lab : lab/uv2
|
Guess the Seams The first problem which comes up when trying to automatically unwrap an object is figuring out which part of the object to cut in order to unwrap it flatly. The edges which are cut when unwrapping the object become seams (Figure 3).
Taking a look at an actual 3D mesh, you can see it may be very difficult to devise a method for automatically finding and cutting seams into a more complicated mesh for nice, easy unwrapping (Figure 4). The contour of the surface, although simple, is irregular, and the topology (the way the edges are connected together) isn't always very pretty.
Now let's take a look at one common solution, sometimes called "Triplanar Mapping". The word "triplanar" refers to the use of three planes in mapping the mesh, each plane intersecting the other plane at a 90 degree angle. The program looks at every polygon in the mesh, and compares the direction it faces to each of the three planes. Each polygon is broken apart and "stuck" to the plane which it faces most. The polygons are then recombined (the three planes are virtually rotated so they sit the same way) and re-arranged in UV space so nothing overlaps, and the resulting new borders are seams (orange lines, Figure 5). This method has several shortcomings. For example, take a look at a single group of polygons which were separated because they faced upwards more than the polygons surrounding them (red arrow, Figure 5). This results in an inconvenient seam being created in the middle of a large, mostly flat section. This would be difficult to paint across in the resulting UV map, since you would have to paint across two seams, while lining the painted details up perfectly (and rather painfully).
A more optimal set of seams would have looked like the image in Figure 6. Notice there are only seams created where distortion would have been inevitable when unwrapping that piece, or where the contour of the shape lended itself to needing to be cut before unwrapping.
|
