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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.
Materials apply the pixel data of each VTF (base map, normal map, specular mask) to same UV area of the model. Therefor the model's material can easily be changed without having to create additional UV maps.
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?)