[Qt-interest] QGL2PaintEngineEx vc QGLContext

[Samuel Rødal]

John Clayton wrote:
>> John Clayton wrote:
>>> Hi All
>>> From testing - I notice that drawing a texture to my GL surface (on 
>>> a  Mac, using 4.6.0, in a QGraphicsItem paint() method) can be done 
>>> in  two different ways.
>>> Method 1 - use QPainter->drawImage(QRectF, QImage)
>>> or
>>> Method 2 - use QGLWidget->drawTexture(QRectF, int, int)
>>> Method 1 works perfectly in my case, and looking at its code it is  
>>> using the new Qt4.6 opengl code.
>>> Method 2 fails to draw anything (it used to work in 4.5.3), and the  
>>> code to draw looks *totally* different compared with Method 1.
>>> So, how can I use Method 1 to draw - given that i have a texture 
>>> ID.   My requirement is to draw textures into a QGraphicsItem, and 
>>> QGLWidget-  >drawTexture() appears to fit the bill - but it doesn't 
>>> draw anything  (unless I call beginNativePainting first - but that 
>>> wipes out the  clipping path).
>>> Is there a way I can use the QPainter based method to draw textures?
>>> Thanks
>>> -- 
>>> John Clayton
>>> Skype: johncclayton
>>
>> For the modelview and projection matrices to be set up as you would 
>> expect you need to call beginNativePainting() in 4.6. How the GL paint 
>> engine handles clipping is an internal detail, it might use the depth 
>> buffer, stencil buffer, or a different approach, so we don't want to 
>> expose that through beginNativePainting(). Instead you should manually 
>> make sure to paint within the clip path, or ideally simplify your 
>> graphics item so it only uses a rectangle clip (path clips are more 
>> expensive in any case).
>>
> 
> How can I manually ensure that? Isn't the whole point of the clip path 
> the fact that I *dont* need to manually ensure this kind of thing while 
> drawing?  I can draw, it gets clipped - that's [I hope] what the API 
> guarantee was/is supposed to be.  But; do tell me if I read the docs 
> wrong or am assuming to much here.

Yes, but using QGLWidget::drawTexture() is on the same level as issuing 
raw OpenGL calls directly, which bypasses the painter completely. Thus 
you can't assume that whatever OpenGL drawing you do is clipped by 
QPainter's clipping state.

>> There _is_ a drawTexture call in QGL2PaintEngineEx (which you can get 
>> by static_cast<QGL2PaintEngineEx *>(painter->paintEngine()) if the 
>> paint engine's type function returns QPaintEngine::OpenGL2, but it's 
>> internal private API, so you shouldn't use it unless you're prepared 
>> that it might break in any future release. For now 
>> QGLWidget::drawTexture() is the only supported way to go, but it 
>> doesn't regard the current painter's state as you've noticed.
>>
> 
> I'm missing something - what do you mean the drawTexture( ) disgregards 
> the painter state - isn't the painter state simply a copy of the GL 
> state?  Hang on; are you saying that the methods used to draw a texture 
> via QGLWidget bypass the painter state used to hold the clipping 
> region?  Putting two & two together from what I've read of the code so 
> far - the clipping stuff looks like it is implemented in 4.6 as 
> shader/vertex programs - and I'm not sure how that interacts in the 
> render pipeline is one just renders a texture ala 
> QGLWidget::drawTexture() - so is that why if I render with QGLWidget the 
> entire texture surface is rendered, whereas the QPainter based routines 
> are combining rendering of a texture with the already constructed 
> shader/vertex code and thus honour clipping?

The painter state is the GL state in addition to some state variables in 
the GL 2 paint engine. The issue is that drawTexture() is independent 
from QPainter, it's original purpose was to simplifying drawing textures 
when using raw OpenGL commands in an overloaded QGLWidget::paintGL().
How clipping is done for drawImage() etc is an implementation detail and 
we don't want to expose that, as that would prevent us from possible 
optimizations in the future.

> I did some more digging, and now I have a concrete error to work with.  
> During the QGLContext::drawTexture() call - the glBindTexture(

Your message seems to have been cut off at this point.

--
Samuel