SAOtng supports the following scale algorithms internally:
Support for other scaling algorithms can be added to SAOtng by writing a program that takes a FITS image as input and generates a scaled (8-bit) FIT image as output. When activated, a scale command line macro-expanded to fill in user-defined arguments and then is executed externally using the system() function (see Macro expansion in SAOtng.
The ASCII file that defines new scaling algorithms consists of one or more scale descriptors, each of which has the following format:
For example, the default image access file currently contains the following:
# # Data scaling descriptions: # help line # name of scale # type (always "scale") # scale command Linear data scaling linear scale $data | linear min=$min max=$max Log data scaling log scale $data | log log=10 min=$min max=$max Square root data scaling sqrt scale $data | pow pow=0.5 min=$min max=$max Linear wrapped data scaling wrap scale $data | wrap min=$min max=$max No data scaling none scale $data | none
Each scale program command line takes $data as stdin and write the scaled FITS image to stdout. When the scale algorithm is executed, $data will be replaced by a command to retrieve the unscaled data which then is passed to the scale program. The output scaled FITS image will be sent back to SAOtng for display using XPA.
The scale programs shown above are written so that the program itself takes FITS data into its stdin and write scaled FITS data to stdout This need not be the case, however, since it the entire command line that must process "stdin" and "stdout" and not just the program. For example, consider a case of a scale program that read and wrote FITS disk files instead of stdin and stdout:
scaleit -i [unscaled_fits] -o [scaled_fits]
This program could use the following command line to scale data in SAOtng:
$data > /tmp/foo; scaleit -i /tmp/foo -o /tmp/goo; cat /tmp/goo
SAOtng will replace $data with an XPA command to retrieve the unscaled
data and also add the appropriate commands on the send output file back
for display. Of course, it clearly is more efficient to read and write
to stdin/stdout and avoid the overhead of disk access.
Return to Main SAOtng Help Page