A book-matched pair of boards is a pair that is cut from the same log and then glued together. The result is a larger board that has symmetry in the grain from one side to the other. We can approximate this effect by mirroring the texture coordinate. For example, we could use the following in place of the first line of the preceding main function:

vec2 tc = TexCoord; 
if( tc.s > 0.5 ) tc.s = 1.0 - tc.s; 
vec4 cyl = Slice * vec4( tc, 0.0, 1.0 ); 

The following image shows an example of the results: