giftoppm

giftoppm [options] [giffile]

-b color_index
    Specifies the background color to use for transparent pixels. color_index is an integer between 0 and 255, representing an index in the GIF's color map. The default is 0.

-t color_index
    Forces the given color_index to be treated as transparent, even if the GIF file specifies a different transparency index (or none).

-s
    (Superimposed) This option is mainly for interlaced GIFs. It uses a "superimposed" algorithm to fill in the missing scanlines for an interlaced GIF, which might result in a slightly different appearance.

-n
    (No background) Prevents giftoppm from filling transparent areas with the background color. Transparent pixels will be output as black (RGB 0,0,0).

-g
    (Gamma) Enables gamma correction for older GIF files, potentially adjusting the brightness or contrast of the output.

-O
    (Old format) Handles very old GIF files that do not explicitly specify a global color map, assuming a default 256-color map.

-a
    Outputs a PAM (Portable Arbitrary Map) file instead of PPM. A PAM file can include an alpha channel, thereby preserving the transparency information from the GIF.

-m
    Maps the GIF's 256-color palette to a more general set of 256 Netpbm colors, potentially optimizing the output color space.

-S
    If the input GIF is an multi-image (animated) GIF, this option ensures that only the first image in the sequence is processed and converted.

The giftoppm command is a utility from the Netpbm project, designed to convert images from the GIF (Graphics Interchange Format) file format to the PPM (Portable PixMap) format. PPM is a simple, uncompressed image format that stores full-color pixel data, making it a suitable intermediate format for further image manipulation within the Netpbm suite or other image processing tools.

giftoppm reads a GIF image from a specified file or standard input and writes the corresponding PPM image to standard output. While GIF is an indexed color format supporting up to 256 colors and optional transparency, PPM is a true-color format. giftoppm handles this conversion, including mapping GIF's color palette to PPM's direct color representation and managing transparency based on command-line options. It is widely used in scripting environments for batch image processing or as part of a larger image conversion pipeline.

• GIF Patent History: Historically, GIF was encumbered by LZW compression patents. While these patents have expired, some older software might still have patent-related restrictions. giftoppm and Netpbm generally handle this transparently.
• Transparency Handling: Converting GIF's indexed transparency to a true-color format like PPM can sometimes be tricky. The -b (background color) and -n (no background) options provide control over how transparent areas are rendered. Using -a to output PAM is the most robust way to preserve full alpha information.
• Multi-Image GIFs: giftoppm by default, or with the -S option, only processes the first frame of a multi-image (animated) GIF. To process all frames, other Netpbm tools or scripting might be required (e.g., using gifsicle).
• Color Depth: GIF images are limited to 256 colors. While giftoppm converts them to a true-color PPM format, the original image quality (in terms of color richness) is limited by the GIF source.

Like many Netpbm tools, giftoppm is designed to work efficiently with standard input and output. This allows it to be easily chained with other commands in a Unix pipeline (e.g., cat input.gif | giftoppm | ppmtopng > output.png).

giftoppm provides informative error messages if the input file is not a valid GIF or if options are used incorrectly.

giftoppm is an integral part of the Netpbm suite of graphics manipulation tools. The Netpbm project originated from Pbmplus utilities developed by Jef Poskanzer in the late 1980s. Its primary aim was to provide a robust, open-source set of tools for converting and manipulating various image formats using a simple, common intermediate format (PBM, PGM, PPM). giftoppm was specifically created to handle the then-proprietary GIF format, enabling users to convert GIFs into the open PPM format for further processing, at a time when GIF was a very popular but also somewhat controversial format due to its LZW compression patent. Its development reflects the open-source community's commitment to interoperability and freedom from proprietary restrictions.

giftopnm(1), ppmtogif(1), pnm(5), ppm(5), gif(5), netpbm(1)