PGM File Format
   
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    GraphicsAcademy.com  >  File Formats  >  Portable Gray Map

       

     

    PGM File Format

    Short Name PGM
    Long Name Portable Gray Map
    Typical File Extensions .pgm .pnm
    Contains Grayscale Bitmap
    The is one of a series of formats, for storing bitmaps, developed by Jef Poskanzer for the pbmplus image toolkit also known as the "enhanced portable bitmap toolkit". The others are PBM (Portable Bitmap) and PPM (Portable Pixel Map).

    These pbmplus utilities work by providing conversion to and from more common formats, and thus by using PBM/PGM/PPM as an intermediate, it is possible to convert images between a wide range of formats.

    For example, to convert from GIF to TIFF, the following sequence of commands is used:
    • giftoppm converts a GIF image to PPM.

    • pnmtotiff converts the PPM to a TIFF.
    The naming conventions used by the various programs in the pbmplus suite, reflects each program's functions:
    • PBM stands for Portable Bitmap (a monochrome bitmap).

    • PGM stands for Portable Gray Map (a gray scale bitmap).

    • PPM stands for Portable Pixel Map (a color bitmap).

    • PNM stands for Portable aNy Map, and indicates a program will work with PGM, PBM and PPM. Additionally, although files are usually named with an extension to indicate their specific type (i.e. .pbm for PBM files, .pgm for PGM files, and .ppm for PPM files), however sometimes the extension .pnm may be generically used to indicate a particular file is in one of these formats - and that the specific type must be identified by reading the file's contents.
    All formats in this set are not compressed, and can in fact be stored in two possible representations:
    1. An ASCII text representation (which is extremely verbose).

      Please Note:

      • For PGM files, when stored in the ASCII representation, there is a limit of 256 levels of gray.

      • For PPM files, when stored in the ASCII representation, there is a limit on the color range, effectively equivalent to 24 bit True Color.

    2. A binary representaton (which is comparatively smaller).
    A technical discussion of this format can be found in Chapter 14 of Graphics File Formats by David C. Kay and John R. Levine.


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