ppmtogif(1) - man - phpMan

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ppmtogif(1) General Commands Manual ppmtogif(1)

NAME
ppmtogif - convert a portable pixmap into a GIF file

SYNOPSIS
ppmtogif [-interlace] [-sort] [-map mapfile]
[-transparent [=]color] [-alpha pgmfile] [-comment text] [-nolzw]
[ppmfile]

All options can be abbreviated to their shortest unique prefix. You may use two hyphens in‐
stead of one to designate an option. You may use either white space or equals signs between
an option name and its value.

DESCRIPTION
Reads a portable pixmap as input. Produces a GIF file as output.

This program creates only individual GIF images. To combine multiple GIF images into an ani‐
mated GIF, use gifsicle (not part of the Netpbm package).

ppmtogif creates either an original GIF87 format GIF file or the newer GIF89 format. It cre‐
ates 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.

OPTIONS
-interlace
Produce an interlaced GIF file.

-sort Produces a GIF file with a sorted color map.

-map mapfile

Uses the colors found in the mapfile to create the colormap in the GIF file, instead
of the colors from ppmfile. The 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 , they are
matched to a "best match." A (much) better result can be obtained by using the follow‐
ing filter in advance:

ppmquant -floyd -map mapfile

-transparent color
ppmtogif marks the specified color as transparent in the GIF image.

If you don't specify -transparent, ppmtogif does not mark any color transparent (ex‐
cept as indicated by the -alpha option).

You specify the color as in ppmmake(1).E.g. red or rgb:ff/00/0d. 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.

-alpha= pgmfile
This option names a PGM file that contains an alpha mask for the image. ppmtogif Cre‐
ates fully transparent pixels wherever the alpha mask indicates transparency greater
than 50%. The color of those pixels is that specified by the -alphacolor option, or
black by default.

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 transpar‐
ent 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 max‐
val. White means opaque and black means transparent.

You cannot specify both -transparent and -alpha.

-alphacolor
See -alpha.

-comment text
Include a comment in the GIF output with comment text text. Without this option,
there are no comments in the output.

-nolzw This option causes the GIF output, and thus ppmtogif, not to use LZW (Lempel-Ziv) com‐
pression. As a result, the image file is larger and no royalties are 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 de‐
compressor would see this as a particular case of LZW compression.

Note that if someone uses an LZW decompressor such as the one in ppmtogif 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 has expressed far less interest in en‐
forcing the patent on decoding than on encoding.

SEE ALSO
giftopnm(1), ppmquant(1), pngtopnm(1), gifsicle(1) <http://www.lcdf.org/gifsicle>, ppm(5).

AUTHOR
Based on GIFENCOD by David Rowley <mgardi AT watdcsu.edu>. Lempel-Ziv compression
based on "compress".

The non-LZW format is generated by code based on djpeg by the Independent Jpeg Group.

LICENSE
If you use ppmtogif without the -nolzw option, you are using a patent on the LZW compression
method which is owned by Unisys, and in all probability you do not have a license from Unisys
to do so. Unisys typically asks $5000 for a license for trivial use of the patent. Unisys
has never enforced the patent against trivial users. The patent expires in 2003.

Rumor has it that IBM also owns a patent covering ppmtogif.

A replacement for the GIF format that does not require any patents to use is the PNG format.

20 May 2000 ppmtogif(1)