Convert hpm to pnm Online: hpm2pnm
You can use Tom's Editor to easily convert images from
Grass' Slideshow .HPM to Portable aNy Map .PNM.
Tom's Editor is a free, online image manipulation and converter program.
Go to Tom's Editor
Grass' Slideshow is an image format with extension HPM.
PNM images are uncompressed 24 bit RGB images. Can cannot store Alpha channel. PNM images come in 2 types: binary and text. Especially popular on Linux.