PBMPlus files are created by Jef Poskanzer's PBMPlus Portable Bitmap Utilities. PBMPlus uses three different file types. These are the Bitmap (PBM), Portable Graymap (PGM), and Portable Pixmap (PPM) formats. PBM can store monochrome (black & white) bitmaps only. PGM additionally stores grayscale bitmaps, and PPM can store color bitmaps. There is also a fourth format; Portable Anymap (PNM). PNM is not a different file format in itself, but a file of type PNM can hold any of the three PBMPlus file types listed above.
The default extension is PPM.
Supporting Platforms and Applications
PBMPlus files are common under UNIX and on Intel-based PCs.
In conformance with Jef Poskanzer's specifications of 1990. The full specification is included below.
PPM Portable Pixmap File Format
The portable pixmap format is a lowest common denominator color image file format. The definition is as follows:
· A "magic number" for identifying the file type. A PPM file's magic number is the two characters "P3".
· Whitespace (blanks, TABs, CRs, LFs).
· A width, formatted as ASCII characters in decimal.
· A height, again in ASCII decimal.
· The maximum color-component value, again in ASCII decimal.
· Width * height pixels, each three ASCII decimal values between 0 and the specified maximum value, starting at the top-left corner of the pixmap, proceeding in normal English reading order. The three values for each pixel represent red, green, and blue, respectively; a value of 0 means that color is off, and the maximum value means that color is maxxed out.
· Characters from a "#" to the next end-of-line are ignored (comments).
· No line should be longer than 70 characters.
Here is an example of a small pixmap in this format:
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
Programs that read this format should be as lenient as possible, accepting anything that looks remotely like a pixmap. There is also a variant on the format, available by saving the image with the RAWBITS option. This variant is different in the following ways:
· The "magic number" is "P6" instead of "P3".
· The pixel values are stored as plain bytes, instead of ASCII decimal.
· Whitespace is not allowed in the pixels area, and only a single character of whitespace (typically a new line) is allowed after the maxval.
· The files are smaller and many times faster to read and write.
Note that this raw format can only be used for maxvals smaller than or equal to 255.