Render knockout transparency groups

src/display/filter_factory.js

@@ -42,6 +42,10 @@ class BaseFilterFactory {
     return "none";
   }
 
+  addKnockoutFilter(alpha = 0) {
+    return "none";
+  }
+
   addHighlightHCMFilter(filterName, fgColor, bgColor, newFgColor, newBgColor) {
     return "none";
   }
@@ -329,6 +333,37 @@ class DOMFilterFactory extends BaseFilterFactory {
     return url;
   }
 
+  addKnockoutFilter(alpha = 0) {
+    // Shape alpha mask: for translucent elements, remove the opacity constant
+    // from the painted alpha while preserving antialias coverage. With no
+    // usable opacity, fall back to a binary mask.
+    const slope = alpha > 0 ? Math.min(1 / alpha, 1e6) : 1e6;
+    const key = `knockout_${slope}`;
+    const value = this.#cache.get(key);
+    if (value) {
+      return value;
+    }
+
+    const id = `g_${this.#docId}_knockout_filter_${this.#id++}`;
+    const url = this.#createUrl(id);
+    this.#cache.set(key, url);
+
+    const filter = this.#createFilter(id);
+    const feComponentTransfer = this.#document.createElementNS(
+      SVG_NS,
+      "feComponentTransfer"
+    );
+    filter.append(feComponentTransfer);
+    const feFuncA = this.#document.createElementNS(SVG_NS, "feFuncA");
+    // Linear feFunc clamps to [0, 1].
+    feFuncA.setAttribute("type", "linear");
+    feFuncA.setAttribute("slope", `${slope}`);
+    feFuncA.setAttribute("intercept", "0");
+    feComponentTransfer.append(feFuncA);
+
+    return url;
+  }
+
   addHighlightHCMFilter(filterName, fgColor, bgColor, newFgColor, newBgColor) {
     const key = `${fgColor}-${bgColor}-${newFgColor}-${newBgColor}`;
     let info = this.#hcmCache.get(filterName);

src/shared/util.js

@@ -657,6 +657,21 @@ class FeatureTest {
     });
   }
 
+  static get isCanvasFilterSupported() {
+    let ctx;
+    if (this.isOffscreenCanvasSupported) {
+      ctx = new OffscreenCanvas(1, 1).getContext("2d");
+    } else if (typeof document !== "undefined") {
+      ctx = document.createElement("canvas").getContext("2d");
+    }
+    // Spec-compliant Canvas2D defaults `ctx.filter` to "none". On
+    // browsers without filter support (Safari) the property is absent
+    // until you assign to it, after which it behaves like an ordinary
+    // JS property and stores whatever string you set without applying
+    // it. Probing the default lets us detect the difference reliably.
+    return shadow(this, "isCanvasFilterSupported", ctx?.filter !== undefined);
+  }
+
   static get isAlphaColorInputSupported() {
     return shadow(
       this,

test/pdfs/.gitignore

@@ -16,6 +16,12 @@
 !bug1727053.pdf
 !issue18408_reduced.pdf
 !bug1907000_reduced.pdf
+!knockout_blend_multiply.pdf
+!knockout_isolated_overlap.pdf
+!knockout_nested.pdf
+!knockout_nested_group_alpha.pdf
+!knockout_nonisolated_sparse.pdf
+!knockout_smask.pdf
 !SimFang-variant.pdf
 !bug1953099.pdf
 !issue11913.pdf
@@ -913,3 +919,4 @@
 !smask_alpha_bc.pdf
 !smask_luminosity_oob_transfer.pdf
 !operator_list_cycle.pdf
+!knockout_groups_test.pdf

test/test_manifest.json

@@ -6740,6 +6740,54 @@
       "maxY": 0.5
     }
   },
+  {
+    "id": "knockout_smask",
+    "file": "pdfs/knockout_smask.pdf",
+    "md5": "d89fbb623f8e5f200a324607dd30449d",
+    "rounds": 1,
+    "type": "eq",
+    "about": "Knockout group element with an active soft mask."
+  },
+  {
+    "id": "knockout_nested",
+    "file": "pdfs/knockout_nested.pdf",
+    "md5": "e3db1591b84f7654d53f553c60ec9fd1",
+    "rounds": 1,
+    "type": "eq",
+    "about": "Nested knockout group used as an element of an outer knockout group."
+  },
+  {
+    "id": "knockout_nested_group_alpha",
+    "file": "pdfs/knockout_nested_group_alpha.pdf",
+    "md5": "cec0133fe020f089a73530274c295fcc",
+    "rounds": 1,
+    "type": "eq",
+    "about": "Nested transparency group with parent alpha used as an element of an outer knockout group."
+  },
+  {
+    "id": "knockout_nonisolated_sparse",
+    "file": "pdfs/knockout_nonisolated_sparse.pdf",
+    "md5": "6c9837d997785b4d7f848fa7a96ff984",
+    "rounds": 1,
+    "type": "eq",
+    "about": "Sparse non-isolated knockout group under a blend mode."
+  },
+  {
+    "id": "knockout_isolated_overlap",
+    "file": "pdfs/knockout_isolated_overlap.pdf",
+    "md5": "b9c5dda6b8c7eb51a904766171290dc0",
+    "rounds": 1,
+    "type": "eq",
+    "about": "Isolated knockout group with two overlapping translucent fills; the overlap area must show only the second fill (no source-over mix)."
+  },
+  {
+    "id": "knockout_blend_multiply",
+    "file": "pdfs/knockout_blend_multiply.pdf",
+    "md5": "e0b68fa675e58685fbf1b916fa03e301",
+    "rounds": 1,
+    "type": "eq",
+    "about": "Non-isolated knockout group with a Multiply blend mode element on a yellow backdrop; the element must blend against the backdrop, not against a transparent temp canvas."
+  },
   {
     "id": "issue6010_1",
     "file": "pdfs/issue6010_1.pdf",
@@ -14196,5 +14244,13 @@
     "md5": "267dc76f33cc0c0b6d36ff4605f60907",
     "rounds": 1,
     "type": "eq"
+  },
+  {
+    "id": "knockout_groups_test",
+    "file": "pdfs/knockout_groups_test.pdf",
+    "md5": "8e085d0dcea38e976f81cbfb50ece6cd",
+    "rounds": 1,
+    "type": "eq",
+    "about": "Knockout groups composite-modes survey."
   }
 ]