Convert hdiff to pnm Online: hdiff2pnm
You can use Tom's Editor to easily convert images from
Horizontal difference image .HDIFF to Portable aNy Map .PNM.
Tom's Editor is a free, online image manipulation and converter program.
Go to Tom's Editor
Horizontal difference image is an image format with extension HDIFF.
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.