Behavior

Both outputs should show a clean rounded-rectangle stroke. Lower resolution should only reduce anti-aliasing detail, not introduce structural artifacts.

Actual Behavior

• ok.png (286×400) — renders correctly:

• glitches.png (143×200) — a visible streak appears in the corners:

Notes

• The bug was originally encountered through resvg + usvg rendering an SVG card. I narrowed it down by reconstructing the same path with PathBuilder and reproducing the artifact without any SVG pipeline involved — so the root cause is in tiny-skia's path rendering (likely stroke tessellation / AA edge accumulation), not in the SVG layer.