[Development] QtCS: OpenGL session notes

[Joe Yu]

Hi,

Is there anybody knows how to enable OpenGL (or OpenGL ES) as QPainter's
backend? I learned from some document there's an existing OpenGL/GLES
backend in QPainter.

Thanks,
Joe Yu

-----Original Message-----
From: development-bounces+joe.yu=arm.com at qt-project.org
[mailto:development-bounces+joe.yu=arm.com at qt-project.org] On Behalf Of
gunnar.sletta at nokia.com
Sent: 2012年6月26日 PM 03:06
To: markg85 at gmail.com
Cc: development at qt-project.org
Subject: Re: [Development] QtCS: OpenGL session notes

>> - simple imperative GLES2 painter suited for Canvas 2D etc, lose all 
>> the poorly / unnecessary performing features of QPainter
>> - QPainterV2 API, reduced QPaintEngine API
>>  - learn from Skia
>>  - improved API that discourages slow use cases

> 
> This is far outside my knowledge area, but i am very curious to know 
> what kind of things can be learned from skia. I know skia is the 
> render engine for chrome and that's all i know about it.

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. 

I was not in the session, so some other points might have been raised there,
but this topic has been raised before in the oslo graphics team. If we were
to strip down the QPainter API into a useful but bare minimum, then we might
as well go for an API that is well adopted by now, and the natural candidate
for that would be the HTML Canvas2D API. 

Then again, we also know that in terms of hardware accelerated graphics,
imperative APIs are not really the way to go. Typical 2D graphics drawing
consists of many tiny bits and pieces here and there and the overhead of
every draw call in the driver outweighs the benefit of using hardware
accelerated APIs in the first place. This is why QPainter on OpenGL is
better than QPainter on raster in only select usecases. Personally, I am
convinced that scene graphs are the right way to accelerate graphics as it
opens up for reordering and batching which in the end results in optimal use
of the graphics driver and thus also the hardware. It might be better to
spend the effort making convenience APIs for the scene graph rather than
trying to reimplement skia with Qt API.

cheers,
Gunnar




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