{ "mode": "man", "parameter": "pgm", "section": "5", "url": "https://www.chedong.com/phpMan.php/man/pgm/5/json", "generated": "2026-11-14T21:37:11Z", "sections": { "NAME": { "content": "pgm - Netpbm grayscale image format\n\n", "subsections": [] }, "DESCRIPTION": { "content": "This program is part of Netpbm(1).\n\nThe PGM format is a lowest common denominator grayscale file format. It is designed to be\nextremely easy to learn and write programs for. (It's so simple that most people will simply\nreverse engineer it because it's easier than reading this specification).\n\nA PGM image represents a grayscale graphic image. There are many pseudo-PGM formats in use\nwhere everything is as specified herein except for the meaning of individual pixel values.\nFor most purposes, a PGM image can just be thought of an array of arbitrary integers, and all\nthe programs in the world that think they're processing a grayscale image can easily be\ntricked into processing something else.\n\nThe name \"PGM\" is an acronym derived from \"Portable Gray Map.\"\n\nOne official variant of PGM is the transparency mask. A transparency mask in Netpbm is rep‐\nresented by a PGM image, except that in place of pixel intensities, there are opaqueness val‐\nues. See below.\n\n", "subsections": [] }, "THE FORMAT": { "content": "The format definition is as follows. You can use the libnetpbm(1) C subroutine library to\nconveniently and accurately read and interpret the format.\n\nA PGM file consists of a sequence of one or more PGM images. There are no data, delimiters,\nor padding before, after, or between images.\n\nEach PGM image consists of the following:\n\n\n\n\n• A \"magic number\" for identifying the file type. A pgm image's magic number is the two\ncharacters \"P5\".\n\n\n• Whitespace (blanks, TABs, CRs, LFs).\n\n\n• A width, formatted as ASCII characters in decimal.\n\n\n• Whitespace.\n\n\n• A height, again in ASCII decimal.\n\n\n• Whitespace.\n\n\n• The maximum gray value (Maxval), again in ASCII decimal. Must be less than 65536, and\nmore than zero.\n\n\n• A single whitespace character (usually a newline).\n\n\n• A raster of Height rows, in order from top to bottom. Each row consists of Width gray\nvalues, in order from left to right. Each gray value is a number from 0 through Max‐\nval, with 0 being black and Maxval being white. Each gray value is represented in\npure binary by either 1 or 2 bytes. If the Maxval is less than 256, it is 1 byte.\nOtherwise, it is 2 bytes. The most significant byte is first.\n\nA row of an image is horizontal. A column is vertical. The pixels in the image are\nsquare and contiguous.\n\nEach gray value is a number proportional to the intensity of the pixel, adjusted by\nthe ITU-R Recommendation BT.709 gamma transfer function. (That transfer function\nspecifies a gamma number of 2.2 and has a linear section for small intensities). A\nvalue of zero is therefore black. A value of Maxval represents CIE D65 white and the\nmost intense value in the image and any other image to which the image might be com‐\npared.\n\nBT.709's range of channel values (16-240) is irrelevant to PGM.\n\nNote that a common variation from the PGM format is to have the gray value be \"lin‐\near,\" i.e. as specified above except without the gamma adjustment. pnmgamma takes\nsuch a PGM variant as input and produces a true PGM as output.\n\nAnother popular variation from PGM is to substitute the newer sRGB transfer function\nfor the BT.709 one. You can use pnmgamma to convert between this variation and true\nPGM.\n\nIn the transparency mask variation from PGM, the value represents opaqueness. It is\nproportional to the fraction of intensity of a pixel that would show in place of an\nunderlying pixel. So what normally means white represents total opaqueness and what\nnormally means black represents total transparency. In between, you would compute the\nintensity of a composite pixel of an \"under\" and \"over\" pixel as under * (1-(alpha/al‐\nphamaxval)) + over * (alpha/alphamaxval). Note that there is no gamma transfer\nfunction in the transparency mask.