QGLWidget 2d painting performance

[amnesiac]

Hi,

I'm doing some experiments on using OpenGL with Qt. My app is a small image viewer, that allows for browsing images by pressing the cursor keys. For the transition from the overview to the single image view I'd like to realize a zooming animation in OpenGL.

According to Trolltechs "Overpainting" example you can mix QPainter commands and OpenGL commands in one class (using paintEvent instead of paintGL). So I started to replace QWidget by QGLWidget, nothing more. Unfortunately the application reacts much slower (same paint method, no OpenGL commands yet). I took a bunch of images and pressed a cursor until I reached the end. The GLWidget took about 30 % more time. So my question is, if there is some kind of overhead that prevents QGLWidget from being as fast as a normal QWidget? I really like to mix OpenGL and normal drawing in one widget!

By the way, this behavior is the same on several machines. And having a look at Trolltech's Overpainting example where they compare a QGLWidget and an QWidget that use the same paint method code, I have the impression that the QWidget variant runs smoother that that GL one.

Regards,
André

[wysota]

Do you have hardware accelerated OpenGL implementation installed? Overpainting is in general slower than plain OGL because it uses overlays as far as I understand it.

[amnesiac]

I'm pretty sure I'm using the hardware accelerated version. The pro file contains the OpenGL flag and when I'm painting using OpenGL I get incredible frame rates.

But in my test case I'm not using any OpenGL commands or overpainting at all, I simply replaced QWidget by QGLWidget in my header file. The paintevent method keeps being the same. Doing so it seems my event loop is much slower (I get less keystroke events per time), like there is some overhead by the QGLWidget.

Regards,
André

[wysota]

Does it happen in every case? Try the chip demo and see if GL based viewport is slower than the regular one.

[ToddAtWSU]

Could the slowdown be occurring because the QGLWidget is making calls to functions such as paintGL( ) and even though there is no real code in there, it is making extra function calls, causing the slowdown you are seeing?

[amnesiac]

This behavior can be reproduced (at least on my Mac, don't have any windows PC here at the moment).

Finally, I did some time measurements within my paint method. And it's definitely drawPixmap that causes the slowdown. What's happening is that QGLWidget tries to cache the image on the first drawPixmap. Of course this takes long but all subsequent paintings are very fast. QWidget does not cache, so all paintings constantly take the same time.

In my test case this means (for a large image), caching time is about 50 milliseconds and after its cached no time can be measured for painting. QWidget takes about 1-2 milliseconds for a drawPixmap command. So in my case doing not that much repaints of the same image this options works a lot better for me.

My question, is there any way of disabling caching of QGLWidget so that it can also be used like a normal QWidget for 2D Painting?

Regards,
André

[wysota]

I think the texture has to be sent into the graphics card before it is rendered thus your choices here might be limited. I'm just guessing though...

[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