Heath Feather wrote:
> Yes Samuel, Thanks for the response.
>
> I completely agree, Once I saw this behavior and diagnosed it I switched form
> something like this.
>
> for(int nY=0; nY < 10; ++nY){
> for(int nX=0 nX < 10; ++nX){
> image.setPixel(nX,nY,color);
> }
> }
>
> To something like this
>
> uchar* pBits = image.bits();
> int nBytesPerLine = image.bytesPerLine();
>
> for(int nY=0; nY < 10; ++nY){
> for(int nX=0 nX < 10; ++nX){
> uchar * scanLine = pBits+nY*nBytesPerLine;
> ((uint *)scanLine)[nX] = color;
> }
> }
>
> And saw an 800x improvement. Even though this is still not optimal, it was
> well within the performance profile I required.
>
> I tried to avoid the QImage::scanLine() method since it too contains a
> detach() call. (see my discussion of the double detach in setPixel())
>
> The other thing not to miss is that when I was monitoring the
> qt_gl_clean_cache() call, I would see it called by Qt as part of their
> internal Backing Store machinery. A simple switching of tabs on a tab widget
> in my application invoked 1800 calls to this (very expensive) method. And
> just dragging your mouse across widgets causes this to be called a tremendous
> amount too.
>
> If you don't mind me asking. What is the exact purpose of this Texture Cache ?
>
> Heath.
Yes, using the scanLine() function per row can also cause some overhead
if the width of the image is small, so the absolute fastest way is to
use bits() and compute offsets as you go, but it's a trade-off of
maintainability/readability and performance.
The texture cache contains a mapping from QImages and QPixmaps to OpenGL
texture ids, and its purpose is to avoid re-uploading data to texture
memory as much as possible when using the QGL-classes. When
images/pixmaps are no longer referenced we remove them from the cache.
The check to see whether they are part of the cache is done when
detach() is called on an image/pixmap that is only referenced from one
place. Currently the check is a lot more expensive than it needs to be,
but we are working on fixing this. As a workaround you could try to free
any images/pixmaps that have been bound as textures when they are no
longer in use, although this might very well be impractical in most cases.
Regards,
Samuel Rødal
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