[Development] OpenGL in Qt 5.1 and onwards

[Sletta Gunnar]

>>> 2) An QOpenGLVertexArrayObject class. This would be just a thin wrapper
>>> around a VAO with possibly some API to make it easier to use with
>>> QOpenGLBuffer and QOpenGLShaderProgram.
>>> 
>>> This could potentially find use in QtQuick 2 so putting it in QtGui
>>> alongside the other QOpenGL* classes makes sense. I think the lack of
>>> VAO's in qtdeclarative is what makes it necessary to still use a
>>> compatibility profile with QtQuick 2. This limits the GL features we can
>>> use in conjunction with QQ2 on Mac as Apple only implement the Core
>>> Profile for GL 3.x. That is another story though.
>> 
>> Could you elaborate on this problem? I'm not familiar with it?
> 
> When I was preparing my devdays talk on rendering an OpenGL underlay with QQ2 
> UI over the top I managed to get it working with a standard OpenGL 2.x context 
> no problem.
> 
> When I wanted to do something more fancy with instanced rendering I needed an 
> OpenGL 3.3 context. Trying this out I found that it works fine with a 
> Compatibility profile context. However, using a Core profile context resulted 
> in no QQ2 elements being drawn.
> 
> I suspect it's due to QQ2 using indexed draw calls but not using vertex array 
> objects which are compulsory in the Core profile since the index buffer is 
> contained in a VAO (whereas the VBOs are just referenced).
> 
> I need to find time to do a quick test to see if this is actually the case. 
> The test could just be creating a single VAO and binding it up front when the 
> SG context is initialized. That way all index buffer changes affect this 
> default VAO.

Thanks for explaining. No need to test, I can confirm that we use index draw calls and no VAO in the default renderer (and also in the overlaprenderer in playground/scenegraph). We'll fix that once the enablers are in.

cheers,
Gunnar