This program is part of Netpbm.
rasttopnm reads a Sun rasterfile as input and produces a PNM image as output. The type of the output file depends on the input file - if it's black and white, rasttopnm writes a PBM image. If it's grayscale, rasttopnm writes a PGM. If it's color, rasttopnm writes a PPM. The program tells you which type it is generating.
In a colormapped Sun rasterfile, the header of the file contains a color map, associating each unique color that occurs in the file with an index number. The raster portion of the file then represents the color of a pixel with one of those colormap indices.
This option has found use as follows: take a regular grayscale Sun rasterfile in which all the grayscale value are represented, so the colormap indices are identical to the color values. Modify its color map so as to alter the colors in the image, in particular to equalize the colors. With that modified rasterfile as input, rasttopnm without -index produces the modified image. But with -index, it produces the original unmodified image.
This option was new in Netpbm 10.56 (September 2011).