ppmtogif - convert a PPM image to a GIF image
[-mapfile mapfile] [-transparent=[=]color]
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.
This program is part of Netpbm(1).
ppmtogif reads a PPM iamge as input and produces a GIF file as output.
This program creates only individual GIF images. To combine multiple GIF images into an animated GIF, use gifsicle (not part of the Netpbm package).
ppmtogif creates either an original GIF87 format GIF file or the newer GIF89 format. It creates GIF89 when you request features that were new with GIF89, to wit the -transparent or -comment options. Otherwise, it creates GIF87. Really old GIF readers conceivably could not recognize GIF89.
Use the colors found in the file mapfile to create the colormap in the GIF file, instead of the colors from ppmfile. mapfile can be any PPM file; all that matters is the colors in it. If the colors in ppmfile do not match those in mapfile, ppmtogif matches them to a 'best match.' You can obtain a much better result by using pnmremap to change the colors in the input to those in the map file.
The mapfile file is not a palette file, just an image whose colors you want to use. The order of colors in the GIF palette have nothing to do with where they appear in the mapfile image, and duplication of colors in the image is irrelevant.
If you don't specify -transparent, ppmtogif does not mark any color transparent (except as indicated by the -alpha option).
Specify the color (color) as described for the argument of the ppm_parsecolor() library routine .
If the color you specify is not present in the image, ppmtogif selects instead the color in the image that is closest to the one you specify. Closeness is measured as a cartesian distance between colors in RGB space. If multiple colors are equidistant, ppmtogif chooses one of them arbitrarily.
However, if you prefix your color specification with '=', e.g. -transparent==red, only the exact color you specify will be transparent. If that color does not appear in the image, there will be no transparency. ppmtogif issues an information message when this is the case.
You cannot specify both -transparent and -alpha.
To do this, ppmtogif creates an entry in the GIF colormap in addition to the entries for colors that are actually in the image. It marks that colormap entry as transparent and uses that colormap index in the output image to create a transparent pixel.
The alpha image must be the same dimensions as the input image, but may have any maxval. White means opaque and black means transparent.
You cannot specify both -transparent and -alpha.
Without this option, there are no comments in the output.
Note that in a command shell, you'll have to use quotation marks around text if it contains characters (e.g. space) that would make the shell think it is multiple arguments:
$ ppmtogif -comment "this is a comment" <xxx.ppm >xxx.gif
This option is mainly of historical interest -- it involves use of a patent that is now expired.
This option causes the GIF output, and thus ppmtogif, not to use LZW (Lempel-Ziv) compression. As a result, the image file is larger and, before the patent expired, no royalties would be owed to the holder of the patent on LZW. See the section LICENSE below.
LZW is a method for combining the information from multiple pixels into a single GIF code. With the -nolzw option, ppmtogif creates one GIF code per pixel, so it is not doing any compression and not using LZW. However, any GIF decoder, whether it uses an LZW decompressor or not, will correctly decode this uncompressed format. An LZW decompressor would see this as a particular case of LZW compression.
Note that if someone uses an LZW decompressor such as the one in giftopnm or pretty much any graphics display program to process the output of ppmtogif -nolzw , he is then using the LZW patent. But the patent holder expressed far less interest in enforcing the patent on decoding than on encoding.
gifsicle http://www.lcdf.org/gifsicle , ppm(1).
Based on GIFENCOD by David Rowley <firstname.lastname@example.org>. Lempel-Ziv compression based on 'compress'.
The non-LZW format is generated by code based on djpeg by the Independent Jpeg Group.
Copyright (C) 1989 by Jef Poskanzer.
If you use ppmtogif without the -nolzw option, you are using a patent on the LZW compression method which is owned by Unisys. The patent has expired (in 2003 in the US and in 2004 elsewhere), so it doesn't matter. While the patent was in force, most people who used ppmtogif and similar programs did so without a license from Unisys to do so. Unisys typically asked $5000 for a license for trivial use of the patent. Unisys never enforced the patent against trivial users.
Rumor has it that IBM also owns or owned a patent covering ppmtogif.
A replacement for the GIF format that never required any patents to use is the PNG format.