OpenGL.GL.ARB.texture_cube_map

OpenGL extension ARB.texture_cube_map

This module customises the behaviour of the OpenGL.raw.GL.ARB.texture_cube_map to provide a more Python-friendly API

Overview (from the spec)

This extension provides a new texture generation scheme for cube map textures. Instead of the current texture providing a 1D, 2D, or 3D lookup into a 1D, 2D, or 3D texture image, the texture is a set of six 2D images representing the faces of a cube. The (s,t,r) texture coordinates are treated as a direction vector emanating from the center of a cube. At texture generation time, the interpolated per-fragment (s,t,r) selects one cube face 2D image based on the largest magnitude coordinate (the major axis). A new 2D (s,t) is calculated by dividing the two other coordinates (the minor axes values) by the major axis value. Then the new (s,t) is used to lookup into the selected 2D texture image face of the cube map.

Unlike a standard 1D, 2D, or 3D texture that have just one target, a cube map texture has six targets, one for each of its six 2D texture image cube faces. All these targets must be consistent, complete, and have equal width and height (ie, square dimensions).

This extension also provides two new texture coordinate generation modes for use in conjunction with cube map texturing. The reflection map mode generates texture coordinates (s,t,r) matching the vertex's eye-space reflection vector. The reflection map mode is useful for environment mapping without the singularity inherent in sphere mapping. The normal map mode generates texture coordinates (s,t,r) matching the vertex's transformed eye-space normal. The normal map mode is useful for sophisticated cube map texturing-based diffuse lighting models.

The intent of the new texgen functionality is that an application using cube map texturing can use the new texgen modes to automatically generate the reflection or normal vectors used to look up into the cube map texture.

An application note: When using cube mapping with dynamic cube maps (meaning the cube map texture is re-rendered every frame), by keeping the cube map's orientation pointing at the eye position, the texgen-computed reflection or normal vector texture coordinates can be always properly oriented for the cube map. However if the cube map is static (meaning that when view changes, the cube map texture is not updated), the texture matrix must be used to rotate the texgen-computed reflection or normal vector texture coordinates to match the orientation of the cube map. The rotation can be computed based on two vectors: 1) the direction vector from the cube map center to the eye position (both in world coordinates), and 2) the cube map orientation in world coordinates. The axis of rotation is the cross product of these two vectors; the angle of rotation is the arcsin of the dot product of these two vectors.

The official definition of this extension is available here: http://www.opengl.org/registry/specs/ARB/texture_cube_map.txt