Posted 11 May 2012 - 03:14 AM
Perhaps the most popular of all UV mapping techniques is planar mapping. It's simple to understand and compute. You simply calculate the bounding box of the model, iterate through each vertex, subtract the minimum bounding box value from the vertex and divide by the (max - min) bounding range. This will assign each vertex a UV within the normalized UVW space. All that remains is deciding which plane you wish to map to. XY, XZ, or YZ. If you want to split the planar space into two, one for the front side and another for the back side of the model, you can use the vertex normal to determine if the vertex is front-facing or back-facing and adjust the UV coordinates accordingly. You could for isntance pack front-facing polygons on the fist half of the X-axis and pack the back-facing polygons on the next half of the X-axis. You may have some stretching or resolution issues along the silhouette of the model, but you can correct for this by treating these polygons as special and giving them additional or dedicated UV space on the map. A similar technique is used for cube mapping, except you analyze all 6 planes +-X, +-Y, and +-Z. You use the vertex normals to decide which cube face the polygon should be mapped to and then make sure each cube face has its own island in the UV space. This is only good for models where each vertex can map to a unique UV coordinate. Cube mapping would not work with a torus, for example.
An extension to the planar mapping algorithm is to give each polygon it's own dedicated, uniformly assigned coordinates. You end up with a UV map that resembles a brick house, but it's great for dealing with very complicated models where you can't be bothered to micro edit UV coordinates from the more "smart" algorithms. It's good in that it maximizes the UV space. It's also great for lightmapping.
Another algorithm is the least squares conformal maps. I heard good things about this and it's used in modeling packages like Blender. If you google on it you can find some more info, or someone else here might shed some light on it.
One thing to keep in mind with UV mapping is that you need to account for texture bleeding. Linear interpolation in graphics hardware will end up sampling neighbouring texels, so make sure to add padding in-between your UV islands to account for that.