QGLWidget 2d painting performance

[amnesiac]

Hmm, QWidgets drawing method has to transfer the image data to the graphic card as well but achieves a constant (quite fast) speed. QGLWidget however takes significant more time for the first drawPixmap command which looks pretty much like a caching operation. Maybe their algorithm isn't the fastest one (bindTexture() seems to me very slow as well).

So, is there no option to influence Qt caching behavior? Once there was a method called setOptimization in QPixmap but this one disappeared in recent versions of Qt.

Regards,
André

[wysota]

Hmm, QWidgets drawing method has to transfer the image data to the graphic card as well but achieves a constant (quite fast) speed.

But it doesn't convert it to textures first.

So, is there no option to influence Qt caching behavior? Once there was a method called setOptimization in QPixmap but this one disappeared in recent versions of Qt.

I don't know. Does the chip demo using GLWidget as a viewport work slow as well? I know I asked it before but I didn't receive a clear answer.

[amnesiac]

But it doesn't convert it to textures first.

Ok, so drawPixmap in GL context has to make a texture conversion before painting and caching it?

I don't know. Does the chip demo using GLWidget as a viewport work slow as well? I know I asked it before but I didn't receive a clear answer.

Sorry, what chip demo do you mean? I didn't find it amongst the Qt examples.

Regards,
André

[wysota]

Ok, so drawPixmap in GL context has to make a texture conversion before painting and caching it?

Yes, that I'm almost sure of.

Sorry, what chip demo do you mean? I didn't find it amongst the Qt examples.

Well... it's in the "demos" directory and it's called "chip"

[amnesiac]

Yes, that I'm almost sure of.

Unfortunately, in my case the first drawPixmap has to be very fast and the image stays at the same position so that caching has no effect anyway. Hmm, OpenGL animation and 2D view in one class would have been an elegant way to share the same resources.

Now I think I have to place the transitions in an extra GLWidget and switch to a normal QWidget after the animation has finished.

Well... it's in the "demos" directory and it's called "chip"

Ok, I looked only in the OpenGL examples
The OpenGL version is definitely faster. But I think there's no problem with initial drawing / caching.

Regards,
André

[wysota]

Try modifying the demo so that it draws some pixmap on each chip.

[amnesiac]

Try modifying the demo so that it draws some pixmap on each chip.

I will try that later on.

Another interesting detail is that a drawPixmap() takes half of the time than a bindTexture() does, which could either mean that drawPixmap does not make a texture conversion or bindTexture() has lots of overhead... or both

[wysota]

You can take a look at sources of the gl paint engine and check it out yourself.

[amnesiac]

You can take a look at sources of the gl paint engine and check it out yourself.

Well, I had a closer look at drawPixmap in class QOpenGLPaintEngine and indeed, they convert a QPixmap into a texture using bindTexture. But under certain circumstances they use the GL_TEXTURE_RECTANGLE_NV instead of GL_TEXTURE_2D option for binding, which seems to make the performance difference. Unfortunately in my code, this does not work (can't bind the texture). Has anyone experience on this?

Still I'd like to avoid this caching behavior, is there any way to avoid this and do a "normal" drawPixmap as QWidget does?

While playing around with QGLWidget I experienced a possible Qt bug. I'm using a QStackedWidget and 3 Widgets( 2 different views and a transition GLWidget). When I switch from the GLWidget to the new view, the former view is shown for one frame. But no paintevent is called in the old view. Looks like there's some old data in the video memory of the graphic card or something like that. Anyone experienced the same or knows how the get around this?

Regards,
André