I've found a PPM image with the header starting with PF (instead of standard P6, P5, P4, ... signatures). The resolution number is stored in floating point (-1.000). No comments are inserted to get how it was produced.
Because the resolution, I derived that each pixel is composed by 12 bytes (4 bytes per component)... I suppose they are float or integer numbers.
The problem is that I cannot get a clear image. Someone has already found this kind of images?
Portable Float Map HDR Shop description.