Optimizing OpenGL canvas with partial updates

[Boudewijn Rempt]

It's worth a try, but at least in some point of time we already didn't update all of the screen, even in in opengl mode, didn't we? I know we had problems with that on AMD. It also sounds like we might be running into the same problems we have with the GPU canvas, namely small artefacts at the update bounds. If we keep that in mind, it's definitely worth a try, anything that improves performance always is!

On Sun, 14 Feb 2016, Dmitry Kazakov wrote:

> Hi, all!
> 
> Today I was talking to Maxim (Salabar) and we found quite an interesting idea, which could optimize our openGL rendering pipeline a lot, and would make our dependency on glSync less
> important (glSync is an extension, and it is not present on some (older) GPUs).
> 
> The point is that starting with Qt 5, our paintGL() implementation does not draw our canvas directly on screen. Instead it is drawn on an internal Qt's framebuffer, which is (later)
> painted on screen buffers, which are swapped double buffering. I'm not going to discuss whether such global decision in Qt is good or bad, but for us it means that now we can use
> partial updates, which (starting with Qt5.5) can be activated with a special call QOpenGLWidget::setUpdateBehavior(). What does it mean? It means that we shouldn't repaint the whole
> canvas every time and we need *not* spend quite a lot of time in our High Quality scaling shader, which is really slow. Instead, we can update only a portion of the screen, spend a
> little time on shading this small portion and store in this internal Qt's buffer. Of course in some special cases glSync will still be needed, but I bet that in 90% of the use cases
> such partial updates will execute really fast!
> 
> What do you think about this idea? 
> 
> 
> --
> Dmitry Kazakov
> 
>

-- 
Boudewijn Rempt | http://www.krita.org, http://www.valdyas.org