ppmtogif
convert a portable pixmap into a GIF file
see also :
giftopnm - ppmquant - pngtopnm
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.
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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.
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.
see also
giftopnm ,
ppmquant , pngtopnm , gifsicle
<http://www.lcdf.org/gifsicle>, ppm.
author
Based on
GIFENCOD by David Rowley
<mgardi[:at:]watdcsu.waterloo[:dot:]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.