Re: Probable bug in WGL implementation (AIGLX) of GLX calls in XWin -wgl

[Jon TURNEY <jon.turney@dronecode.org.uk>]

On 16/10/2012 21:34, Tim Edwards wrote:
>> The current implementation of GLX using WGL takes a few shortcuts, basically
>> anything that is drawn with OpenGL isn't composed into the screen, it's just
>> drawn on top of it.
> 
> I wouldn't want to sound too peevish, as I was quite happy to find that
> OpenGL hardware acceleration was even possible, as it was not on the
> previous version of Cygwin that I had downloaded.  That it fails in
> obscure applications is sort of to be expected.
> 
>> This works well enough when the GLX window is a top-level window, or is
>> non-top-level and has no occluding relatives and is drawn after anything it
>> occludes, but mis-renders in more complex scenarios.
>>
>> This is discussed a bit more in [1]
>>
>> As mentioned there I have done a bit of work fix the mis-rendering in some
>> cases.  I can't quite tell from you description exactly what's going wrong, so
>> I am not sure if those changes are going to help in this particular case.
>>
>> I have built a test release including the changes discussed there, available
>> at [2], if you would like to test if it makes things better/worse/no
>> difference.
> 
> I downloaded the link, ran the X server, ran my application, and get no
> difference in the behavior.

Ok. Thanks for testing, anyhow.

> 
>> The proper solution is probably something like rendering the OpenGL to an
>> offscreen buffer, and then composing it into the un-occluded area of the
>> window, but that is considerably more complex to implement.
> 
> The two problems with this approach are that (1) I have found more
> buggy implementations in OpenGL servers by doing offscreen rendering;
> and (2) this particular tool is a VLSI layout editor and must be
> rendered directly on the front buffer.  The general approach is to
> render everything as fast as possible and always be willing to break
> on key interrupt to start over.  And I have misappropriated the back
> buffer for backing store purposes. . .

Sorry, I wasn't clear here.  I'm not suggesting that the application should be
changed.  I'm just describing a possible approach to fixing this problem in
the X server.


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