PGM file (Portable Graymap)
Netpbm greyscale bitmap.
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Convert PGM to JPG, free and without uploading it.
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.
| Windows | IrfanView or GIMP. |
|---|---|
| macOS | GIMP or ImageMagick; Preview will not. |
| Linux | GIMP, ImageMagick and the netpbm tools. |
| Phone | Needs a dedicated viewer. |
| In a browser | No browser displays PGM. |
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.
| Full name | Portable Graymap |
|---|---|
| Magic bytes | P5 for binary, P2 for ASCII |
| Channels | One — greyscale only |
| Bit depth | 8 or 16 bits per pixel |
| Compression | None |
| Common in | Computer vision, medical and scientific imaging |
Not sure the file really is a PGM? Identify it by its contents, whatever its extension says.
Questions
What is a PGM file?
PGM stands for Portable Graymap. Netpbm greyscale bitmap. Files use the .pgm extension.
What does PGM stand for?
PGM stands for Portable Graymap.
How do I open a PGM file?
On Windows: IrfanView or GIMP. On a Mac: GIMP or ImageMagick; Preview will not. On a phone: Needs a dedicated viewer. Or convert a PGM here to a format that opens everywhere, free and without uploading it.
Can I convert PGM for free?
Yes. Every conversion here is free and unlimited, because your own device does the work rather than a server we have to pay for.
Is PGM lossless?
Partly. The compression itself loses nothing, but PGM first reduces the image to greyscale, discarding all colour, and that step cannot be undone. Flat graphics survive it well; photographs visibly do not.