Texture coordinates lock a to geometry. Environment coordinates, on the other hand, lock a map to the world. If you move the object, the map remains in place. If you move the point of view, the map changes. This type of mapping system is used with reflection, refraction, and environment maps.
Above: Image uses a picture in screen coordinates as a background.
Below: Image shows spherical mapping coordinates applied using a checker map.
The first three are the same as those used by the . If you imagine a sphere, infinite in size, surrounding your scene and mapped with spherical , you can visualize the effect you get with spherical environment mapping. Shrink-wrap wraps the map around a giant sphere, leaving only one singularity. Cylindrical is like a giant cylinder.
The Screen system maps the image directly to the view, with no distortion. It's similar to planar, in that it's like a giant backdrop hung in the scene. Unlike the other environment mapping methods, Screen is locked to the view. When you move the camera, the map moves with it. Therefore, you can only use it for still renderings, or animations in which the camera doesn't move.
An environment map is not assigned in the , because it's not applied to the geometry of an object, but rather to the scene itself. Environment maps appear in the background, as seen from the camera or perspective view.
When you assign a map to the environment, it's the same as if you'd assigned a mapped material to an object in your scene. To edit or adjust the assigned map, you need to place it in one of the sample slots in the Material Editor. You can do that in one of two ways: