fix(render): key soft masks by inherited context (#716)

Co-authored-by: Brandon Thomas <[email protected]>

crates/render/src/raster.rs

@@ -1986,6 +1986,47 @@ trailer << /Root 1 0 R >>
         assert_ne!(second_pixel, [255, 255, 255, 255]);
     }
 
+    #[test]
+    fn shared_soft_mask_uses_each_inherited_resource_context() {
+        let page_body = "/A Do /B Do";
+        let form_a = "/GS gs 1 0 0 rg 0 0 40 40 re f";
+        let form_b = "/GS gs 0 0 1 rg 60 0 40 40 re f";
+        let mask_body = "/FILL Do";
+        let white = "1 g 0 0 100 100 re f";
+        let black = "0 g 0 0 100 100 re f";
+        let pdf = format!(
+            "%PDF-1.7\n\
+             1 0 obj << /Type /Catalog /Pages 2 0 R >> endobj\n\
+             2 0 obj << /Type /Pages /Kids [3 0 R] /Count 1 /MediaBox [0 0 100 100] >> endobj\n\
+             3 0 obj << /Type /Page /Parent 2 0 R /Resources << /XObject << /A 4 0 R /B 5 0 R >> >> /Contents 6 0 R >> endobj\n\
+             4 0 obj << /Type /XObject /Subtype /Form /BBox [0 0 100 100] /Resources << /ExtGState << /GS 8 0 R >> /XObject << /FILL 9 0 R >> >> /Length {} >> stream\n{form_a}\nendstream endobj\n\
+             5 0 obj << /Type /XObject /Subtype /Form /BBox [0 0 100 100] /Resources << /ExtGState << /GS 8 0 R >> /XObject << /FILL 10 0 R >> >> /Length {} >> stream\n{form_b}\nendstream endobj\n\
+             6 0 obj << /Length {} >> stream\n{page_body}\nendstream endobj\n\
+             8 0 obj << /Type /ExtGState /SMask 11 0 R >> endobj\n\
+             9 0 obj << /Type /XObject /Subtype /Form /BBox [0 0 100 100] /Length {} >> stream\n{white}\nendstream endobj\n\
+             10 0 obj << /Type /XObject /Subtype /Form /BBox [0 0 100 100] /Length {} >> stream\n{black}\nendstream endobj\n\
+             11 0 obj << /Type /Mask /S /Luminosity /G 12 0 R >> endobj\n\
+             12 0 obj << /Type /XObject /Subtype /Form /BBox [0 0 100 100] /Group << /S /Transparency /CS /DeviceGray >> /Length {} >> stream\n{mask_body}\nendstream endobj\n\
+             trailer << /Root 1 0 R >>\n%%EOF",
+            form_a.len(),
+            form_b.len(),
+            page_body.len(),
+            white.len(),
+            black.len(),
+            mask_body.len()
+        )
+        .into_bytes();
+        let mut renderer = PageRenderer::new(Arc::new(pdf), RenderConfig::default());
+        let page = renderer.render(RenderRequest { page: 0, scale: 1.0, ..Default::default() });
+        assert!(page.error.is_none(), "{:?}", page.error);
+        let pixel = |x: u32, y: u32| {
+            let offset = ((y * page.width + x) * 4) as usize;
+            [page.rgba[offset], page.rgba[offset + 1], page.rgba[offset + 2], page.rgba[offset + 3]]
+        };
+        assert_eq!(pixel(20, 80), [255, 0, 0, 255], "the white inherited mask makes form A visible");
+        assert_eq!(pixel(80, 80), [255, 255, 255, 255], "the black inherited mask must not reuse form A's cached pixels");
+    }
+
     #[test]
     fn renders_a_blue_rectangle() {
         let pool = RenderPool::new(Arc::new(ONE_PAGE.to_vec()), 1, RenderConfig::default());