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 polarized 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 delineated 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.

P6 1024 788 255

P6 
1024 788 
# A comment
255

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.

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.

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".

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

Question

I came across your name on the web while browsing for PPM file format 
and viewer. I really appreciate if you can answer my question.
 
I want a simple image file format in RGB with color depth more than 8 bits 
(MaxVal more than 255 in PPM file format). I created a simple file as shown 
below in 10 bits per color. My question is about which viewer can display 
the 10 bits? I used xnview and Irfanview, both of them truncate color to 
8 bits. Can you suggest me a viewer which can display image without any manipulation?
---
P3
# example.ppm
4 4
1023
0  0  0    0  0  0    0  0  0   1023  0 1023
0  0  0    0 1023  70    0  0  0    0  0  0
0  0  0    0  0  0    0 1023  70    0  0  0
1023  0 1023    0  0  0    0  0  0    0  0  0
---
 
Please do not hesitate me to suggest other image file format which can support 
more than 8 bits per color and associated viewer/tool.

I would not base a solution on the ppm, pbm, etc range of
formats. They are very old, having been developed in the 80's, are not
and probably never will be well supported, and certainly were never
intended for higher bit depth images even though there have been
binary variants (P5 and P6) since that do support higher bit depths.

If I want higher bit depths then I generally write to a boring raw
format and open/edit the images in PhotoShop which has a very general
raw reader. Sure it isn't free software but sometimes the savings from
good quality software that meets your needs is a cost effective
solution in time saving.

Unfortunately at the time of writing GIMP doesn't support 16bit colour
although I believe that is coming in version 3.0. Other free options,
untested by the author with respect to reading 16 bit raw images are
"krita" and "cinepaint", I'm sure there are others.

Once read into PhotoShop I would generally save in a standard file
format, my preference would be TIFF or PNG. Of course you can write
directly to those formats yourself using the libtiff or libpng library
API's, but of course that is a level of additional learning and
development that may exceed the time constraints of your project.