This program is part of Netpbm. ppmrainbow generates a PPM image that fades from one color to another to another from left to right, like a rainbow.
If you want a vertical or other non-horizontal rainbow, run the output through pnmrotate or pamflip.
One use for such a rainbow is to compose it with another image under a transparency mask in order to add a rainbow area to another image. In fact, you can make rainbow-colored text by using pbmtext, pamcomp, and ppmrainbow.
pgmramp does a similar thing for grayscale images.
If you just want an image containing all the possible colors (for some kind of processing; not to look at), see pamseq.
color ... is the list of colors, in order from left to right, to go into the rainbow.
The first color is added again on the right end of the image unless you specify the -norepeat option. This means you can concatenate multiple copies (tile, as with pnmtile) to make a continuous larger image.
color is as described for the argument of the ppm_parsecolor() library routine.
All options can be abbreviated to their shortest unique prefix. You may use two hyphens instead of one to designate an option. You may use either white space or equals signs between an option name and its value.
Default is 600.
Default is 8.
Default is the value of the TMPDIR environment variable, or /tmp if TMPDIR is not set.
ppmrainbow always creates a directory within this directory and creates all its files within that directory.
Arjen Bax wrote ppmrainbow in June 2001 and contributed it to the Netpbm package. Bryan Henderson wrote this manual in July 2001.