Merge pull request #21372 from calixteman/issue7998

Render gray transparency groups in grayscale

src/core/evaluator.js

@@ -523,6 +523,7 @@ class PartialEvaluator {
         isolated: false,
         knockout: false,
         needsIsolation: false,
+        isGray: false,
       };
 
       const groupSubtype = group.get("S");
@@ -541,6 +542,10 @@ class PartialEvaluator {
         }
       }
 
+      // When the group color space is gray (a single component) the group's
+      // content must be rendered in grayscale, see issue 7998.
+      groupOptions.isGray = colorSpace?.numComps === 1;
+
       if (smask?.backdrop) {
         colorSpace ||= ColorSpaceUtils.rgb;
         smask.backdrop = colorSpace.getRgbHex(smask.backdrop, 0);

src/display/canvas.js

@@ -3262,6 +3262,7 @@ class CanvasGraphics {
     if (
       !group.needsIsolation &&
       !group.knockout &&
+      !group.isGray &&
       this.#knockoutGroupLevel === 0 &&
       currentCtx.globalAlpha === 1 &&
       currentCtx.globalCompositeOperation === "source-over" &&
@@ -3473,6 +3474,13 @@ class CanvasGraphics {
       return;
     }
 
+    if (group.isGray) {
+      // The group color space is gray (a single component), so its rendered
+      // content must be converted to grayscale before being composited onto
+      // the parent canvas, see issue 7998.
+      this.#convertGroupToGray(groupCtx);
+    }
+
     this.ctx = ctx;
     // Turn off image smoothing to avoid sub pixel interpolation which can
     // look kind of blurry for some pdfs.
@@ -3604,6 +3612,38 @@ class CanvasGraphics {
     }
   }
 
+  #convertGroupToGray(groupCtx) {
+    const { canvas } = groupCtx;
+    const { width, height } = canvas;
+
+    if (FeatureTest.isCanvasFilterSupported) {
+      // Draw the canvas onto itself with the grayscale filter applied (which
+      // preserves the alpha channel), using the "copy" composite operation so
+      // the filtered content fully replaces the original.
+      groupCtx.save();
+      groupCtx.setTransform(1, 0, 0, 1, 0, 0);
+      groupCtx.filter = "grayscale(1)";
+      groupCtx.globalAlpha = 1;
+      groupCtx.globalCompositeOperation = "copy";
+      groupCtx.drawImage(canvas, 0, 0);
+      groupCtx.restore();
+      return;
+    }
+
+    // Fallback when canvas filters aren't supported: convert each pixel to
+    // grayscale by hand, using the same luminance coefficients as the
+    // "grayscale(1)" filter while leaving the alpha channel untouched.
+    const imageData = groupCtx.getImageData(0, 0, width, height);
+    const { data } = imageData;
+    for (let i = 0, ii = data.length; i < ii; i += 4) {
+      const gray =
+        (data[i] * 0.2126 + data[i + 1] * 0.7152 + data[i + 2] * 0.0722 + 0.5) |
+        0;
+      data[i] = data[i + 1] = data[i + 2] = gray;
+    }
+    groupCtx.putImageData(imageData, 0, 0);
+  }
+
   #destroyKnockoutPools(groupMeta) {
     if (!groupMeta) {
       return;

test/pdfs/issue7998.pdf.link

@@ -0,0 +1 @@
+https://bugs.ghostscript.com/attachment.cgi?id=13156

test/test_manifest.json

@@ -3822,6 +3822,14 @@
     "link": false,
     "type": "eq"
   },
+  {
+    "id": "issue7998",
+    "file": "pdfs/issue7998.pdf",
+    "md5": "6cf4622cbcb61fb07525dfb0ab23f9c3",
+    "rounds": 1,
+    "link": true,
+    "type": "eq"
+  },
   {
     "id": "issue11279",
     "file": "pdfs/issue11279.pdf",