pgm-interlace

Vertically interlace n PGM frames, each one width/n pixels in height into one square image.

Syntax

pgm-interlace [frame 0] [frame 1] [frame 2] ...

The order in which the frames are specified on the command line is critical for the reconstruction of the image. pgm-interlace has no way of telling in which order a handful of pgm frames should be interlaced unless you tell it how to stitch them together.