Ubuntu Manpage: pamtogif - convert a Netpbm image to a GIF image

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 ⟨http://www.lcdf.org/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 ⟨index.html#commonoptions⟩ ), 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 ⟨libnetpbm_image.html#colorname⟩ .

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 ⟨libnetpbm_image.html#colorname⟩ .

-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.

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