Creating and Editing Texture Coordinates (UVs) - An Introduction to Computer Graphics for Artists - page 148

Fig. 10.4 The spherical projection shown on left projects inward toward the object to be mapped.
A planar projection projects in the direction of the face normal. A planar projection used on a
sphere will be distorted
through it, take with them the color of the fi lm at the point of intersection. The
colors follow the ray until it hits an opaque object, the screen, and bounce off.
Projecting texture coordinates requires a projection matrix type; planar, cubic,
cylindrical, spherical, or any other options your application may have (Fig. 10.4 ).
There are a variety of projection shapes because models come in a variety of shapes
as well. If the projection rays are parallel, as in the movie projector example, they
will distort if they hit anything other than a perpendicular fl at surface. If a movie-
goer walks into the path of the light, the image will be projected onto their body, but
it will be distorted. The different projection matrices project coordinates through
different shapes that approximate the shape of the objects the coordinates are
projected onto. The way they do this is to project rays along the normals of the
projection matrix shape until they contact the surface of your model.
To achieve the best projection, your projection matrix should be the right size,
shape, and orientation to match your model. Any discrepancy will cause distortion.
Distortion can be fi xed in editing, but it is easier if the projection is as good as pos-
sible before editing begins. Most programs can do a good job of automatically
aligning the projection matrix with the object it is mapping, but you will occasion-
ally need to modify the orientation, position, or scale manually.
Once the projection matrix is aligned and the project function is invoked,
your object, or the part of it you have selected, will have new UVs. These UVs will
probably, but not always, require further editing. This is the primary stumbling
block for less experienced artists. They want a projection type that results in perfect
UVs, thus sparing themselves any painstaking UV editing. Get used to the idea that
a single projection or even multiple unedited projections will not be enough. Expect