noel gordon Reporter
	Description • 10 years ago

User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.95 Safari/537.36

Steps to reproduce:

For example, encoding <canvas> content via toDataURL as image/png (lossless) into a new Image(), where the canvas contains alpha values near 0.5, and then drawing that Image() onto a new canvas with drawImage(), and repeating this process over and over, should be idempotent.  For some alpha values, the image is slowly destroyed.

Test case attached: for Firefox, set alpha=0.498 for example, and keep pressing the test button.


Actual results:

The canvas image slowly fades to gray with each test button press.

Other browsers:
  IE9+: works.
  Chrome 39 stable: broken.
  Firefox 33.0.3: broken.
  Safari 6: broken.
  Safari 7: works.



Expected results:

The canvas image should not fade away for any alpha in the neigbourhood of 0.5.

	noel gordon Reporter
	Comment 1 • 10 years ago

Test-case result for alpha=0.498 on Firefox 33.0.3

	noel gordon Reporter
	Comment 2 • 10 years ago

Chrome bug https://code.google.com/p/chromium/issues/detail?id=450189

	:Gijs (he/him)
	Comment 3 • 10 years ago

Seth, can you help place/prioritize/find-a-someone for this? Not sure if this is canvas, imagelib, gfx, or somewhere in the middle. :-)

	Seth Fowler [:seth] [:s2h]
	Comment 4 • 10 years ago

This probably exists at the intersection of canvas and ImageLib, which is good news since I'm about to start focusing on canvas work once I get the recent ImageLib regressions cleaned up.

I'll come back to this once I switch over to canvas.

	noel gordon Reporter
	Comment 5 • 9 years ago

Fixed in chromium: https://code.google.com/p/chromium/issues/detail?id=450189

	noel gordon Reporter
	Comment 6 • 9 years ago

I believe that Gecko uses (or once used) a table-driven method to compute pixel alpha pre-multiplication. Perhaps the table data was computed from a truncating pre-multiply routine?

Perhaps try a rounding one.  That was the problem in Chrome a least - a truncating premultiply in the image decoders. I changed the decoders to use a rounding pre-multiply and the alpha channel instability was gone.

HTH.

	Bas Schouten (:bas.schouten)
	Comment 7 • 9 years ago

I actually suspect this is more likely to be in the Canvas code. We should probably just fix this.

	Jonathan Watt [:jwatt]
	Comment 8 • 6 years ago

Mass bug change to replace various 'parity' whiteboard flags with the new canonical keywords. (See bug 1443764 comment 13.)