Netpbm format
From Wikipedia, the free encyclopedia
 |
This article or section is missing information about: The exact definition of Netpbm format, an explanation as to what it refers to, and the reason this article is called "Netpbm format" whilst it is describing formats none of which are explicitly called "Netpbm format".. This concern has been noted on the talk page where it may be discussed whether or not to include such information. (May 2009) |
| Filename extension | .ppm, .pgm, .pbm, .pnm |
|---|---|
| Internet media type | image/x-portable-pixmap, -graymap, -bitmap, -anymap all unofficial |
| Developed by | Jef Poskanzer |
| Type of format | Image file formats |
The phrase Netpbm format commonly refers to any or all of the members of a set of closely related graphics formats. The portable pixmap format (PPM), the portable graymap format (PGM) and the portable bitmap format (PBM) are image file formats originally designed to be easily exchanged between platforms. They are also sometimes referred to collectively as the portable anymap format (PNM)[1].
Contents |
[edit] History
The PBM format was invented by Jef Poskanzer in the 1980s as a format for monochrome bitmaps that could be transmitted within an email message as plain ASCII text, allowing it to survive any changes in text formatting.
The first library of tools to handle the PBM format was Pbmplus. It was developed by the format's creator, Jef Poskanzer, and released in 1988. It mainly contained tools, written by Jef, to convert between PBM and other already-existing graphics formats. By the end of 1988, Jef had developed the PGM and PPM formats along with their associated tools and added them to Pbmplus. The final release of Pbmplus was December 10, 1991.
In 1993, the Netpbm library was developed to replace the unmaintained Pbmplus. It was simply a repackaging of Pbmplus with additions and fixes submitted by people all over the world. It is probably the library package used most nowadays to deal with the PBM, PGM, and PPM formats.[2]
[edit] File format description
Each format differs in what colors it is designed to represent:
- PBM is monochrome
- PGM is grayscale
- PPM is RGB color.
Each file starts with a two-byte file descriptor (in ASCII) that explains the type of file it is (PBM, PGM, and PPM) and it's encoding (ASCII or binary). The descriptor is a capital P followed by a single digit number.
| File Descriptor | Type | Encoding |
|---|---|---|
P1 |
Portable bitmap | ASCII |
P2 |
Portable graymap | ASCII |
P3 |
Portable pixmap | ASCII |
P4 |
Portable bitmap | Binary |
P5 |
Portable graymap | Binary |
P6 |
Portable pixmap | Binary |
The ASCII based formats allow for human-readability and easy transport to other platforms (so long as those platforms understand ASCII), while the binary formats are more efficient both at saving space in the file, as well as being easier to parse due to the lack of whitespace.
When using the binary formats, PBM uses 1 bit per pixel, PGM uses 8 bits per pixel, and PPM uses 24 bits per pixel: 8 for red, 8 for green, 8 for blue.
[edit] PBM example
A simple example of the PBM format is as follows:
P1 # This is an example bitmap of the letter "J" 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 string P1 identifies the file format. The hash sign introduces a comment. 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).
Here is the resulting image: 
Here it is again magnified 20 times:
[edit] PGM example
The PGM and PPM formats (both ASCII and binary versions) have an additional parameter for the maximum value in a line between the X and Y dimensions and the actual pixel data.
P2 # Shows the word "FEEP" (example from Netpbm man page on PGM) 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
[edit] PPM example
P3 # The P3 means colors are in ASCII, then 3 columns and 2 rows, then 255 for max color, then RGB triplets 3 2 255 255 0 0 0 255 0 0 0 255 255 255 0 255 255 255 0 0 0
The image (expanded): 
The P6 binary format of the same image will store each color component of each pixel with one byte (thus three bytes per pixel) in the order red, green, then blue. The file will be smaller but the color information will not be readable by humans.
The PPM format is not compressed and thus requires more space and bandwidth than a compressed format would require. For example, the above 192x128 PNG image has a file size of 187 bytes. When converted to a 192x128 PPM image, the file size is 73848 bytes. The PPM format is generally an intermediate format used for image work before converting to a more efficient format, for example the PNG (Portable Network Graphics) format, without any loss of information in the intermediate step.
[edit] 16-bit extensions
The original definition of the PGM and the PPM binary formats (the P5 and P6 formats) did not allow bit depths greater than 8 bits. One can of course use the ASCII format, but this format both slows down reading and makes the files much larger. Accordingly, many programmers have attempted to extend the format to allow higher bit depths. Using higher bit depths encounters the problem of having to decide on the endianness of the file. Unfortunately it appears that the various implementations could not agree on which byte order to use, and some connected the 16-bit endianness to the pixel packing order.[3]. In Netpbm, the de facto standard implementation of the PNM formats, the most significant byte is first.
[edit] References
- ^ PBM, PGM, PNM, and PPM: Summary
- ^ Netpbm history
- ^ "Pnmtotiff User Manual". netpbm doc at SourceForge. 27 March 2005. http://netpbm.sourceforge.net/doc/pnmtotiff.html.
