Transparent Texture - Artifacts

OpenGL OpenGL: Basic Coding
1

Hi,
I’m trying to produce a transparent world … land masses solid and seas semi-transparent. I want to be able to see the reverse land masses through the sphere so I disable GL_CULL_FACE.

Now, it works … but it generates transparent artifacts that are not related to lighting but are related to the view angle of the object.

I’m using the texture to control the alpha.

glDisable(GL_CULL_FACE);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE);
glPushMatrix;
glColor4f(1.0, 0.0, 0.0, 1.0);
glBindTexture(GL_TEXTURE_2D, LogoWorld]);
glCallList(3DWorld]);
…
…

Any idea of what I’m doing wrong and why I’m getting these artifacts ? I’ve reproduced the same problem in Deep Exploration (Right Hemisphere).

Thanks Andrew

2

This is “z-fighting” : 2 pieces of geometry fighting for the same Z distance.

Solutions :

  • increase zbuffer precison : go from 16 to 24 bits, bring near projection plane farther (never at 0 !), bring far projection plane nearer
  • make your geometry better, ie specify the exact same coordinates for both land masses and seas, so depending on your depth test, the last one will show.
  • in your case it seems that both “spheres” were not tesselated the same way, maybe do something about it ?
3

Yes … that’s what I thought at first and you’re correct, the shape of the artifacts do relate to the actual mesh.

In the end, I found that rendering it in two passes fixed the problem completely … first pass with glCullFace(GL_FRONT) and then glCullFace(GL_BACK).

Not actually certain as to whether this might possibly be a limitation of my graphics card / driver (NVidia GForce 2) - especially as I can reproduce the same problem in a commercial 3D explorer application. I am fairly certain that it’s not z-fighting as the problem does not appear to be effected by the geometrical size of the sphere nor z-buffer depth etc.

Thanks

Andrew