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OpenGL Particle Flicker in the distance

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Amitola85    122
I read the FAQ on Z-Fighting, but am unable to alter the Z-near / Z-far planes to adjust the issue (for good reasons). Also, 24-bit double-buffering appears to already be in use. I've landed in this mess of a project. For a brief description: There is map over which a plane flies. The map renders fine. There are also little 'Targets' on the map, which are drawn seperately from the map. They are small, and they move. At a distance they flicker, that is: in one frame they are drawn, in the next they are not (somewhat randomly). I must eliminate this flicker. The project will be used to run simulations of spotting those Targets, and flickering (of course) immediately draws the eye's attention, which would make any results from the simulations useless, since in real life things don't just disappear and reapper at random. I am fairly new to OpenGL, and am learning as much as possible, but I can't seem to find anything to help me out on this problem. Unfortunately we have no OpenGL expert around to help (Academic Research....). Anyone have any advice on eliminating, or at least drastically reducing the flicker that is occurring? Once the Targets get close enough to the camera the flicker effectively stops. As I already said, I've tweaked the Znear and Zfar planes as much as possible, but it's only helped some. Thanks for any help!

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Kalidor    1087
Original post by Glaiel_Gamer
i am new to openGL too, but try mipmapping
If the problem is z-fighting, which it certainly sounds like, mipmapping won't help. That only affects the texture quality. It is a good suggestion to use mipmapping with textures that will be seen from varying distances though, so if the map and targets are textured I would use mipmapping.

If you haven't read it yet, check out Learning to Love Your Z-buffer to get a better understanding of how the depth buffer works and how the near and far planes relate to it.

You may be able to get good results using glPolygonOffset. It needs to be enabled with one of the GL_POLYGON_OFFSET_* contants. See glEnable for more info on those. It is a pain to find good values for it however so I would avoid if it at all possible.

You shouldn't need to use polygon offset though if you have reasonable near and far plane values, so I'm curious why you can't adjust them anymore. Is it because the airplane and targets are too far apart? If that's the case you should be able to scale the world so that they are within a closer range so that you can use better values for the near and far planes.

EDIT: Another possible solution is to render with two separate frustums. The basic idea is you set the near and far planes so that they contain the farther away objects (the map and targets), render them, clear the depth buffer (important!), set the near and far plane values so that they contain the nearer objects (the airplane), then render that.

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Amitola85    122
Thanks for the suggestions. I don't think mipmapping will help any, since no textures are being applied to the Targets.

I'll read through the Z-buffer information you linked to; once I understand it better I may be able to tweak things more. I can't change the values now, since the plane is being shown from a chase perspective (so it's about 1 meter from the camera), flying at an altitude of 50-500 meters, and the camera is pointing at an angle of about 30 (Deg) from horizontal. So you see things out to the horizon.

I think the idea of rendering with two seperate frustrums sounds pretty promising, if I don't figure something better out from the readings.

Once again, thanks for the help. I'll try to remember to post a solution if I find one.

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Amitola85    122
Actually... Thinking about it a little more, I may be able to ignore the fact that we're in a chase view, since I won't need it for the purpose of my simulations (I'll just need to remember to change back the zNear value before someone else uses the code...)

I'll try some more tinkering...

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Amitola85    122
I've moved the near clipping plane out; but it doesn't seem to affect the ficker at all.

Even when the Targets are just about to come inside the clipping plane (ie they are very close to it, but still in front of it) they still flicker. Maybe the problem is something different?

So, umm.. I think the problem may actually just be that at the distance, the Targets only fill 1 or 2 pixels; so sometimes it decides that the pixel will be the Target color, and sometimes it decides the pixel will be the ground color. So there may not be a real solution to my flicker problem... maybe anti-aliasing on the Targets, so they get blurred over a couple of pixels?

[Edited by - Amitola85 on March 15, 2006 5:55:12 PM]

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Guest Anonymous Poster   
Guest Anonymous Poster
I'm going to throw out the term 'Subpixel Accuracy'
because I think it may be relevant, Quake used to to avoid wierd jaggy skippy lines.
I have no idea how it works though, but hey, you never know...

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