The initial image is the overlay of two complementary masks and . This overlay evolves according to two rules and . The result of evolving by the first rule is masked by multiplication with bitmap mask . The result of evolving by the second rule is masked by multiplication with the inverted bitmask which is taken to be . The sum of the results and gives the next image .
The rule numbers and specify two of the 1024 totalistic 9-neighbor two-color cellular automaton rules.
In this Demonstration mask and its negative can display (1) the selected rule numbers, (2) a number which gives the number of the current step, (3) static random patterns, or (4) a new random pattern for each step.
Note that the sums of the masked automaton results are always positive. Differences of those intermediary masked results can give negative values. However, taking the absolute value makes negative results of differences acceptable for the next step of evolution by the binary automaton rules and .