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zasdarq

OpenGL Display slowdowns without framerate slowdown [Solved?]

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Hello everyone,

I've had a bizarre problem plaguing me. My project uses SDL with OpenGL. I'm relatively new with OpenGL so the more detailed inner workings are still a little fuzzy so I apologize in advance for any of my ignorance.

While the game is running, I get severe slowdowns for what appears to be just the display. The mouse icon will stick in place, or moving objects will stop moving, but the code is chugging along just fine. The mouse position / object positions are being correctly updated (checked via print lines) without any slowdown, and they are apparently going through their rendering code without any errors, yet they don't update in the display itself.

Even more strangely, this problem can be "fixed" by either no longer displaying one or two textures (randomly chosen), or by arbitrarily rendering a couple additional plain triangles. Also, not clearing the Depth / Color / Stencil buffer every frame will solve the problem (It's a 2D project and I'm not relying on the depth buffer to order things)The problem occurs only on this computer. The project runs fine on my laptop and another desktop... both of which are much older than this one.I'm using SDL 1.2.x with OpenGL 3.x (though using only pre 3.0 OpenGL), Windows 7 and an ATI HD4800.

The erratic nature of the problem leads me to believe that it's sloppy code leading to undefined behaviour (bad data, overwritten memory, etc.) but I thought throwing it out to the forums might still be helpful. Given the size of the project it's hard for me to supply useful code examples, but I'll do my best to supply any additional helpful information on demand. So I apologize all around for the vagueness of this post.

Thanks for reading.

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Small update: The problem also does not occur in fullscreen mode... only in windowed. I've been told OpenGL can be so unreliable in windowed mode you shouldn't even bother?

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[quote name='zasdarq' timestamp='1310522833' post='4834595']
Small update: The problem also does not occur in fullscreen mode... only in windowed. I've been told OpenGL can be so unreliable in windowed mode you shouldn't even bother?
[/quote]
I never encounter any unreliable behavior of open gl in windowed mode. I would say that you have some unhandled OpenGL error. Unhandled errors must not result in a crash, but it could result in screen artifacts or screen freezes (this is what seems to happen here). Behavior of such error depends on the driver, so running it on an other PC or laptop could produce other or none artifacts.

I would sugguest to add several check error sniplets at your code to detect the error and track it down (-> glGetError). But remember to remove the get error checks later, atleast deactivate them (#ifdef ...) to prevent slowdowns.

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Are you, by chance, changing window title rapidly? Like every frame? I remember that slowed down window updates significantly for me some time ago (can't tell the exact SDL version alas), and only on Windows.

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I had no OpenGL errors and was not changing the title quickly. However I do have updates to this problem. Maybe someone can explain...

Further testing made it seem clear to me that it was Windows 7 fault there was a slowdown. As I mentioned before, fullscreen mode functioned fine. Also, turning off all the fancy effects in Windows 7 by switching into Windows Classic mode also fixed the problem. And I found this article: [url="http://www.opengl.org/pipeline/article/vol003_7/"]http://www.opengl.org/pipeline/article/vol003_7/ [/url] . Apparently Windows 7 and OpenGL don't get along very well?

Now on to my "solution." After reading that article, I added glFinish() right before SDL_GL_SwapBuffers(), and the problem went away. From some quick searches I've gained little information about if that's a good idea or not. It seems many people say you should never do that. Others say it fixed problems for them. And still others say that it shouldn't do anything at all because SwapBuffers should be doing it automatically. Perhaps there's one out there, but I could find no definite conclusion on the issue. All I know is for me that it fixed my problem and it did not cause any additional slowdowns (as some people have complained about).

Any enlightenment on either the complications with Windows 7 or the proper use of glFinish would be appreciated.

Thank you.

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