hayro: port PdfCraft's page transparency group (#2972)

The page group composites over a transparent backdrop and is then put on the background, so a blend mode over an empty part of the page keeps its color. With a color replacement, and on a plate render, the background stays inside the group as its backdrop, so plate films are as before. Ledger hayro (18); the PdfCraft raster test is adapted in upstream_raster.rs.

crates/pdf/tests/upstream_raster.rs

@@ -618,3 +618,37 @@ fn visibility_expressions_decide_what_a_membership_dictionary_shows() {
     let deep = format!("<< /Type /OCMD /OCGs [5 0 R] /VE {}5 0 R{} >>", "[/Not ".repeat(41), "]".repeat(41));
     assert!(!render(&deep, &[5]), "too deep: /OCGs decides");
 }
+
+/// PdfCraft ledger hayro (18) (#2972): a page's objects composite with a transparent backdrop
+/// and the page group is then put on the paper, so a blend mode only blends where something
+/// was drawn. A green fill with `/BM /Screen` (or Lighten, Difference…) half over a red square
+/// takes the mode's color over the red and stays plain green over the empty page.
+#[cfg_attr(not(target_arch = "wasm32"), test)]
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+fn blend_modes_over_an_empty_part_of_the_page_keep_their_color() {
+    for (mode, over_red) in [
+        ("Normal", Some([0, 255, 0])),
+        ("Multiply", Some([0, 0, 0])),
+        ("Screen", Some([255, 255, 0])),
+        ("Lighten", Some([255, 255, 0])),
+        ("Difference", Some([255, 255, 0])),
+        ("Exclusion", Some([255, 255, 0])),
+        ("ColorDodge", Some([255, 0, 0])),
+        ("Overlay", Some([255, 0, 0])),
+        // The non-separable modes mix hue, saturation and luminosity; over an empty part of
+        // the page they too keep the source color.
+        ("Hue", None),
+        ("Saturation", None),
+        ("Color", None),
+        ("Luminosity", None),
+    ] {
+        let page = format!("/MediaBox [0 0 300 200] /Resources << /ExtGState << /G1 << /Type /ExtGState /BM /{mode} >> >> >>");
+        let p = render(&one_page(&page, b"1 0 0 rg 20 20 150 150 re f /G1 gs 0 1 0 rg 100 50 150 120 re f", b""));
+        // Pixel rows run top down: PDF (210, 100) and (130, 100) are rows 100 from the top.
+        assert_eq!(pixel(&p, 210, 100), [0, 255, 0, 255], "{mode} over the empty page");
+        if let Some(over_red) = over_red {
+            assert_eq!(pixel(&p, 130, 100)[..3], over_red, "{mode} over the red square");
+        }
+        assert_eq!(pixel(&p, 280, 10), [255, 255, 255, 255], "{mode}: the paper stays white");
+    }
+}

vendor/README.craft-reader.md

@@ -210,8 +210,13 @@ each marked `PdfCraft patch` in the source: hayro-interpret (40) (PdfCraft #2987
 content membership dictionary with a visibility expression (`/VE`: `/And`, `/Or` and `/Not`
 over groups the document lists in `/OCProperties /OCGs`) shows its content when the
 expression is true, ahead of `/OCGs` and `/P`; a malformed expression, or one nested deeper
-than 32 levels, is ignored and `/OCGs` and `/P` decide. Its PdfCraft raster test is adapted in
-`tests/upstream_raster.rs`.
+than 32 levels, is ignored and `/OCGs` and `/P` decide; and hayro (18) (PdfCraft #2972), the
+page renders as one transparency group over a transparent backdrop that is then put on the
+background, so a blend mode over an empty part of the page keeps its color; with a color
+replacement (reading colors) the background stays inside the group as its backdrop. A plate
+render (`InterpreterSettings::plate`) also keeps the background inside, marked `Craft patch`,
+so blend modes on a separation's film blend with the paper. The PdfCraft raster tests of both
+are adapted in `tests/upstream_raster.rs`.
 
 This remains the patched 0.7 backend. It retains the existing Bradford Lab conversion,
 floating component precision and codec versions, rather than claiming full 0.8

vendor/hayro/SOURCE.json

@@ -62,7 +62,7 @@
       "path": "src/lib.rs",
       "source_sha256": "682db2a32fbf4d875c0e9dd7ac5dc875a6b09ade375efbce6f104de9197de078",
       "source_git_blob": "6333ed5808ae0cf9cf158e4730e973c1d7089bab",
-      "sha256": "682db2a32fbf4d875c0e9dd7ac5dc875a6b09ade375efbce6f104de9197de078"
+      "sha256": "8396b90516603079bb43012621e8089621fec58fdd3d9c9aa9a8d2aa840265da"
     },
     {
       "path": "src/renderer.rs",

vendor/hayro/src/lib.rs

@@ -254,15 +254,22 @@ pub fn render_into<'a>(
         interpreter_settings.preview,
     );
 
-    // PdfCraft patch (18): the page renders as one transparency group with the background
-    // painted *inside* it, as the initial backdrop. A group isolates what it wraps from what
-    // was painted before it, so with the background outside, a Multiply highlight blended
-    // only with the group's transparent backdrop and stayed its own colour — over a dark
-    // reading-colours background the whole quad became one flat bar and the words under it
-    // vanished. Inside the group the highlight multiplies with the background too: the words
-    // keep the highlight's colour and the band between them takes the background (white
-    // backgrounds are unaffected — multiplying with white is the identity).
-    device.push_transparency_group(1.0, None, BlendMode::Normal);
+    // PdfCraft patch (18): the page renders as one transparency group (the page group). Its
+    // objects composite with an initially transparent backdrop, and the group is then put on
+    // the paper (ISO 32000-2, page group), so a blend mode only blends where something was
+    // drawn: a /BM /Screen fill over an empty part of the page keeps its color (#2970).
+    // Under reading colors the background is painted *inside* the group instead, as its
+    // initial backdrop, so a Multiply highlight blends with the dark background: outside it,
+    // the highlight kept its own flat color and the words under it vanished into the band
+    // (#1548). Reading colors replace the page's colors anyway; for document colors the
+    // background stays outside, or every separable mode but Normal, Multiply, Darken and
+    // HardLight would blend with the white paper. Craft patch: a plate render keeps the
+    // background inside as well, so blend modes on a separation's film blend with the paper.
+    let backdrop_inside =
+        render_settings.color_replacement.is_some() || interpreter_settings.plate.is_some();
+    if backdrop_inside {
+        device.push_transparency_group(1.0, None, BlendMode::Normal);
+    }
 
     device.ctx.set_paint(render_settings.bg_color);
     device
@@ -274,11 +281,19 @@ pub fn render_into<'a>(
         path: clip_path,
         fill: FillRule::NonZero,
     });
+    if !backdrop_inside {
+        device.push_transparency_group(1.0, None, BlendMode::Normal);
+    }
 
     interpret_page(page, &mut state, &mut device);
 
+    if !backdrop_inside {
+        device.pop_transparency_group();
+    }
     device.pop_clip_path();
-    device.pop_transparency_group();
+    if backdrop_inside {
+        device.pop_transparency_group();
+    }
 
     let mut resources = vello_cpu::Resources::default();
     device.ctx.render_to_buffer(