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JoshG

Screen Artifacts

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OK People, I figured i would edit the first post with all the updated information that I have now.. I am writing a GL app. When running the app in fullscreen mode I get an artifact appearing at the top right of the screen, it appears to be some black line, that is randomly broken into pieces. After some playing with the code, I have found that the colour of this phantom line is actually the colour specified by glClearColor. and can usually be observed in 1024x768, 1280x1024, or even the 512 rez I have found a way to get rid of this line.. which involves pressing Alt-Tab, several times untill the window cycles back to itself (ie without actually changing windows at all). This removes the line for the remainder of the program's life. I tried changing my app so that it doesn't render anything except a single white polygon in the problem area, and this is enough to bring out the artifact. I have tried my app on 4 machines, one of them seems to work... but it uses a widescreen (laptop). 2 of those machines have ATI Radeon X800's in them, and the other is some non-3d intel thing. I've looked at NEHE's tutes and can't find ANY substantial difference (besides structure) and even found that by making lesson 10 run in fullscreen 1280x1024 it also produces the same artifact i am seeing. Some reason lesson 16 doesn't show the artifact though. I've tried some SetFocus, SetForegroundWindow etc calls in various places, but ultimatly i cant get rid of this annoying artifact! Please help! Any ideas what this could be from? thanks --Josh [Edited by - JoshG on March 15, 2006 12:45:03 AM]

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I just tried a few things and found that if whilst running my app i do this:

Alt-Tab, then keep going through till i come back to my app
then the artifact dissapears....

So it would seem that my window isn't completely in charge of the screen or something..

Any ideas?

I've tried putting SetForegroundWindow into as many places as i can think
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Update:
I've just found that this black line is actually the glClearColor() colour, so if I change that then i get get it to be red/blue etc..

Still no ideas?

--Josh

[Edited by - JoshG on March 14, 2006 10:40:57 PM]

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On, I stand corrected about the NEHE app, it doesn't run fine,
If i change the resolution that it runs at to 1280x1024 then it also displays the same symptoms...

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Yep, updated drivers a few hours ago... no change....

Another thing i've noticed though.. Sometimes my app will goto fullscreen and the start bar will still be visible at the bottom, other times it'll have the startbar invisible,

When the startbar is there, the artifact doesn't appear...

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Hmmm, others seem to have had your problem, but I really can't comprehend what exactly is happenning--could you post a screenshot of each anomaly you are having?

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Well, am trying to get a screenshot of the issue...
Tried running Fraps.. it says the framerate is 999 fps, and when i try to record this drops to 19fps and doesn't seem to have the artifact...

This could indicate tearing.. or the framerate being too high... but I am using double buffering! this shouldn't happen should it???

will keep trying to get a video of the behaviour

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Still no luck getting a video of the behaviour, or a screenshot... maybe i can take a digi photo on a camera or something..

I can still remove the issue using that Alt-Tab trick, and once that is gone Fraps also still says that it's running at 999 fps

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Ok, some success in documenting my issues, on this site you can see the probs i am having:

http://www.ug.it.usyd.edu.au/~jgre5893/index.html

Notice that the ClearColor is red and I have drawn a white quad in the top right of the screen. Sorry about the quicktime format, 's all my camera likes doing. Damn olympus :S
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Also updated the site to have a movie showing how the Alt-Tab trick seems to work, (For those of you that must think i'm crazy)
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Also uploaded a copy of the modified project of Lesson 10 of NEHE that
brings out the artifact.. NOTE: I have only changed line 547, where the window resolution is specified, to 1280x1024


--Josh

[Edited by - JoshG on March 16, 2006 3:16:34 AM]

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Allthough I cant atm acces the files on the site, I might be able to help a bit...

I have had a problem with white lines as well ages ago with a older card, trying to render the bump effect from nehe. It was the z-buffer...

Now glClearColor might be called more then 1 time between buffering? Either way, there really seems to be a z-fight / switch going on....

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Well, as for the glClear in my code, i did a search on all my files and put breakpoints on all the glClear points in the code, the only one that was being run during the application was the one in the loop.

Also, looking at NeHe's code, (lesson10) there is only one glClear call in the entire code...

Soo then the z-buffer.... when you say z-buffer do you mean the depth buffer? cos I've tried playing a little bit with the depth clear value...I just tried to switch off the depth testing in NeHe's tute and that didn't do anything to the artifact (only screwed up what was being drawn)

--Josh

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