Adding New File Formats to SAOtng

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SAOtng supports FITS files internally, so that a standard FITS image file can be sent directly to SAOtng for display using XPA:

  xpaset SAOtng < foo.fits

Also, when a new file name is sent to SAOtng, the program checks the first 2880 bytes of the file to see if it is a FITS file and if so, processes it accordingly. Support for other image file formats (or any other image format) is added to SAOtng by writing two external file access programs:

a header access program to generate a FITS header describing the full image.
a data access program to generate a FITS image of a specified data section (given the center, dimensions, and block factor). Note that the block factor should be applied after the image extraction so that if xdim=ydim=1024 and block=2, the resulting image is 512x512.

These program pairs are made known to SAOtng at start-up time in a user-configurable ASCII file. When loading an image, the program command lines first are macro-expanded to fill in user-defined arguments and then is executed externally using the system() function (see Macro expansion in SAOtng.

When a new file name is sent to SAOtng, the appropriate header access program is run to gather the overall image dimensions.
The data access program then is run to extract the desired image section, which is scaled to the image display depth and displayed on the screen.
Finally, the data access program again extracts the data section and stores it as raw FITS data for later use (e.g., in order to re-scale the image without having to run the data extraction program again).

The ASCII file that defines the known image files consists of one or more file descriptors, each of which has the following format:

A help line describing the file types
A space-separated list of templates
A space-separated list of file types (not currently used)
The command-line of the image section program
The command-line of the FITS header generation program

Note that blank lines separate the file descriptions and should not be used as part of a description. Also, the '#' character is a comment character.

For example, the default image access file currently contains the following:

#
# File access descriptions:
#	help explanation
#	file template
#	file type
#	header access command
#	data access command

FITS image files
*.fit *.fits
FITS
fitsextract fits='$filename' xcen=$xcen ycen=$ycen xdim=$xdim ydim=$ydim block=$block option=$blocking_preference checkNaN=$donan ofits=stdout type=image mode=h 2>$errorlog
fitsheader fits='$filename' ofits=stdout type=image mode=h

ARRAY files (use: foo.arr[xdim=x,ydim=y,bitpix=b,skip=s])
*.arr[*]
ARRAY
fitsextract fits='$filename' xcen=$xcen ycen=$ycen xdim=$xdim ydim=$ydim block=$block option=$blocking_preference checkNaN=$donan ofits=stdout type=array mode=h 2>$errorlog
fitsheader fits='$filename' ofits=stdout type=array mode=h

IRAF image files (qpoe, imh, etc.)
*.qp *.imh *.stf *.pl
QPOE IMH STF
uiraf x_fmgr.e -b blkfits image='$filename' xcen=$xcen ycen=$ycen xdim=$xdim ydim=$ydim block=$block option=$blocking_preference
uiraf x_fmgr.e -b hdrfits image='$filename'

VIEW image files
*.sdt 
VIEW
viewextract view='$filename' xcen=$xcen ycen=$ycen xdim=$xdim ydim=$ydim block=$block option=$blocking_preference 2>$errorlog
viewheader view='$filename'

binary event lists
*.ev *.nepr
BINTABLE
fitsextract fits='$filename' xcen=$xcen ycen=$ycen xdim=$xdim ydim=$ydim block=$block option=$blocking_preference checkNaN=$donan ofits=stdout type=events mode=h 2>$errorlog
fitsheader fits='$filename' ofits=stdout type=events mode=h

FITS files over the Net
ftp://* http://*
WWW
wwwtng '$root' | fitsextract fits='stdin' ext='$filename' xcen=$xcen ycen=$ycen xdim=$xdim ydim=$ydim block=$block option=$blocking_preference checkNaN=$donan ofits=stdout type=image mode=h 2>$errorlog
wwwtng '$root' | fitsheader fits='stdin' ext='$filename' ofits=stdout type=image mode=h

compressed FITS files
*.Z *.gz
COMPRESSED
unziptng '$root' | fitsextract fits='stdin' ext='$filename' xcen=$xcen ycen=$ycen xdim=$xdim ydim=$ydim block=$block option=$blocking_preference checkNaN=$donan ofits=stdout type=image mode=h 2>$errorlog
unziptng '$root' | fitsheader fits='stdin' ext='$filename' ofits=stdout type=image mode=h

Note that you do not actually have to write full image extraction programs for your new file type. It is sufficient to write a program that generates a FITS file from your data and then pass that FITS file to the fitsextract program for sectioning. Use of this technique can be seen above in the support given to compressed FITS and FITS on the Web. For example, in the case of a compressed FITS file (containing either images or binary tables), the script tngunzip takes a compressed FITS file as input, uncompresses it and pipes the FITS result to fitsextract, which handles the extraction and blocking.

In such a case, the $root macro is used on the command line to pass to pass filename info to tngunzip (so that irrelevant filter information is not sent to the script) and the full name including filter then is passed to fitsextract via the ext= keyword. In this way, a program such as gunzip can uncompress the file and pass the result to fitsextract, where the filter can be applied appropriately.

New files are added to SAOtng using the -files [file access list] option on the command line (or the SAOTNG_FILES environment variable). The files in the file access list are then added to the system-support file types.

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Adding New File Formats to SAOtng

Last Updated June 14, 1997