Cube map

A cubemap is six images that are mapped onto a cube. The cube is always centered on the current viewpoint. This means, when used as a backdrop or skybox it always appears infinitely distant.

If you turn on wireframe mode in any 3d engine using a cubemap skybox, you'll see the geometry of the box. Cubemaps are supported in hardware so they're really quick to transform and render, there's a unified method for rendering them in games.

The bitmaps for all six sides need to be square and each of the six need to be the same size. You can store the bitmaps as six separate files, or you can store all six in a single DDS file (compressed or uncompressed). The NVIDIA DDS plugin for Photoshop expects you to lay out the faces of the cubemap horizontally: +X -X +Y -Y +Z -Z. In 3ds Max terms it is:

When used for reflections, the reflection cubemap is always centered on the mesh object. The viewing angle bounces off the mesh surface at an angle opposite to the surface normals, so it points to particular parts of the cube map. These angles determine what parts of the cubemap are rendered for reflection, and where on the surface they are projected.

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