I've been using the PFM format (http://netpbm.sourceforge.net/doc/pfm.html) for a project recently.

It's the most obscure member of the Netpbm family. Almost no image viewer understands it, and no image editor can read or write it. Luckily there's the incredible CImg (http://www.cimg.eu/) lib.

PFM is stored in left to right, bottom to top order pixel. It has exactly one or three channels. One pixel is four bytes. There's an obscure "scale/endianness" parameter to tell you the byte order.

I love it, and I hate it.