OpenGL Mipmapping: how does OpenGL decide on map level?

Question

Hi,

I am having trouble implementing mipmapping in OpenGL. I am using OpenFrameworks and have modified the ofTexture class to support the creation and rendering of mipmaps.

The following code is the original texture creation code from the class (slightly modified for clarity):

  glEnable(texData.textureTarget);
      glBindTexture(texData.textureTarget, (GLuint)texData.textureID);
      glTexSubImage2D(texData.textureTarget, 0, 0, 0, w, h, texData.glType, texData.pixelType, data);
      glTexParameteri(texData.textureTarget, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
      glTexParameteri(texData.textureTarget, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
  glDisable(texData.textureTarget);

This is my version with mipmap support:

  glEnable(texData.textureTarget);
       glBindTexture(texData.textureTarget, (GLuint)texData.textureID);
       gluBuild2DMipmaps(texData.textureTarget, texData.glTypeInternal, w, h, texData.glType, texData.pixelType, data);
       glTexParameteri(texData.textureTarget, GL_TEXTURE_MAG_FILTER, GL_LINEAR_MIPMAP_LINEAR);
       glTexParameteri(texData.textureTarget, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
  glDisable(texData.textureTarget);

The code does not generate errors (gluBuild2DMipmaps returns '0') and the textures are rendered without problems. However, I do not see any difference.

The scene I render consists of "flat, square tiles" at z=0. It's basically a 2D scene. I zoom in and out by using "glScale()" before drawing the tiles. When I zoom out, the pixels of the tile textures start to "dance", indicating (as far as I can tell) unfiltered texture look-up. See: http://www.youtube.com/watch?v=b_As2Np3m8A at 25s.

My question is: since I do not move the camera position, but only use scaling of the whole scene, does this mean OpenGL can not decide on the appropriate mipmap level and uses the full texture size (level 0)?

Paul

Answer 1

You should probably switch to automatic mipmap generation if you're targeting OpenGL >= 1.4.

You could try changing GL_TEXTURE_MIN/MAX_LOD to force a particular mip level.

Answer 2

Mipmapping will compensate for scene scale in addition to perspective distance. The vertex shader outputs (which the driver will still create even if you aren't using your own shader) specify the screenspace coordinates of each vertex and the texture coordinates of those vertices. The GPU will decide which mip level to use based on the texel-to-pixel ratio of the fragments that will be generated.

Are you setting GL_LINEAR_MIPMAP_LINEAR when you render your tiles as well? It only matters when you render things, not when you create/load the texture. Your bracketing glEnable/glDisable calls may need to be moved too, depending on what state you are actually passing in there.