[Development] QtCS: OpenGL session notes

[Uwe Rathmann]

On Tue, 26 Jun 2012 07:05:35 +0000, gunnar.sletta wrote:

 > The lines left above highlights the general problem... The QPainter API
 > is very feature rich and opens up for a number of use patterns that are
 > less than ideal for performance.

Please don't forget that QPainter is more then rendering to screen: f.e. 
when you need to generate PDF documents and you don't want to implement 
the render code twice.
IMHO being an abstract API for very different backends is as important 
as performance issues ( that you won't even notice in most use cases ).

One note about performance: QPainter often does pointless calculations - 
in many applications I had much more effect with doing polygon clipping 
in application code - before the points have been passed to the painter 
- than with using a hardware accelerated backends. And these type of 
algorithmic optimizations work on any system - even on those, where you 
don't have any hardware accelerated option !

I bet that introducing a QPainter::ClipPolygon render hint would improve 
the performance of many Qt widgets showing graphics significantly - 
without any major redesign neither in Qt nor in application code.

Uwe