[Qt-interest] Performance issues with Graphics View Framework

[Enrico Ros]

On Tuesday 21 July 2009 11:41:28 Constantin Makshin wrote:
> Your animation method is very "GPU-unfriendly".
>
> Since all widget handling (including drawing) is done on CPU, their
> off-screen buffers are stored in RAM. If you use the default graphics
> system, which is not hardware-accelerated, then pixmaps are also stored in
> RAM and your QWidget<->QPixmap copy operations are relatively cheap. But
> if you mix hardware- and software-accelerated things, e.g. QGLWidget and
> software QPixmap-s, then you do a lot of RAM<->VRAM copy and probably
> image processing (conversion between CPU and GPU formats, etc.) operations.

On linux the default graphicssystem is "native" so X11 based and QPixmaps are 
in video memory. However what you say still apply ;-)

> If you want to use available hardware acceleration, then do what Enrico
> suggests (load the image as a texture and scroll it by modifying texture
> coordinates). Updating 4 vertices is MUCH cheaper than uploading new image
> to VRAM (assuming each vertex has 3 coordinates (x, y, z) and 2 texture
> coordinates (s, t), 4 vertices'll occupy only 80 bytes).

Yes, and he could go even further, by using a vertexbuffer with the data and 
just scrolling the texture matrix :D 1 operation ~ 16 bytes ;-) Impressive 
savings.

Enrico

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