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.
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 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.
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
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.
Response
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.