The portable pixmap / greymap / bitmap file formats (PPM, PGM, PBM) are file formats for exchanging graphics files. They are very basic and serve as a least common denominator for converting pixmap, greymap , or bitmap files between different platforms. They are used under the collective name PNM format in the Netpbm file conversion package.

Let's take the example of the letter J from the bitmap article

....X.
....X.
....X.
....X.
....X.
....X.
X...X.
.XXX..
......
......

It is represented in the most basic (monochrome) PBM format as follows

P1
# This is an example bit map file j.pbm
6 10
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 0
0 0 0 0 1 0
1 0 0 0 1 0
0 1 1 1 0 0
0 0 0 0 0 0
0 0 0 0 0 0

The P1 is the identificaton of the file format. The hash sign marks a commentary. And the next two numbers give the width and the height. Then follows the matrix with the pixel values (in the monochrome case here only zeros and ones).

This file can be converted by two conversion programs from the Netpbm package for example to a bmp file:

pgmtoppm "#FFFFFF" j.pbm  > j.ppm
ppmtobmp j.ppm > j.bmp

Depending on the identification of the file format three similar file formats are distinguished, each with two versions:

• pbm - portable bitmap file format (P1/P4) - 1 bit per pixel
• pgm - portable graymap file format (P2/P5) - 8 bits per pixel
• ppm - portable pixmap file format (P3/P6) - 24 bits per pixel, 8 for red, 8 for green, 8 for blue

In each case, the lower-numbered version refers to a human-readable, ASCII-based format similar to the one in the example above and the higher-numbered version refers to a binary format which is not human-readable but saves some space in the file, as well as being easier to parse due to the lack of whitespace.

External link

• File format details for all 3 formats:
  • http://www-info2.informatik.uni-wuerzburg.de/mitarbeiter/wolfram/lehre/bildformate.html