Displace the pixels in a PPM image by a randomized amount. More information: https://netpbm.sourceforge.net/doc/ppmspread.html.
ppmspread a path/to/input_file.ppm > path/to/output_file.ppm
ppmspread a path/to/input_file.ppm -randomseed seed > path/to/output_file.ppm