Texture in triangle and quad
pri...@gmail.com
Bruce Sherwood
texpos: If you intend to apply a texture or bumpmap to a mesh object, you must specify in the mesh how you want the texture to be mapped onto your object. You might want the texture to occupy only a portion of the object, or be stretched or distorted in some way. This is done by specifying "texel" coordinates, given as texpos vectors in the vertex objects.
Texel coordinates for a surface texture are specified by an "x" and a "y", both ranging from 0 to 1 if the entire texture is to be displayed, left to right and bottom to top. (In many discussions texel coordinates are referred to as "u" and "v", or as "s" and "t".) In anticipation that WebGL will eventually enable 3D textures, which permit filling a textured volume, VPython requires that in texpos elements the third component must be zero.
For example, to fill a quad with a texture, the texpos vectors in the vertex objects v0, v1, v2, and v3 would be vec(0,0,0), vec(1,0,0), vec(1,1,0), vec(0,1,0), no matter what the 3D pos attributes are. If you want only the lower quarter of the texture to fill the quad, the texpos vectors would be vec(0,0,0), vec(0.5,0,0), vec(0.5,0.5,0), vec(0,0.5,0). Note that <0,0,0> is at the lower left, and <1,1,0> is at the upper right.