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(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
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 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
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 enforcing the patent on decoding than on encoding.
Based on GIFENCOD by David Rowley <mgardi@watdcsu.waterloo.edu>.
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.
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.