This program is part of Netpbm.
ppmmix reads two PPM images as input and mixes them together using the specified fade factor. The fade factor may be in the range from 0.0 (only ppmfile1's image data) to 1.0 (only ppmfile2's image data). Anything in between specifies a smooth blend between the two images.
The two pixmaps must have the same size.
pamcomp is a more general alternative. It allows you to mix images of different size and to have the fade factor vary throughout the image (through the use of an alpha mask).