opengl - texturing a glutSolidSphere

Question

I need to add an earth texture to a glutSolidSphere. The problem is that I cannot figure out how to make the texture stretch over the entire sphere and still be able to rotate.

We're enabling the textures with.

glTexGeni(GL_S, GL_TEXTURE_GEN_MODE,GL_OBJECT_LINEAR);
glTexGeni(GL_T, GL_TEXTURE_GEN_MODE,GL_OBJECT_LINEAR);

glEnable(GL_TEXTURE_2D);

glEnable(GL_TEXTURE_GEN_S);
glEnable(GL_TEXTURE_GEN_T);

//drawcode...

using GL_SPHERE_MAP in param instead of GL_OBJECT_LINEAR makes the textures look proper, but they cannot rotate.

The parameters I use for the texture are

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

I understand that GL_REPEAT tiles the texture, while using GL_CLAMP instead gives me the texture once on the object, but I cannot get it to stretch over the whole sphere.

Does anyone know how to properly texture a glutSolidSphere?

Answer 1

glutSolidSphere doesn't provide proper texture coordinates, and OpenGL built in texture generation allows only for linear mappings from vertex position to vertex texture coordinate, which essentially means that you can not use them to texture a 3-sphere with a 2-flat, bounded texture (for a mathematical explanation look up the topics of topology of manifolds and map theory).

So what can you do? There are a number of possible solutions:

• Don't use glutSolidSphere, but some other geometry generator that does provide proper texture coordinates (though texturing a sphere with just a single bounded 2D texture is a difficult topic, there are several mappings, each with their problems)

• Use a texture with the same topology as a sphere, a cube map, then you can use the GL_NORMAL_MAP for the texture gen mode, i.e.

  glTexGeni(GL_S, GL_TEXTURE_GEN_MODE, GL_NORMAL_MAP);
  glTexGeni(GL_T, GL_TEXTURE_GEN_MODE, GL_NORMAL_MAP);
  glTexGeni(GL_R, GL_TEXTURE_GEN_MODE, GL_NORMAL_MAP);
  

  Look up tutorials about cube mapping. But in a essence a cube map consists of 6 texture faces, arranged in a cube about the origin and texture coordinates are not points on the cube itself, but a direction from the origin and the addressed texel is the one, where the direction ray intersects with the cube.

• Use a vertex shader, generating texture coordinates from vertex positions. Since a vertex shader is freely programmable, the mapping isn't required to be linear. Of course will run into the peculiarities of mapping a 3-sphere with a bounded 2-flat again.