PPM / PGM / PBM image files

This note describes the format of PPM (Portable PixMap), PGM (Portable GreyMap), PBM (Portable BitMap) files. These formats are a convenient (simple) method of saving image data, they are equally easy to read in ones own applications. Unfortunately the standards aren't always implemented as well as they could.

These formats were popularised by the pbmplus image toolkit otherwise known as the "enhanced portable bitmap toolkit". The description of the toolkit from the man page is given below

DESCRIPTION
     The pbmplus toolkit allows conversions between  image  files
     of  different format.  By means of using common intermediate
     formats, only 2*N conversion filters are required to support
     N  distinct  formats,  instead  of  the  N^2  which would be
     required to convert directly between any one format and  any
     other.   The  package also includes simple tools for manipu-
     lating portable bitmaps.

     The package consists of four upwardly compatible sections:

     pbm  Supports monochrome bitmaps (1 bit per pixel).

     pgm  Supports greyscale images.  Reads  either  pbm  or  pgm
          formats and writes pgm format.

     ppm  Supports full-color images.  Reads either pbm, pgm,  or
          ppm formats, writes ppm format.

     pnm  Supports content-independent manipulations  on  any  of
          the  three  formats  listed  above, as well as external
          formats having multiple types.  Reads either pbm,  pgm,
          or  ppm  formats, and generally writes the same type as
          it read (whenever a pnm tool  makes  an  exception  and
          "promotes"  a  file  to a higher format, it informs the
          user).

PPM
A PPM file consists of two parts, a header and the image data. The header consists of at least three parts normally delinineated by carriage returns and/or linefeeds but the PPM specification only requires white space. The first "line" is a magic PPM identifier, it can be "P3" or "P6" (not including the double quotes!). The next line consists of the width and height of the image as ascii numbers. The last part of the header gives the maximum value of the colour components for the pixels, this allows the format to describe more than single byte (0..255) colour values. In addition to the above required lines, a comment can be placed anywhere with a "#" character, the comment extends to the end of the line.

The following are all valid PPM headers.

Header example 1

P6 1024 788 255

Header example 2

P6
1024 788
# A comment
255

Header example 3

P3
1024 # the image width
788 # the image height
# A comment
1023

The format of the image data itself depends on the magic PPM identifier. If it is "P3" then the image is given as ascii text, the numerical value of each pixel ranges from 0 to the maximum value given in the header. The lines should not be longer than 70 characters.

PPM example 4

P3
# example from the man page
4 4
15
 0  0  0    0  0  0    0  0  0   15  0 15
 0  0  0    0 15  7    0  0  0    0  0  0
 0  0  0    0  0  0    0 15  7    0  0  0
15  0 15    0  0  0    0  0  0    0  0  0

If the PPM magic identifier is "P6" then the image data is stored in byte format, one byte per colour component (r,g,b). Comments can only occur before the last field of the header and only one byte may appear after the last header field, normally a carriage return or line feed. "P6" image files are obviously smaller than "P3" and much faster to read. Note that "P6" PPM files can only be used for single byte colours.

While not required by the format specification it is a standard convention to store the image in top to bottom, left to right order. Each pixel is stored as a byte, value 0 == black, value 255 == white. The components are stored in the "usual" order, red - green - blue.

PGM
This format is identical to the above except it stores greyscale information, that is, one value per pixel instead of 3 (r,g,b). The only difference in the header section is the magic identifiers which are "P2" and "P5", these correspond to the ascii and binary form of the data respectively.

PGM example
An example of a PGM file of type "P2" is given below

P2
24 7
15
0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
0  3  3  3  3  0  0  7  7  7  7  0  0 11 11 11 11  0  0 15 15 15 15  0
0  3  0  0  0  0  0  7  0  0  0  0  0 11  0  0  0  0  0 15  0  0 15  0
0  3  3  3  0  0  0  7  7  7  0  0  0 11 11 11  0  0  0 15 15 15 15  0
0  3  0  0  0  0  0  7  0  0  0  0  0 11  0  0  0  0  0 15  0  0  0  0
0  3  0  0  0  0  0  7  7  7  7  0  0 11 11 11 11  0  0 15  0  0  0  0
0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0

PBM
PBM stores single bit pixel image as a series of ascii "0" or "1"'s. Traditionally "0" refers to white while "1" refers to black. The header is identical to PPM and PGM format except there is no third header line (the maximum pixel value doesn't have any meaning. The magic identifier for PBM is "P1".

PBM example
Here is an example of a small bitmap in this format, as with PPM files there can be no more than 70 characters per line.

P1
# PBM example
24 7
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 1 1 1 1 0 0 1 1 1 1 0 0 1 1 1 1 0 0 1 1 1 1 0
0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 1 0
0 1 1 1 0 0 0 1 1 1 0 0 0 1 1 1 0 0 0 1 1 1 1 0
0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0
0 1 0 0 0 0 0 1 1 1 1 0 0 1 1 1 1 0 0 1 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0