Using a Sphere Map

 

OpenGL provides a mechanism to generate s and t texture coordinates at vertices based on the current normal and the direction to the eyepoint. The generated coordinates are then used to index a sphere map image which has been bound as a texture.

We denote the vector from the eye point to the vertex as u, normalized to u'. Since the computation is performed in eye coordinates, the eye is located at the origin and u is equal to the location of the vertex. The current normal n is transformed to eye coordinates, becoming n'. The reflected vector r can be computed as:

We define:

Then the texture coordinates are calculated as:

This computation happens internally to OpenGL in the texture coordinate generation step.

To use sphere mapping in OpenGL, the following steps are performed:

• Bind the texture containing the sphere map
• Set sphere mapping texture coordinate generation(glTexGenGL_S, GL_TEXTURE_GEN_MODE, GL_SPHERE_MAP(GL_S, GL_TEXTURE_GEN_MODE, GL_SPHERE_MAP)) and glTexGenGL_T, GL_TEXTURE_GEN_MODE, GL_SPHERE_MAP(GL_T, GL_TEXTURE_GEN_MODE, GL_SPHERE_MAP))
• Enable texture coordinate generation (glEnable_GEN_S(TEXTURE_GEN_S) and glEnable_GEN_T(TEXTURE_GEN_T))
• Draw the object, providing correct normals on a per-face or per-vertex basis