This input format is used to read 2D regular grids encoded in a popular picture format readable by SDL-image library (jpg, png, gif, ...). When using this format as input to mse, the field whose topology is computed is the luminosity L of the image, a weighted average of the RED, GREEN and BLUE components of the image:

L = 0.2989*RED + 0.5870*GREEN + 0.1140*BLUE