Each piece is grown from a 12 x 12 arrangement of right triangles. They’re controlled primarily by each tile’s likelihood of being connected to its neighbors. The two patterns below are at 0% and 95% connectivity, respectively.
Adding black squares or empty white squares to the palette of shapes placed in each location helps allow some more interesting patterns to emerge. Without them the pieces tend to grow into a sort of checkerboard as connectivity increases.
Overlapping two of these patterns on top of one another, with varying offsets, produces some of the most interesting results. It often unearths the hidden “triangleness” in some of the more ordered patterns.