PGM file format (.pgm)

What is a PGM file?

In short: A PGM file (.pgm) is an image file in the PGM format. PGM stores a single brightness value per pixel and nothing else. It can be converted to JPG, PNG or WEBP for free in a web browser with iHateFiles, without uploading it.

The greyscale member of the Netpbm family, from the same late-1980s Unix toolkit.

PGM stores a single brightness value per pixel and nothing else. It is used wherever colour is irrelevant or would get in the way: depth maps, height fields, masks, medical and scientific imaging, and computer vision test data.

What PGM is good at

• Trivially simple to read and write
• Lossless, with 8 or 16 bits per pixel
• A third the size of the colour equivalent

Where PGM falls short

• No colour, and converting to it discards colour permanently
• No compression
• Not viewable in a browser

When to use PGM

Use PGM for masks, depth data and computer vision work where a single channel is what you want.

How to open a PGM file

Scientific and image processing tools.

The thing that usually goes wrong

Converting a colour image to PGM throws the colour away for good. Keep the original if you might want it back.

Inside a PGM file

PGM is the greyscale member of the Netpbm family: identical to PPM in every respect except that each pixel is one value rather than three. The header is P5 and then the same plain-text width, height and maximum value.

Its real home is computer vision and medical imaging, where images are frequently single-channel to begin with and where a format that adds nothing between the algorithm and the data is a feature. OpenCV, ImageJ and most academic imaging tools read and write it without a thought.