This picture is constructed in Postscript by a subroutine that evaluates a function on a triangular lattice and interpolates linearly in between (the edges of the lattice have been left visible here). As the function varies from -1 to 1, the color varies from black to white through various hues that can be adjusted (here red = 0).

As a function we chose the superposition of waves emitted by two sources with slightly different frequences and intensities. Close to the sources the function overflows, becoming greater than one.

You should see the 3-source case!