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A UV map is a part of an object's geometry. It defines which areas (using UV coordinates) of a 2D pixelmap should be rendered on which polygonal surfaces (using XYZ coordinates) of a particular 3D mesh. A UV map does not contain any pixel data (eg color, etc). It is only a matrix relating 2D to 3D coordinates.
In Source, the object's UV map is stored in a VMT file and reads the pixel data from the same UV area of each VTF in the texture stack. Therefore one UV map imports data from the colormap, bumpmap, opacitymap, specularmap, etc as though they were one image (with enormous bitdepth). Thus the object's Material can easily be changed without having to rebuild the UV map.
UV Mapping, or Texture Mapping, is the process of aligning Polygons to Pixels.
- To UV map Models use XSI Texture Editor (or equivalent).
- To UV map Brushes use Hammer Face Edit Dialog (and the Paint Alpha Dialog for blending Materials on Displacements).
- To UV map Decals use Hammer Decal Tool.
- To UV map Overlays use Hammer Overlay Tool.
- Valve Material Type files can also specify UV mapping adjustments for each VTF seperately, via the $basetexturetransform, $bumptransform, etc VMT parameters. (To do: confirm: this only works for VMTs applied to Brushes, not to Models?)