We get asked this question a lot, so I thought I'd post it here in the FAQ!
The file format specification for PTM is available from the HP website here:
http://www.hpl.hp.com/research/ptm/downloads/PtmFormat12.pdf
The file Format Specification for the RTI file format was previously posted here in the forums, and I am attaching it here as well. Note that it is marked "draft" because it contemplates a .xmp structure embedded in the file structure, and that part was still under discussion when this document was created. The format for the RTI data is final, and this is what is used by the RTIViewer and other viewers that support this format.
Carla
rti_format_final-draft.pdf