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 images must have the same dimensions and the same maxval. Before Netpbm 10.54 (March 2011), they must also have the same type (PBM/PGM/PPM).
The fade factor is applied to brightness, not light intensity. That means for example that if you have a series of images you generated using ppmmix of a black and a white image with a linearly increasing fade factor, you will see an image getting linearly brighter, but the light intensity will increase faster at the end. That is because it requires more intensity change at the bright end of the scale than at the dark end for the human eye to perceive the same brightness change. This also means that if the original images aren't all one color, the mixed image is distorted, since the intensity relationship between pixels is different from the original image.
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). It does not have the same-maxval and same-type restrictions. It mixes light intensity, not brightness.