Breeding combines two user-selected chromosomes, replacing the other (unlocked) chromosomes in the gene pool with their offspring. Breeding is done as in genetic algorithms, with three-way cross-over (see Figure 3). In other words, the two parent chromosomes are cut into three pieces each and a new chromosome is formed from the first and last portions of one parent and the middle portion of the other.
Figure 3: The two-way crossover operation used in the breed operation.
Breeding is a discrete operation; if one has two parent facial expressions, one is not going to get an expression halfway between them in expression space. In geometric terms, possible offspring are at the vertices of a N-cube one of whose diagonals is the line between the two parents.