[Qt-interest] Performance issues with Graphics View Framework

[Constantin Makshin]

OpenGL uses RGB order internally, but most, if not all, GPUs store RGB  
textures in RGBA format for alignment purposes, so RGB<->RGBA conversion  
is more likely to happen during copying than RGB<->BGR.

On Tue, 21 Jul 2009 13:29:12 +0400, Enrico Ros <enrico.qt at email.it> wrote:
> On Tuesday 21 July 2009 10:20:54 Simone Artesino wrote:
>> Ok Enrico, but let's consider now these tests result, made on atom  
>> chipset
>> polusbo. running a 30 fps timer,
>> when use as viewport a Widget I obtain 30 fps and 19% cpu load.
>> when use as viewport a QGLWidget I obtain 5 fps and 67% cpu load.
>> these results are not influenced from switching viewport update policy.
>>
>> What I can infer from this result?
>> my qpixmap copy is well hardware accelerated when I use a Widget (and is
>> stored in gpu internal memory I guess), instead when I use a QGLWidget  
>> the
>> copy is translated in a very bad open-gl code that makes several copies  
>> of
>> memory from ram-cpu memory to gpu.
>>
>> Is a correct deduction? what you think about it?
>
> It seems so, yes. Moreover, there may be pixel conversion stuff involved  
> (RGB (qimage/qpixmap) to BGR (opengl), I think). Uploading a texture to
> OpenGL takes time, maybe it's less accelerated than a simple memory->gpu
> pixmap transfer.
>
> In any case if you want to get the maximum performance for you  
> application,
> use the old school trick I explained you in the previous mail. Just  
> upload the
> pixmap once to the OpenGL memory and draw a fixed rectangle with  
> scrolling
> texture coordinates on vertices (to select the right 'scroll portion').
>
> Without looking at the code this is the best I can tell.
>
> Ciao,
> Enrico
>
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-- 
Constantin "Dinosaur" Makshin