Merge pull request #21604 from calixteman/issue21593

Copy the backdrop for non-isolated groups with a soft mask

Author: timvandermeij

src/display/canvas.js

@@ -3333,18 +3333,53 @@ class CanvasGraphics {
     groupCtx.translate(-offsetX, -offsetY);
     groupCtx.transform(...currentTransform);
 
-    if (
-      !group.isolated &&
-      !group.smask &&
-      inSMaskMode &&
-      group.needsIsolation
-    ) {
-      // For non-isolated groups that need isolation and are entered from SMask
-      // mode, copy the current canvas background so that inner blend modes
-      // (e.g. "screen") interact correctly with the background rather than
-      // compositing onto a transparent canvas.
-      // Groups without needsIsolation have no inner blend modes; their content
-      // is composited correctly via the SMask in endGroup without a copy.
+    const needsBackdropCopy =
+      !group.isolated && !group.smask && group.needsIsolation;
+    const replaceBackdrop =
+      needsBackdropCopy &&
+      !inSMaskMode &&
+      savedKnockoutLevel === 0 &&
+      !group.knockout &&
+      !group.isGray &&
+      group.hasSoftMask &&
+      currentCtx.globalAlpha === 1 &&
+      currentCtx.globalCompositeOperation === "source-over" &&
+      this.current.transferMaps === "none";
+    if (needsBackdropCopy && (inSMaskMode || replaceBackdrop)) {
+      // A non-isolated group that needs isolation (because of an inner blend
+      // mode and/or a soft mask) can't use the direct path above, so it
+      // renders on a transparent intermediate canvas. Copy the current
+      // backdrop into it so the inner blend modes (e.g. "multiply", "screen")
+      // interact with the real background instead of transparency; otherwise a
+      // /Multiply highlight, say, would be painted opaquely over the text
+      // behind it, hiding it (bug 1873345 -- same as the direct path, but here
+      // the soft mask forces an intermediate canvas).
+      // This is needed both when entered from SMask mode and at the top level;
+      // a non-isolated subgroup nested inside a knockout group instead gets its
+      // backdrop through the hasInnerBackdrop path in endGroup, so it's
+      // excluded here via savedKnockoutLevel.
+      //
+      // Outside SMask mode the copy is limited to a group that *would* have
+      // qualified for the direct path (source-over, group alpha 1, not
+      // knockout/gray) but was forced onto the intermediate canvas by its soft
+      // mask (`hasSoftMask`). Restricting to that domain keeps the change
+      // surgical:
+      //   - Only the soft-mask case is the bug 1873345 gap; a non-isolated
+      //     blend group pushed onto the intermediate canvas merely by a
+      //     non-default group alpha (no soft mask) is a separate pre-existing
+      //     case, left untouched to avoid changing its rendering (issue 13520).
+      //   - source-over/alpha 1 let endGroup replace the backdrop region with
+      //     the copied-and-updated result; compositing it normally would blend
+      //     partially transparent backdrop pixels twice. Under an outer blend
+      //     mode the outer blend would combine the backdrop with itself (e.g.
+      //     a /Multiply group with an inner soft mask darkens, issue 12798); a
+      //     non-1 group alpha would likewise mix the copied backdrop back in.
+      //     Those groups keep the transparent canvas and blend against the
+      //     real backdrop instead.
+      //   - isGray groups are grayscaled wholesale in endGroup, and an
+      //     inherited transfer filter (`transferMaps`) would be applied on
+      //     write-back, both of which would corrupt the copied backdrop, so
+      //     they're excluded too.
       groupCtx.save();
       groupCtx.setTransform(1, 0, 0, 1, 0, 0);
       groupCtx.drawImage(currentCtx.canvas, -offsetX, -offsetY);
@@ -3413,6 +3448,7 @@ class CanvasGraphics {
       offsetX,
       offsetY,
       hasInnerBackdrop,
+      replaceBackdrop,
       knockoutMaskEntry,
       // Per-group scratch pools, lazily filled and freed in endGroup.
       knockoutTempEntry: null,
@@ -3566,6 +3602,14 @@ class CanvasGraphics {
           });
         }
       } else {
+        if (groupMeta.replaceBackdrop) {
+          // "copy" clears the destination outside the source within the
+          // current clip, so limit it to the intermediate canvas bounds.
+          const clip = new Path2D();
+          clip.rect(0, 0, groupCtx.canvas.width, groupCtx.canvas.height);
+          this.ctx.clip(clip);
+          this.ctx.globalCompositeOperation = "copy";
+        }
         this.ctx.drawImage(groupCtx.canvas, 0, 0);
       }
       this.ctx.restore();

test/pdfs/.gitignore

@@ -947,3 +947,4 @@
 !issue21579.pdf
 !saslprep-r6.pdf
 !jpx_smaskindata.pdf
+!nonisolated_blend_smask.pdf

test/pdfs/nonisolated_blend_smask.pdf

@@ -14501,5 +14501,12 @@
     "md5": "f7a8628b14ca176700247f106c16812a",
     "rounds": 1,
     "type": "eq"
+  },
+  {
+    "id": "nonisolated_blend_smask",
+    "file": "pdfs/nonisolated_blend_smask.pdf",
+    "md5": "4b87611416be34894eeb19c158fafbad",
+    "rounds": 1,
+    "type": "eq"
   }
 ]