\n\n\n\nStrings starting with \"#\" may be comments, the same as with PBM(1).\n\nNote that you can use pamdepth to convert between a the format with 1 byte per gray value and\nthe one with 2 bytes per gray value.\n\nAll characters referred to herein are encoded in ASCII. \"newline\" refers to the character\nknown in ASCII as Line Feed or LF. A \"white space\" character is space, CR, LF, TAB, VT, or\nFF (I.e. what the ANSI standard C isspace() function calls white space).\n\n", "subsections": [ { "name": "Plain PGM", "content": "There is actually another version of the PGM format that is fairly rare: \"plain\" PGM format.\nThe format above, which generally considered the normal one, is known as the \"raw\" PGM for‐\nmat. See pbm(1) for some commentary on how plain and raw formats relate to one another and\nhow to use them.\n\nThe difference in the plain format is:\n\n\n\n•\n\nThere is exactly one image in a file.\n\n•\n\nThe magic number is P2 instead of P5.\n\n•\n\nEach pixel in the raster is represented as an ASCII decimal number (of arbitrary\nsize).\n\n•\n\nEach pixel in the raster has white space before and after it. There must be at least\none character of white space between any two pixels, but there is no maximum.\n\n•\n\nNo line should be longer than 70 characters.\n\n\nHere is an example of a small image in the plain PGM format.\n\nP2\n# feep.pgm\n24 7\n15\n0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0\n0 3 3 3 3 0 0 7 7 7 7 0 0 11 11 11 11 0 0 15 15 15 15 0\n0 3 0 0 0 0 0 7 0 0 0 0 0 11 0 0 0 0 0 15 0 0 15 0\n0 3 3 3 0 0 0 7 7 7 0 0 0 11 11 11 0 0 0 15 15 15 15 0\n0 3 0 0 0 0 0 7 0 0 0 0 0 11 0 0 0 0 0 15 0 0 0 0\n0 3 0 0 0 0 0 7 7 7 7 0 0 11 11 11 11 0 0 15 0 0 0 0\n0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0\n\n\nThere is a newline character at the end of each of these lines.\n\nPrograms that read this format should be as lenient as possible, accepting anything that\nlooks remotely like a PGM.\n\n\n" } ] }, "INTERNET MEDIA TYPE": { "content": "No Internet Media Type (aka MIME type, content type) for PGM has been registered with IANA,\nbut the value image/x-portable-graymap is conventional.\n\nNote that the PNM Internet Media Type image/x-portable-anymap also applies.\n\n\n", "subsections": [] }, "FILE NAME": { "content": "There are no requirements on the name of a PGM file, but the convention is to use the suffix\n\".pgm\". \"pnm\" is also conventional, for cases where distinguishing between the particular\nsubformats of PNM is not convenient.\n\n\n", "subsections": [] }, "COMPATIBILITY": { "content": "Before April 2000, a raw format PGM file could not have a maxval greater than 255. Hence, it\ncould not have more than one byte per sample. Old programs may depend on this.\n\nBefore July 2000, there could be at most one image in a PGM file. As a result, most tools to\nprocess PGM files ignore (and don't read) any data after the first image.\n\n", "subsections": [] }, "SEE ALSO": { "content": "pnm(1), pbm(1), ppm(1), pam(1), libnetpbm(1), programs that process PGM(1),\n\n", "subsections": [] }, "AUTHOR": { "content": "Copyright (C) 1989, 1991 by Jef Poskanzer.\n", "subsections": [] }, "DOCUMENT SOURCE": { "content": "This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master\ndocumentation is at\n\nhttp://netpbm.sourceforge.net/doc/pgm.html\n\nnetpbm documentation 09 October 2016 PGM Format Specification(5)", "subsections": [] } }, "summary": "pgm - Netpbm grayscale image format", "flags": [], "examples": [], "see_also": [ { "name": "pnm", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/pnm/1/json" }, { "name": "pbm", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/pbm/1/json" }, { "name": "ppm", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/ppm/1/json" }, { "name": "pam", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/pam/1/json" }, { "name": "libnetpbm", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/libnetpbm/1/json" }, { "name": "PGM", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/PGM/1/json" } ] }