This program is part of Netpbm.
ppmrelief reads a PPM image as input, and writes a relief of that image as a PPM image as output.
The relief process is described in "Beyond Photography" by Holzmann, equation 3.19. It's a sort of edge-detection and is essentially a convolution with this matrix:
| 1 0 0 |
| 0 0 0 |
| 0 0 -1 |