MipMapping problems in OpenGL

Question

I'm loading 3D objects (obj or 3ds or collada files) into my openGL application. The 3 environment is quite large (a few hundred metres in all axis'). My problem is that smaller 3D objects (i.e. in the order of ~< 1-2m ) don't appear to be depth-tested properly. Depending on the zoom of the camera, I can sometimes see the back face of the object (I have been using a simple cube for testing) or other faces becoming visible/invisible/torn. Please see the attached images for a better explanation. I am led to believe the problem is due to mipmapping being enabled. I would either like to disable mipmapping (can someone suggest a simple, fast way to do this) or set the resolution to be greater for the mipmapped objects. Or am I barking up the wrong tree completely?

Thanks

Chris

Answer 1

That's the result of insufficient z-buffer precission, which is an issue in games that have huge worlds but (relatively) small objects. The immediate solution would be to try using a 24 bit z-buffer instead of a 16 bit one. Another way to tackle this would be to render the game world it two steps, first the big distant objects, then clearing the zbuffer and then drawing the closer objects.

This specific problem is called z-fighting by the way, here's a great resource on this issue: http://www.codermind.com/articles/Depth-buffer-tutorial.html

The take-away is the last paragraph of the article above:

the true issue is that you can't draw both objects that are very far and objects that are very near with the same depth buffer equations. If you want to draw very far objects then you need to sacrifice your near view by pushing it further. To avoid clipping artifacts you can make your collision envelope large enough so that your clip plane will never intercept an existing object within your frustum. Or you can make object gradually disappear with transparency as they come near your clip plane.

If you want to keep near objects and at the same time draw mountains (or planets) in the far distance, then you can cut your rendering in parts. First drawing your far objects, then clearing the depth buffer and rendering the near objects with a different z buffer.

Answer 2

Like Julio, I believe that this is a depth precision issues, not something related to mip-mapping. However, I suggest you start by adjusting your near and far clipping plane before changing anything else (You are probably already using a 24-bit depth buffer anyways, as that is the default on most drivers/cards). Particularly the near plane should be as far away as possible for your scene. Look for calls to glFrustum or gluPerspective.