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Re: [Public WebGL] Weird alpha issue.

Thx for helping identify the issue and providing a nice test case
(please don't delete it.  At least keep it for a few days, as I used
the link in my bug report).


On Mon, Nov 1, 2010 at 11:16 AM,  <steve@sjbaker.org> wrote:
> Yep - it looks fine in the Canary build under Windows with --use-gl=desktop.
> Presumably that's the difference though.  Under Linux, Chrome is forced to
> use OpenGL because it's the only game in town.  If there is no
> destination-alpha bug in Chrome/OpenGL then that explains what I'm seeing.
> You're not seeing my problem with ANGLE right now because it's broken and
> you don't see anything at all (neither do I since it auto-upgraded
> sometime yesterday).  The older Windows build I have been using (sorry - I
> don't have the version number to hand right now) runs ANGLE OK - and there
> the background-leakage bug is more or less the same as in Minefield...but
> I don't think there is a common cause because Minefield under Linux uses
> OpenGL and it's also broken.
> I'd guess that ANGLE is ignoring "alpha:false" in getCompositor and
> returning me a destination-alpha canvas come-what-may - but OpenGL is
> doing the right thing.
>  -- Steve
>> I tried your test case with top-of-tree Chrome on Mac and Windows.
>> Both do not have the background leaking issue you saw here.  Can you
>> try the same build and confirm?
>> For the Windows build, you need to pass in --use-gl=desktop flag.
>> Apparently there is a bug in ANGLE, so without the flag you won't see
>> the WebGL canvas at all - at least that's the case on my machine.
>> Mo
>> On Sun, Oct 31, 2010 at 2:02 PM, Steve Baker <steve@sjbaker.org> wrote:
>>> OK - I finally have a reasonably simple test case at:
>>>    http://www.sjbaker.org/test/index.php
>>> ...I apologize if some of the code doesn't look exactly pretty - it's
>>> been brutally ripped out of a 5000 line application.  The checkerboard
>>> background helps you see the blending issues clearly - but it's a bit
>>> hard on the eyes!
>>> We should see a dark brown canvas on a checkerboard background - with a
>>> chair and a grey translucent 'smoke puff' slicing through it.
>>> * The only setup that I've found seems to display it "correctly" (ie
>>> without dest-alpha) is Chrome under Linux/64bit...but I don't have a Mac
>>> to test on so we'll give Safari the benefit of the doubt right now.
>>> * Minefield lets the background 'leak' through the image (presumably
>>> because we can't shut off destination-alpha because of bug 539771.
>>> (That's a **REALLY** serious bug!)
>>> * Chrome under Windows7/32bit also lets the background leak through -
>>> but does a weird compositing job.
>>> * None of the browsers are getting the alpha=0 thing right.
>>> * If you remove the "alpha:false" from the getContext call - you can
>>> also see that Chrome and Minefield composite the resulting dest-alpha
>>> canvas differently.
>>>> If you go to about:config and set
>>>>    layers.accelerate-all=false
>>>>    layers.accelerate-none=true
>>>> Does that make a difference?
>>> No...no difference at all.
>>>> If yes, you have found a bug in our accelerated compositing code.
>>>> If no, we can't conclude anything.
>>> Oh.   :-(
>>>  -- Steve

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