Provided by: netpbm_11.10.02-1build1_amd64 
NAME
pamtogif - convert a Netpbm image to a GIF image
SYNOPSIS
pamtogif
[-interlace]
[-sort]
[-mapfile=mapfile] [-transparent=[=]color]
[-alphacolor=color]
[-comment=text]
[-noclear]
[-nolzw]
[-aspect=fraction]
[-verbose] [netpbmfile]
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 an equals sign between an option name and its
value.
DESCRIPTION
This program is part of Netpbm(1).
pamtogif reads a Netpbm image 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).
pamtogif creates either an original GIF87 format GIF file or the
newer GIF89 format. It creates GIF89 when the output needs to have features
that were new with GIF89, to wit transparency or comments. Otherwise, it
creates GIF87. Really old GIF readers conceivably could not recognize
GIF89. The output needs to have transparency when either the input has a
transparency information or you specify the -transparent option. It
needs to have comments when you specify the
-comment option.
pamtogif generates a GIF image with a single image block, which means the image cannot have more than 256
colors in it (it contains a single color map with a maximum size of 256). If the image you want to
convert has more colors than that (ppmhist can tell you), you can use pnmquant to reduce it to 256. Or
use the more complex but faster method described under the -mapfile option.
If your input image is a PAM with transparency information, pamtogif uses one entry in the GIF colormap
specifically for the transparent pixels, so you can have at most 255 opaque colors. In contrast, if you
use the -transparent option, one of the colors from the input becomes transparent, so the limit is still
256.
pamtogif recognizes transparency information in the input by the
tuple type being RGB_ALPHA, GRAYSCALE_ALPHA, or
BLACKANDWHITE_ALPHA. This is the case for any image that has
transparency information and was created by a Netpbm program that
manipulates visual images. If, on the other hand, you have a PAM generated
some other way, but you know the planes have the same meaning as implied by
these tuple types, you can make pamtogif process the transparency
information by changing the tuple type accordingly before you pass it
to pamtogif. You can use pamstack to change the tuple type.
pamtogif was new in Netpbm 10.37 (December 2006). In older Netpbm, use ppmtogif.
OPTIONS
In addition to the options common to all programs based on libnetpbm (most notably -quiet, see Common
Options ), pamtogif recognizes the following command line options:
-interlace
Produce an interlaced GIF file.
-sort Produce a GIF file with a color map sorted in a predictable order.
This does not produce the sorted color map which is part of the GIF format. That kind of sorted
color map is one where the colors are sorted according to how important they are, and the GIF
header tells the viewer that it is sorted that way. Its purpose is to allow the viewer to use
fewer colors than are in the color map if it is not capable of displaying all the colors.
What this option produces is a color map sorted by red value, then green, then blue. That can be
useful in analyzing GIF images, particularly those made with two versions of the program, because
it removes some of the variability.
-mapfile=mapfile
Use the colors found in the file mapfile to create the colormap in the GIF file, instead of the
colors from netpbmfile. mapfile can be any PPM file; all that matters is the colors in it. If
the colors in netpbmfile do not match those in mapfile, pamtogif 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.
The map file's depth must match the number of color components in the input (which is not
necessarily the same as the input's depth -- the input might have a transparency plane in
addition). If your map file does not, or it might not, run your input through pnmremap using the
same map file so that it does.
You can use -mapfile to speed up conversion of an image where you already have a map file because
of earlier processing of your image. For example, it is common to start with an image that has
more than 256 colors and remap its colors to a set of 256 colors so that pamgtogif can convert it
(a GIF can have only 256 colors; pamtogif without -mapfile fails on any image that has more than
that) with pnmquant. When you do this, pnmquant generates a palette to do the color quantization,
then pamtogif generates an identical palette from the quantized image. You can save computation
by generating the palette once:
$ pnmcolormap 256 myimage.ppm >/tmp/colormap.ppm
$ pamtogif myimage.ppm -mapfile=/tmp/colormap.ppm >output.gif
-transparent=color
pamtogif marks the specified color as transparent in the GIF image.
If you don't specify -transparent, pamtogif does not mark any color transparent (except as
indicated by the transparency information in the input file).
Specify the color (color) as described for the argument of the pnm_parsecolor() library routine .
If the color you specify is not present in the image, pamtogif 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, pamtogif 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. pamtogif issues an information message when this is the case.
When you specify -transparent, pamtogif ignores explicit transparency information (the "alpha
channel") in the input image.
-alphacolor=color
This specifies the foreground color for transparent pixels. A viewer may use the foreground color
for a transparent pixel if it chooses not to have another color "show through.". The default is
black.
