This program draws a white pixel if the corresponding pixel number is prime, black otherwise. The default width is 800, so there are lots of vertical black rows (factors of 5). If you left click the width will go up by 1. If you right click the width will go down by 1. Type "s" to make all the square pixels white, type "t" to make all the triangular pixels white, and type "p" to go back to just the prime numbers white.