Draw non-isolated blend-mode groups against their backdrop (bug 1873345)

A non-isolated transparency group must blend with its backdrop, but a group
containing a blend mode was forced onto a transparent intermediate canvas;
e.g. a /Multiply highlight then painted opaquely over the text behind it,
 making that text invisible.

src/core/annotation.js

@@ -5025,16 +5025,7 @@ class HighlightAnnotation extends MarkupAnnotation {
 
     const quadPoints = (this.data.quadPoints = getQuadPoints(dict, null));
     if (quadPoints) {
-      const resources = this.appearance?.dict.get("Resources");
+      if (!this.appearance) {
-
-      if (!this.appearance || !resources?.has("ExtGState")) {
-        if (this.appearance) {
-          // Workaround for cases where there's no /ExtGState-entry directly
-          // available, e.g. when the appearance stream contains a /XObject of
-          // the /Form-type, since that causes the highlighting to completely
-          // obscure the PDF content below it (fixes issue13242.pdf).
-          warn("HighlightAnnotation - ignoring built-in appearance stream.");
-        }
         // Default color is yellow in Acrobat Reader
         const fillColor = getPdfColorArray(this.color, [1, 1, 0]);
         const fillAlpha = dict.get("CA");

src/core/evaluator.js

@@ -523,6 +523,7 @@ class PartialEvaluator {
         isolated: false,
         knockout: false,
         needsIsolation: false,
+        hasSoftMask: false,
         isGray: false,
       };
 
@@ -573,6 +574,7 @@ class PartialEvaluator {
 
     if (group) {
       groupOptions.needsIsolation = newOpList.needsIsolation || !!smask;
+      groupOptions.hasSoftMask = newOpList.hasSoftMask || !!smask;
       operatorList.addOp(OPS.beginGroup, [groupOptions]);
       operatorList.addOp(OPS.paintFormXObjectBegin, args);
       operatorList.addOpList(newOpList);

src/core/operator_list.js

@@ -830,24 +830,29 @@ class OperatorList {
  * operations require being drawn in isolation (i.e. on a separate canvas).
  * A group/pattern needs isolation when it uses non-default compositing
  * (blend mode) or a soft mask. The result is exposed via `needsIsolation`.
+ *
+ * `hasSoftMask` separately flags the use of a soft mask: unlike a plain blend
+ * mode, which a non-isolated group can apply directly against its backdrop, a
+ * soft mask always requires a real intermediate canvas (see bug 1873345).
  */
 class CheckedOperatorList extends OperatorList {
   needsIsolation = false;
 
+  hasSoftMask = false;
+
   addOp(fn, args) {
-    if (!this.needsIsolation) {
+    if (!this.needsIsolation || !this.hasSoftMask) {
       if (fn === OPS.beginGroup) {
         // Propagate isolation only if the nested group itself needs it.
-        this.needsIsolation = args[0].needsIsolation;
+        this.needsIsolation ||= args[0].needsIsolation;
+        this.hasSoftMask ||= args[0].hasSoftMask;
       } else if (fn === OPS.setGState) {
         for (const [key, val] of args[0]) {
           if (key === "BM" && val !== "source-over") {
             this.needsIsolation = true;
-            break;
+          } else if (key === "SMask" && val !== false) {
-          }
-          if (key === "SMask" && val !== false) {
             this.needsIsolation = true;
-            break;
+            this.hasSoftMask = true;
           }
         }
       }

src/display/canvas.js

@@ -3210,12 +3210,13 @@ class CanvasGraphics {
     }
 
     const currentCtx = this.ctx;
-    if (!group.isolated && !group.knockout && this.#knockoutGroupLevel === 0) {
-      info("TODO: Fully support non-isolated non-knockout groups.");
-    }
-
     if (
-      !group.needsIsolation &&
+      // A non-isolated group blends with its backdrop, so drawing it directly
+      // on the parent canvas (rather than on a transparent intermediate one)
+      // is correct even when it contains blend modes (bug 1873345). A soft
+      // mask still needs its own canvas though, and an isolated group requires
+      // a transparent backdrop, so both keep the intermediate canvas.
+      (!group.needsIsolation || (!group.isolated && !group.hasSoftMask)) &&
       !group.knockout &&
       !group.isGray &&
       this.#knockoutGroupLevel === 0 &&
@@ -3234,12 +3235,24 @@ class CanvasGraphics {
         }
         currentCtx.clip(clip);
       }
+      // Unlike the intermediate-canvas path below, the content is drawn
+      // straight onto the parent canvas with no later compositing step, so the
+      // inherited blend mode, alpha constants and transfer function must stay
+      // active here rather than being reset (issue 20722); the conditions
+      // above already guarantee a Normal blend and an opaque (ca === 1) state.
       this.groupStack.push(null); // null = no intermediate canvas
       this.#groupStackMeta.push(null);
       this.groupLevel++;
       return;
     }
 
+    // Reached only when the direct path above didn't apply, e.g. a soft mask,
+    // non-default group alpha or blend mode: we still composite on a
+    // transparent intermediate canvas rather than the real backdrop.
+    if (!group.isolated && !group.knockout && this.#knockoutGroupLevel === 0) {
+      info("TODO: Fully support non-isolated non-knockout groups.");
+    }
+
     const currentTransform = getCurrentTransform(currentCtx);
     if (group.matrix) {
       currentCtx.transform(...group.matrix);

test/pdfs/.gitignore

@@ -936,3 +936,4 @@
 !bug1802506.pdf
 !checkbox_no_appearance.pdf
 !opt_demo.pdf
+!bug1873345.pdf

test/test_manifest.json

@@ -6786,6 +6786,13 @@
     "type": "eq",
     "about": "Every blend mode that PDF supports."
   },
+  {
+    "id": "bug1873345",
+    "file": "pdfs/bug1873345.pdf",
+    "md5": "03483b94a2c02ac5f98c17ccf14de8ea",
+    "rounds": 1,
+    "type": "eq"
+  },
   {
     "id": "transparency_group",
     "file": "pdfs/transparency_group.pdf",