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4.8 Support for IEEE Special Values

The ANSI/IEEE-754 floating-point number standard defines certain special values that are used to represent such quantities as Not-a-Number (NaN), denormalized, underflow, overflow, and infinity. (See the Appendix in the NOST FITS standard or the NOST FITS User's Guide for a list of these values). The CFITSIO routines that read floating point data in FITS files recognize these IEEE special values and by default interpret the overflow and infinity values as being equivalent to a NaN, and convert the underflow and denormalized values into zeros. In some cases programmers may want access to the raw IEEE values, without any modification by CFITSIO. This can be done by calling the fits_read_img or fits_read_col routines while specifying 0.0 as the value of the NULLVAL parameter. This will force CFITSIO to simply pass the IEEE values through to the application program without any modification. This is not fully supported on VAX/VMS machines, however, where there is no easy way to bypass the default interpretation of the IEEE special values. This is also not supported when reading floating-point images that have been compressed with the FITS tiled image compression convention that is discussed in section 5.6; the pixels values in tile compressed images are represented by scaled integers, and a reserved integer value (not a NaN) is used to represent undefined pixels.


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