Portable Pix Map, an image format of the PNM family.  It is a part of
PBM Plus http://www.acme.com/software/pbmplus/ for which in the Netpbm project
[http://sourceforge.net/projects/netpbm/]
a large numbers of converters and manipulators are available.

Every pixel is represented by three bytes for red, green, and blue channel. No compression, no transparency, as far as [RS] is aware.

Tk images can be read and written in PPM format using [photo].  See also [PGM].
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[Category Graphics]