Giftopnm User Manual(1) General Commands Manual Giftopnm User Manual(1)
NAME
giftopnm - convert a GIF file into a PNM image
SYNOPSIS
giftopnm [--alphaout={alpha-filename,-}] [-verbose] [-comments] [-im-
age={N,all}] [-repair] [-quitearly] [GIFfile]
Minimum unique abbreviation of option is acceptable. You may use dou-
ble hyphens instead of single hyphen to denote options. You may use
white space in place of the equals sign to separate an option name from
its value.
DESCRIPTION
This program is part of Netpbm(1).
This is a graphics format converter from the GIF format to the PNM
(i.e. PBM, PGM, or PPM) format.
If the image contains only black and maximally bright white, the output
is PBM. If the image contains more than those two colors, but only
grays, the output is PGM. If the image contains other colors, the out-
put is PPM.
A GIF image contains rectangular pixels. They all have the same as-
pect ratio, but may not be square (it's actually quite unusual for them
not to be square, but it could happen). The pixels of a Netpbm image
are always square. Because of the engineering complexity to do other-
wise, giftopnm converts a GIF image to a Netpbm image pixel-for-pixel.
This means if the GIF pixels are not square, the Netpbm output image
has the wrong aspect ratio. In this case, giftopnm issues an informa-
tional message telling you to run pamscale to correct the output.
OPTIONS
In addition to the options common to all programs based on libnetpbm
(most notably -quiet, see
Common Options ⟨index.html#commonoptions⟩ ), giftopnm recognizes the
following command line options:
--alphaout=alpha-filename
giftopnm creates a PBM file containing the transparency infor-
mation from the input image. This transparency image is the
same dimensions as the input image, and each pixel of the trans-
parency image tells whether the corresponding pixel of the input
image is transparent. Black means transparent; white means
opaque. If you don't specify --alphaout, giftopnm does not gen-
erate a transparency file, and if the input image has a trans-
parency channel, giftopnm simply discards it.
If you specify - as the filename, giftopnm writes the transpar-
ency output to Standard Output and discards the image.
See pamcomp(1) for one way to use the transparency output file.
-verbose
Produce verbose output about the GIF file input.
-comments
With this option, giftopnm issues messages showing the GIF com-
ments (A GIF89 stream can contain comments in comment exten-
sions).
By default, giftopnm ignores comment extensions.
-image={N,all}
This option identifies which image from the GIF stream you want.
You can select either one image or all the images. Select all
the images with all. Select one image by specifying its se-
quence number in the stream: 1, 2, 3, etc.
The default is just Image 1.
A GIF stream normally contains only one image, so you don't need
this option. But some streams, including animated GIFs, have
multiple images.
When you select multiple GIF images, the output is a PNM stream
with multiple images.
If you specify a single image, giftopnm must read and partially
validate the images before that in the stream. It may or may
not do the same for the images after it; see -quitearly.
The all value was added in Netpbm 10.16 (June 2003). Earlier
giftopnm can extract only one image.
-repair
This option makes giftopnm try to salvage what it can from an
invalid GIF input.
In particular, when giftopnm detects that the GIF input is in-
valid so that it is impossible to determine what the pixels are
intended to be, it produces a single arbitrary color for all
further pixels in the image. giftopnm processes the image from
top to bottom, left to right, so this means the bottommost pix-
els will be this padding.
giftopnm issues warning messages when it salvages an image in
this way.
Without this option, giftopnm fails when it detects invalid GIF
input. Any output it produces is arbitrary, and typically is
not a valid PNM image.
It is fairly common for an image to be corrupted such that is
started off as a valid GIF, but had the end of the file cut off.
An interrupted network transfer tends to do this. In this case,
giftopnm's salvage operation will produce a valid PNM image of
the proper dimensions, but with a single arbitrary color for the
pixels that were left out of the file.
This option was new in Netpbm 10.38 (March 2007). From 10.32
through 10.37, giftopnm always fails if it detects invalid GIF
input. Before 10.32, it succeeds in the case of a truncated im-
age, and replaces the missing pixels with arbitrary colors, not
necessarily all the same (The pre-10.32 behavior wasn't actually
intended by the design).
-quitearly
This option makes giftopnm stop reading its input file as soon
as it has converted and output the images from the input that
you requested. By default, giftopnm reads until the end of the
GIF stream, ignoring any data after the images you requested.
Two reasons not to use this option:
• The input file is a pipe and the process that is filling that
pipe expects the pipe to take the entire stream and will fail or
get stuck if it doesn't.
• You want to validate the entire GIF stream.
Two reasons to use this option:
• It saves the time and other resources to read the end of the
stream.
• There are errors in the end of the stream that make giftopnm
fail.
This option has no effect if you also specify -image=all
This option was new in Netpbm 10.35 (August 2006). Before that,
giftopnm always reads the entire stream.
RESTRICTIONS
This does not correctly handle the Plain Text Extension of the GIF89
standard, since I did not have any example input files containing them.
SEE ALSO
pamtogif(1), ppmcolormask(1), pamcomp(1), http://www.lcdf.org/gifsicle
⟨http://www.lcdf.org/gifsicle⟩ , ppm(1).
AUTHOR
Copyright (c) 1993 by David Koblas (koblas@netcom.com)
LICENSE
As a historical note, for a long time if you used giftopnm, you were
using a patent on the LZW compression method which was owned by Unisys,
and in all probability you did not have 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, and
made statements that it is much less concerned about people using the
patent for decompression (which is what giftopnm does than for compres-
sion. The patent expired in 2003.
Rumor has it that IBM also owns a patent covering giftopnm.
A replacement for the GIF format that has never required any patent li-
cense 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/giftopnm.html
netpbm documentation 13 September 2012 Giftopnm User Manual(1)
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