This applies only to pixels that are transparent in the GIF because they are transparent in the
Netpbm input. If a GIF pixel is transparent because of the -transparent option, the foreground
color is the color indicated by that option.
Note that in GIF, all transparent pixels have the same foreground color. (There is only one entry
in the GIF colormap for transparent pixels).
Specify the color (color) as described for the argument of the pnm_parsecolor() library routine .
-comment=text
Include a comment in the GIF output with comment text text.
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:
$ pamtogif -comment "this is a comment" <xxx.ppm >xxx.gif
-noclear
This option causes the output not to contain any GIF clear codes.
In GIF, the stream defines codes that represent strings of pixels as it goes. The stream contains
definitions of codes mixed in with the references to those codes that describe the pixels of the
image. GIF specifies a maximum number of codes that can be defined; when the stream has defined
that many, the stream can either just use those for the rest of the image or include a clear code,
deleting all the string codes so that the stream can start over defining new ones.
By far the most common choice is the clear code. This usually results in a smaller stream because
the set of strings of pixels that occur in an image vary over the parts of the image. Hardly any
GIF encoders produce streams that don't use the clear code.
But it is conceivable that a stream could be smaller without the use of the clear code because it
saves the stream having to redefine the same string codes over and over. It could even avoid a
thrashing situation where the stream continually defines a set of strings that never get used
again before the maximum is reached.
The default is to use the clear codes.
This option was new in Netpbm 10.82 (March 2018). Before that, the program aways uses the clear
codes.
-nolzw
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 pamtogif, 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, pamtogif 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 pamtogif -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.
-aspect=fraction
This is the aspect ratio of the pixels of the image. Its only effect is to record that
information in the GIF for use by whatever interprets the GIF. Note that this feature of GIF is
hardly ever used and most GIF decoders ignore this information and assume pixels are square.
Pixels in a Netpbm image do not have aspect ratios; there is always a one-one correspondence
between GIF pixels and Netpbm pixels.
The aspect ratio is the quotient of width divided by height. GIF allows aspect ratios from 0.25
(1:4) to 4 (4:1) in increments of 1/64. pamtogif implements a natural extension of GIF that
allows an aspect ratio up to 4 14/64. If you specify anything outside this range, pamtogif fails.
pamtogif rounds fraction to the nearest 1/64.
The default is square (1.0).
This option was new in Netpbm 10.38 (March 2007). Before that, the pixels are always square.
-verbose
This option causes pamtogif to display information about the conversion process and the image it
produces.
SEE ALSO
giftopnm(1), pnmremap(1), ppmtogif(1),
gifsicle http://www.lcdf.org/gifsicle , pnm(1), pam(1).
HISTORY
pamtogif was new in Netpbm 10.37 (December 2006). It replaced ppmtogif, which created GIF images for
Pbmplus/Netpbm users since 1989.
The main outward change in the conversion from ppmtogif to pamtogif was that pamtogif was able to use
transparency information ("alpha channel") in PAM input, whereas with ppmtogif, one had to supply the
transparency mask in a separate pseudo-PGM image (via the -alpha option).
Jef Poskanzer wrote ppmtogif in 1989, and it has always been a cornerstone of Pbmplus/Netpbm because GIF
is such a popular image format. Jef based the LZW encoding on GIFENCOD by David Rowley
<mgardi@watdcsu.waterloo.edu>. Jef included GIFENCOD's GIFCOMPR.C file pretty much whole. Rowley, in
turn, adapted the LZW compression code from classic Unix compress, which used techniques described in
IEEE Computer, June 1984.
Jef's ppmtogif notably lacked the ability to use a transparency mask with it. You could create
transparent pixels in a GIF, but only with the -transparent option, which allowed one to specify that all
pixels of a certain color in the input were to be transparent. Bryan Henderson added the -alpha option
in July 2001 so you could supply a mask image that indicates exactly which pixels are to be transparent,
and those pixels could have the same color as other opaque ones.
Bryan Henderson added another significant piece of code and function in October 2001: the ability to
generate a GIF without using the LZW patent -- an uncompressed GIF. This was very important to many
people at the time because the GIF patent was still in force, and this allowed them to make an image that
any GIF viewer could display, royalty-free. Bryan adapted code from the Independent JPEG Group's djpeg
for that.
There is no code in pamtogif from Jef's original, but Jef may still hold copyright over it because of the
way in which it evolved. Virtually all of the code in pamtogif was written by Bryan Henderson and
contributed to the public domain.
LICENSE
If you use pamtogif 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 pamtogif 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 pamtogif.
A replacement for the GIF format that never required any patents to use is the PNG format.
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master documentation
is at
http://netpbm.sourceforge.net/doc/pamtogif.html
netpbm documentation 09 June 2021 Pamtogif User Manual(1)