Bug 278454 – CanvasRenderingContext2D globalAlpha property is ignored for some values of globalCompositeOperation

• Component: Canvas
• Status: RESOLVED FIXED

[Alf Eaton]

Created attachment 472255 [details]
Screenshot of demo in Safari

When drawing in a canvas, some values of globalCompositeOperation (source-in, source-out, destination-in, destination-atop) cause the current globalAlpha value to be ignored, which results in transparency being lost.

This bug is preventing PDF.js from rendering soft masks in PDF images, when used in Safari (unlike both Chrome and Firefox, which handle this correctly).

https://github.com/mozilla/pdf.js/issues/18634

A demonstration of all the globalCompositeOperation values:

https://codepen.io/invisiblecomma/pen/poXLPaZ

Minimal code to reproduce:

  const canvas = document.createElement("canvas");
  canvas.width = "40";
  canvas.height = "60";
  document.body.append(canvas);

  const ctx = canvas.getContext("2d");
  ctx.font = "50px Impact";
  ctx.fillStyle = "#777777";
  ctx.fillText("A", 10, 40);

  ctx.globalCompositeOperation = 'destination-atop';
  ctx.globalAlpha = 0.5;
  ctx.fillStyle = "#ff0000";
  ctx.fillText("B", 10, 60);

[Karl Dubost]

Created attachment 472262 [details]
rendering in safari, firefox, chrome

Safari Technology Preview  201           20620.1.2
Firefox Nightly            131.0a1       13124.8.19
Google Chrome Canary       130.0.6670.0  6670.0

[Karl Dubost]

This is a duplicate of Bug 209802

*** This bug has been marked as a duplicate of bug 209802 ***

[Karl Dubost]

I will reopen here. 
Alex noticed rightly that this is different.
Bug 209802 is about performance.

[Radar WebKit Bug Importer]

<rdar://problem/134840885>

[Said Abou-Hallawa]

Pull request: https://github.com/WebKit/WebKit/pull/32845

[Said Abou-Hallawa]

Created attachment 472336 [details]
test case

[EWS]

Committed 282965@main (40b19269f440): <https://commits.webkit.org/282965@main>

Reviewed commits have been landed. Closing PR #32845 and removing active labels.

[Said Abou-Hallawa]

Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/47906

[Said Abou-Hallawa]

Re-opening for pull request https://github.com/WebKit/WebKit/pull/32966

[EWS]

Committed 282996@main (1e29db507d16): <https://commits.webkit.org/282996@main>

Reviewed commits have been landed. Closing PR #32966 and removing active labels.