sgitopnm - convert a SGI image file to PNM
This program is part of Netpbm(1).
sgitopnm reads an SGI image file as input and produces a PGM image for a 2-dimensional (1- or 2-channel) input file, and a PPM image for a 3-dimensional (3 or more channels) input file.
Alternatively, the program produces a PGM image of any one of the channels in the input file.
Before Netpbm 10.67 (June 2014), sgitopnm does not work on 2-channel SGI images. It fails if you try.
If you don't specify the SgiFileName argument, input is from Standard Input.
Before Netpbm 10.67 (June 2014), sgitopnm requires its input to be a seekable file, so for example you can't feed it from a pipe.
Without this option, sgitopnm extracts the first 3 channels as a PPM image or, if the input has 1 or 2 channels, extracts the first channel as a PGM image.
A 2-channel image is grayscale plus transparency, so you can get the transparency information with -channel=2. You could then combine them into a PAM image of tuple type GRAYSCALE_ALPHA with pamstack.
The SGI image format specification version 1.0 is at ftp://ftp.sgi.com/graphics/grafica/sgiimage.html .
There is an example SGI file at https://github.com/ZaaLabs/ZaaIL-TestImages/tree/master/SGI .
Copyright (C) 1994 by Ingo Wilken ( Ingo.Wilken@informatik.uni-oldenburg.de)