See #34 - the FITS standard mentions the possibility of using NULL_PIXEL_MASK for compression algorithms that don't preserve ints. However I think there are no such algorithms at the moment so NULL_PIXEL_MASK is never needed (and not implemented in CFITSIO).
See #34 - the FITS standard mentions the possibility of using NULL_PIXEL_MASK for compression algorithms that don't preserve ints. However I think there are no such algorithms at the moment so NULL_PIXEL_MASK is never needed (and not implemented in CFITSIO).