pamundice

Combine a grid of Netpbm images into one. See also: pamdice. More information: https://netpbm.sourceforge.net/doc/pamundice.html.

pamundice filename_%1d_%1a.ppm -across grid_width -down grid_height > path/to/output.ppm

pamundice filename_%1d_%1a.ppm -across x_value -down y_value -hoverlap value -voverlap value > path/to/output.ppm

pamundice -listfile path/to/file.txt -across x_value -down y_value > path/to/output.ppm