In the fragment shader, cubemap textures are accessed using a samplerCube.

3D texture coordinates, [s, t, r]T are used in conjunction with the samplerCube. 

Each texture coordinate is interpreted as a direction vector from the centre of the cube.  The direction vector is normally in world coordinates.

The line defined by the vector and the centre of the cube is extended to intersect one of the cube faces. 

The image that corresponds to that face is then accessed at the location of the intersection.

High