Netpbm header: P1 to P6, width, height and maxval

A Netpbm file starts with a short header written as text. It gives the type, the size and, except for black and white images, the largest sample value. The raster follows it directly.

Magic numbers

Magic	Bytes	Format	Name	Raster	Usual extension
`P1`	`50 31`	Plain PBM	Bitmap	ASCII digits	`.pbm`
`P2`	`50 32`	Plain PGM	Greyscale	ASCII decimal	`.pgm`
`P3`	`50 33`	Plain PPM	Colour	ASCII decimal	`.ppm`
`P4`	`50 34`	Raw PBM	Bitmap	Packed bits	`.pbm`
`P5`	`50 35`	Raw PGM	Greyscale	Binary samples	`.pgm`
`P6`	`50 36`	Raw PPM	Colour	Binary samples	`.ppm`
`P7`	`50 37`	PAM	General	Binary samples	`.pam`

.pnm is used when a file may be any of PBM, PGM or PPM. None of these formats has a registered Internet media type; the usual informal ones are image/x-portable-bitmap, image/x-portable-graymap, image/x-portable-pixmap and image/x-portable-anymap.

Header fields

Order	Field	Form	In	Description
1	magic	2 ASCII characters	all	One of the values above.
2	whitespace	1 or more	all	Separates the fields.
3	width	ASCII decimal	all	Width in pixels.
4	whitespace	1 or more	all
5	height	ASCII decimal	all	Height in pixels.
6	whitespace	1 or more	PGM, PPM	Only before maxval.
7	maxval	ASCII decimal	PGM, PPM	The largest sample value: greater than 0 and less than 65536. PBM has no maxval.
8	whitespace	exactly 1	all	One character, usually a newline, ends the header. The raster starts at the next byte.

In a raw file the single whitespace character matters: any further byte belongs to the raster, even if it looks like whitespace.

Whitespace

Byte	Name
`20`	Space
`09`	Tab
`0A`	Line feed
`0D`	Carriage return
`0B`	Vertical tab
`0C`	Form feed

The PBM and PGM pages give all six, the set C’s isspace() recognises. The PPM page lists only blanks, tabs, carriage returns and line feeds.

Comments

A # starts a comment that runs to the next carriage return or line feed, or to the end of the file. Comments are ignored and can appear only before the single whitespace character that ends the header. Before June 2024 the specification said that the line end was part of the comment, although Netpbm never worked that way.

Maxval and sample size

Maxval	Bytes per raw sample
1 to 255	1
256 to 65535	2, most significant byte first

Before April 2000 a raw file could not have a maxval above 255.

Several images in one file

A raw file may hold more than one image, placed back to back with nothing between them; each has its own header. Before July 2000 a file could hold only one, and many programs still ignore everything after the first. A plain file holds exactly one image.

Widest compatibility

The specification says readers should be lenient. For the simplest readers, write only the magic and a newline, width and height separated by one space and a newline, a maxval of 255 and a newline where there is one, then the raster, with one image per file.

.PPM File Format – Header