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tickbox variable: $tiffoutflag -- tiff vs. rgb
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default: 0 (FALSE)

The unlabeled tickbox to the right of the "rgb:"
entry controls $tiffoutflag.  When it is ticked
(=1, the default), the bitmap output file format
is a 3 bytes per pixel, LZW compressed, tiff
file.  If unticked (=0), the original SGI rgb
bitmap format is used instead (rarely used).
