This program is part of
Netpbm(1).
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 images must have the same dimensions.
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).