ppmtogif Man page

ppmtogif General Commands Manual ppmtogif

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 instead 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 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.

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 following 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 (except as indicated by the -alpha option).

You specify the color as in ppmmake.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 arbitrar‐
ily.

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 trans‐
parency. 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 Creates 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 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.

-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) compression. 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 pix‐
els into a single GIF code. With the -nolzw option, ppmtogif
creates one GIF code per pixel, so it is not doing any compres‐
sion 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
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 enforc‐
ing the patent on decoding than on encoding.

SEE ALSO

giftopnm, ppmquant, pngtopnm, gifsicle(1)
, ppm(5).

AUTHOR

Based on GIFENCOD by David Rowley . Lem‐
pel-Ziv compression based on “compress”.

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

Copyright (C) 1989 by Jef Poskanzer.

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

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