6.5 Data Scaling and Undefined Pixel Parameters

These subroutines define or modify the internal parameters used by FITSIO to either scale the data or to represent undefined pixels. Generally FITSIO will scale the data according to the values of the BSCALE and BZERO (or TSCALn and TZEROn) keywords, however these subroutines may be used to override the keyword values. This may be useful when one wants to read or write the raw unscaled values in the FITS file. Similarly, FITSIO generally uses the value of the BLANK or TNULLn keyword to signify an undefined pixel, but these routines may be used to override this value. These subroutines do not create or modify the corresponding header keyword values.

1

Reset the scaling factors in the primary array or image extension; does not change the BSCALE and BZERO keyword values and only affects the automatic scaling performed when the data elements are written/read to/from the FITS file. When reading from a FITS file the returned data value = (the value given in the FITS array) * BSCALE + BZERO. The inverse formula is used when writing data values to the FITS file. (NOTE: BSCALE and BZERO must be declared as Double Precision variables).

        FTPSCL(unit,bscale,bzero, > status)

2

Reset the scaling parameters for a table column; does not change the TSCALn or TZEROn keyword values and only affects the automatic scaling performed when the data elements are written/read to/from the FITS file. When reading from a FITS file the returned data value = (the value given in the FITS array) * TSCAL + TZERO. The inverse formula is used when writing data values to the FITS file. (NOTE: TSCAL and TZERO must be declared as Double Precision variables).

        FTTSCL(unit,colnum,tscal,tzero, > status)

3

Define the integer value to be used to signify undefined pixels in the primary array or image extension. This is only used if BITPIX = 8, 16, 32. or 64 This does not create or change the value of the BLANK keyword in the header. FTPNULLL is identical to FTPNUL except that the blank value is a 64-bit integer instead of a 32-bit integer.

        FTPNUL(unit,blank, > status)
        FTPNULLL(unit,blankll, > status)

4

Define the string to be used to signify undefined pixels in a column in an ASCII table. This does not create or change the value of the TNULLn keyword.

        FTSNUL(unit,colnum,snull > status)

5

Define the value to be used to signify undefined pixels in an integer column in a binary table (where TFORMn = 'B', 'I', 'J', or 'K'). This does not create or change the value of the TNULLn keyword. FTTNULLL is identical to FTTNUL except that the tnull value is a 64-bit integer instead of a 32-bit integer.

        FTTNUL(unit,colnum,tnull > status)
        FTTNULLL(unit,colnum,tnullll > status)