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This program is part of Netpbm.
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 output is PPM.
A GIF image contains rectangular pixels. They all have the same aspect 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 otherwise, 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 informational message telling you to run pamscale to correct the output.
In addition to the options common to all programs based on libnetpbm (most notably -quiet, see Common Options), giftopnm recognizes the following command line options:
If you specify - as the filename, giftopnm writes the transparency output to Standard Output and discards the image.
See pamcomp for one way to use the transparency output file.
By default, giftopnm ignores comment extensions.
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.
In particular, when giftopnm detects that the GIF input is invalid 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 pixels 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 image, 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).
Two reasons not to use this option:
Two reasons to use this option:
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.
This does not correctly handle the Plain Text Extension of the GIF89 standard, since I did not have any example input files containing them.
Copyright (c) 1993 by David Koblas (koblas@netcom.com)
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 compression. 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 license to use is the PNG format.