The PNM file (.pnm, .pbm, .pgm, .ppm, .pam)

PNM stands for Portable Anymap, a name for a small family of image formats that belong to Netpbm, an open-source toolkit for converting and editing images on the command line. Each format stores a picture as plainly as possible: a few lines of text saying how big the image is, then every pixel, uncompressed.

The formats go back to Jef Poskanzer, who designed PBM in the 1980s so that pictures could be sent inside plain-text email. In 1988 he released Pbmplus, a set of converters between PBM and other formats, and added PGM and PPM the same year. Netpbm replaced Pbmplus in 1993 and is still maintained; Bryan Henderson has looked after it since 1999. The PAM format arrived in August 2000.

• PBM (.pbm): black and white, one bit per pixel.
• PGM (.pgm): greyscale.
• PPM (.ppm): colour, red, green and blue per pixel.
• PAM (.pam): any number of channels, for example RGB plus alpha.
• PNM (.pnm): not a separate format, but the name for "PBM, PGM or PPM, whichever it is". Programs that accept PNM read all three.

Netpbm's own manual calls PPM a "lowest common denominator" format: it carries almost no metadata and wastes space, but any program can read or write it. That makes PNM a common go-between in image pipelines and a favourite in teaching, where students can write an image file in a few lines of code without a library.

The first two bytes of a Netpbm file say exactly what it is. Each of the three classic formats has a plain variant, with pixels written as decimal text, and a raw variant, with pixels in binary. Netpbm programs write raw files unless you ask for plain ones with -plain.

A PAM header uses names instead of positions, which makes it easy to extend:

P7
WIDTH 227
HEIGHT 149
DEPTH 3
MAXVAL 255
TUPLTYPE RGB
ENDHDR

TUPLTYPE names the meaning of the channels: BLACKANDWHITE, GRAYSCALE, RGB, or the same with _ALPHA added, where the last channel is opacity. Older tools that only know PNM can't read PAM, and the P7 magic number is also used by xv thumbnails, an unrelated format.

Both variants share the same header. The difference is only in the pixels:

Raw files are what tools normally write and expect. Plain files are for people: for debugging, for tests you can read in a diff, and for exercises where students write the pixels themselves. Readers are asked to be generous with plain files, so extra spaces and line breaks don't matter.

PPM's manual says the samples are gamma-encoded with the ITU-R BT.709 curve, but adds that many programs write sRGB or linear values instead. In practice, treat an 8-bit PPM like any other 8-bit RGB image. Because there's no colour profile field, the file can't tell you which it is.

Use PNM inside a pipeline, between tools, or when you need a format you can write without a library. Use PNG to keep or share the result: it's lossless too, much smaller, and opens everywhere. BMP is the nearest Windows equivalent; TIFF is the choice when metadata, 16-bit colour and compression all matter.

• Command-line conversion. Netpbm has hundreds of programs, most named after what they convert: pngtopam, pnmtopng, jpegtopnm, pnmscale. Each reads PNM, does one thing and writes PNM, so they chain with pipes: jpegtopnm photo.jpg | pnmscale 0.5 | pnmtopng > small.png.
• Document tools. Some programs write Netpbm by default. Poppler's pdftoppm renders PDF pages to PPM unless told otherwise, and DjVuLibre's ddjvu can write PNM, as on our DjVu page.
• Teaching and research. Image-processing courses use PPM and PGM because a student can write a blur filter or a ray tracer and save the result with a few printf calls.
• Tests and fixtures. A plain PNM file is a readable, diffable test image: the expected output of a filter can sit in a repository as text.
• Old Unix software and fax tools, where Netpbm was the standard way to move images between programs.

• On Windows: the built-in Photos app doesn't support Netpbm files. Use GIMP or IrfanView, or convert the file first.
• On macOS: GIMP opens all four formats; so do ImageMagick and the Netpbm tools, installed with Homebrew or MacPorts.
• On Linux: most image viewers read PNM, and the Netpbm tools are in every distribution's packages (usually netpbm).
• In a browser: no browser displays PNM. Convert it to PNG first.
• With ImageMagick: magick input.ppm output.png, or the other way round. Add -compress none to write a plain (P1, P2, P3) file.
• With Netpbm: pnmtopng input.ppm > output.png, pngtopam input.png > output.pam, or pnmtopnm -plain input.ppm > plain.ppm to turn a raw file into a plain one.
• In Python: Pillow reads and writes PBM, PGM and PPM: Image.open("input.ppm").save("output.png"). It doesn't support PAM.

• The image is skewed or shifted diagonally. The reader is out of step with the bytes, usually because a raw file was opened in text mode on Windows (which changes line endings) or because a writer put two whitespace characters after maxval instead of one.
• Colours are garbled in a 16-bit file. Two-byte samples are big-endian. A program that writes them little-endian, as the computer stores them in memory, produces noise. Swap the bytes or use a library.
• A PBM comes out inverted. In PBM, 1 is black. Code that treats 1 as white (as in PGM, or in PAM's black-and-white type) produces a negative.
• A program says "bad magic number". It may be a PAM (P7) file and the program only reads PNM, or a PFM (PF), or not a Netpbm file at all. Look at the first two bytes.
• Only the first image is read. Raw files can hold several images in a row, but most programs stop after the first.

A PNM file holds pixels and comments, nothing that runs. The risks are in the programs that read it:

• Huge dimensions. A header of a few bytes can claim an image of 100,000 × 100,000 pixels. A reader that allocates memory before checking the file length can be pushed into using gigabytes. Check width × height against a limit before decoding.
• Short files. A truncated raw file has fewer bytes than the header promises. Hand-written readers must check, not assume, how much data arrived.
• Comments travel with the file. They can contain a user name, a path or a command line. They're easy to miss and easy to strip: converting the file usually removes them.