This program is part of Netpbm.
pnmtofits reads a PNM image as input and produces a FITS (Flexible Image Transport System) file as output. The resolution of the output file is either 8 bits/pixel, or 16 bits/pixel, depending on the value of maxval in the input file. If the input file is a portable bitmap or a portable graymap, the output file consists of a single plane image (NAXIS = 2). If instead the input file is a portable pixmap, the output file will consist of a three-plane image (NAXIS = 3, NAXIS3 = 3). You can find a full description of the FITS format in Astronomy & Astrophysics Supplement Series 44 (1981), page 363.
You can use options -min and -max to set DATAMAX, DATAMIN, BSCALE and BZERO in the FITS header, but not cause the data to be rescaled.