1PPM Format Specification(5)   File Formats Manual  PPM Format Specification(5)
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NAME

6       PPM - Netpbm color image format
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DESCRIPTION

10       This program is part of Netpbm(1).
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12       The PPM format is a lowest common denominator color image file format.
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14       It  should be noted that this format is egregiously inefficient.  It is
15       highly redundant, while containing a lot of information that the  human
16       eye  can't  even  discern.   Furthermore, the format allows very little
17       information about the image besides basic color, which  means  you  may
18       have to couple a file in this format with other independent information
19       to get any decent use out of it.  However, it is very easy to write and
20       analyze programs to process this format, and that is the point.
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22       It  should  also  be  noted  that files often conform to this format in
23       every respect except the precise semantics of the sample values.  These
24       files are useful because of the way PPM is used as an intermediary for‐
25       mat.  They are informally called PPM files, but to be  absolutely  pre‐
26       cise,  you  should  indicate the variation from true PPM.  For example,
27       'PPM using the red, green, and blue colors that the scanner in question
28       uses.'
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30       The name "PPM" is an acronym derived from "Portable Pixel Map."  Images
31       in this format (or a precursor of it) were once also  called  "portable
32       pixmaps."
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34       The format definition is as follows.  You can use the libnetpbm(1)Csub‐
35       routinelibrarytoreadand interpret the  format  conveniently  and  accu‐
36       rately.
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38       A  PPM file consists of a sequence of one or more PPM images. There are
39       no data, delimiters, or padding before, after, or between images.
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41       Each PPM image consists of the following:
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45       ·      A 'magic number' for identifying the file type.  A  ppm  image's
46              magic number is the two characters 'P6'.
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48       ·
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50              Whitespace (blanks, TABs, CRs, LFs).
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52       ·
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54              A width, formatted as ASCII characters in decimal.
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56       ·
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58              Whitespace.
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60       ·
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62              A height, again in ASCII decimal.
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64       ·
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66              Whitespace.
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68       ·
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70              The  maximum color value (Maxval), again in ASCII decimal.  Must
71              be less than 65536 and more than zero.
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73
74       ·      A single whitespace character (usually a newline).
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77       ·      A raster of Height rows, in order from top to bottom.  Each  row
78              consists  of  Width  pixels,  in order from left to right.  Each
79              pixel is a triplet of red, green,  and  blue  samples,  in  that
80              order.  Each sample is represented in pure binary by either 1 or
81              2 bytes.  If the Maxval is less than 256, it is 1 byte.   Other‐
82              wise, it is 2 bytes.  The most significant byte is first.
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84              A  row  of  an  image is horizontal.  A column is vertical.  The
85              pixels in the image are square and contiguous.
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87              In the raster, the sample values are 'nonlinear.' They are  pro‐
88              portional  to  the  intensity of the ITU-R Recommendation BT.709
89              red, green, and blue in the pixel, adjusted by the BT.709  gamma
90              transfer  function.   (That  transfer function specifies a gamma
91              number of 2.2 and has a linear section for  small  intensities).
92              A value of Maxval for all three samples represents CIE D65 white
93              and the most intense color in the color universe  of  which  the
94              image  is  part  (the  color  universe  is all the colors in all
95              images to which this image might be compared).
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97              ITU-R Recommendation BT.709 is a renaming  of  the  former  CCIR
98              Recommendation  709.   When  CCIR  was  absorbed into its parent
99              organization, the ITU, ca. 2000, the standard was renamed.  This
100              document  once  referred to the standard as CIE Rec. 709, but it
101              isn't clear now that CIE ever sponsored such a standard.
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103              Note that another popular color space is the newer sRGB.  A com‐
104              mon  variation  on PPM is to substitute this color space for the
105              one specified.
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107              Note that a common variation on the PPM format is  to  have  the
108              sample  values be 'linear,' i.e. as specified above except with‐
109              out the gamma adjustment.  pnmgamma takes such a PPM variant  as
110              input and produces a true PPM as output.
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114       Strings starting with '#' may be comments, the same as with PBM(1).
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116       Note  that  you can use pamdepth to convert between a the format with 1
117       byte per sample and the one with 2 bytes per sample.
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119       There is actually another version of the  PPM  format  that  is  fairly
120       rare: 'plain' PPM format.  The format above, which generally considered
121       the normal one, is known as  the  'raw'  PPM  format.   See  pbm(1)for‐
122       somecommentaryonhowplain  and raw formats relate to one another and how
123       to use them.
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125       The difference in the plain format is:
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129       -      There is exactly one image in a file.
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131       -      The magic number is P3 instead of P6.
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133       -      Each sample in the raster is represented  as  an  ASCII  decimal
134              number (of arbitrary size).
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136       -      Each  sample  in the raster has white space before and after it.
137              There must be at least one character of white space between  any
138              two  samples,  but  there is no maximum.  There is no particular
139              separation of one pixel from another -- just the required  sepa‐
140              ration  between the blue sample of one pixel from the red sample
141              of the next pixel.
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143       -      No line should be longer than 70 characters.
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145
146       Here is an example of a small image in this format.
147       P3
148       # feep.ppm
149       4 4
150       15
151        0  0  0    0  0  0    0  0  0   15  0 15
152        0  0  0    0 15  7    0  0  0    0  0  0
153        0  0  0    0  0  0    0 15  7    0  0  0
154       15  0 15    0  0  0    0  0  0    0  0  0
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156       There is a newline character at the end of each of these lines.
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158       Programs that read this  format  should  be  as  lenient  as  possible,
159       accepting anything that looks remotely like a PPM image.
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161       All  characters  referred  to  herein  are encoded in ASCII.  'newline'
162       refers the the character known in ASCII as Line Feed or LF.   A  'white
163       space'  character  is space, CR, LF, TAB, VT, or FF (I.e. what the ANSI
164       standard C isspace() function calls white space).
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COMPATIBILITY

169       Before April 2000, a raw format  PPM  file  could  not  have  a  maxval
170       greater than 255.  Hence, it could not have more than one byte per sam‐
171       ple.  Old programs may depend on this.
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173       Before July 2000, there could be at most one image in a PPM file.  As a
174       result,  most  tools  to  process PPM files ignore (and don't read) any
175       data after the first image.
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SEE ALSO

179       pnm(1), pgm(1), pbm(1), pam(1), programsthatprocessPPM(1)
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183netpbm documentation            03 October 2003    PPM Format Specification(5)