Jpeg-recompress is an unusual open-source tool which can cut JPEG file sizes by 50% or more.

The program works by intelligently selecting the lowest possible quality setting for your JPEG, which also has the minimum effect on its perceived visual quality.

This is still lossy compression, and not something to use with images you might want to edit later. But it’s ideal for shrinking pictures you’d like to share elsewhere, perhaps on social networks, by email or on your own websites.

The program is portable, a command line tool included in a project called jpeg-archive. There are versions available for Windows, Linux and Mac, and setup is as easy as unzipping the download into a spare folder.

The program syntax is just as straightforward, thanks to some well-chosen default settings. Here’s the simplest possible example.

jpeg-recompress in.jpg out.jpg

The program drops JPEG levels as low as possible, without making any obvious visual changes to your images

This tells jpeg-recompress to take the image in.jpg and process it at several quality levels.

At each stage the program uses an algorithm (structural similarity, or SSIM by default) to measure the perceived change in the image, comparing this against an adjustable quality figure.

If the change is below the acceptability limit, jpeg-recompress knows it can drop the level further; when it’s over the limit, the program writes your out.jpg image using the lowest acceptable quality level it tried.

We tried this out with a few sample HD photos, and found they shrunk by between 50% and 78%, with no obvious visual difference between the “before” and “after” images.

If this doesn’t quite work for you, or you’re hoping for even more compression, there are various options to try. You could use a different algorithm to check your images, set a new “acceptable” quality level to further shrink the files, and there’s a “strip metadata” switch to save the maximum amount of space.

The command line interface means jpeg-recompress won’t be for everyone, but it’s an interesting way to take the guesswork out of setting JPEG quality levels, and could cut file sizes by 50% or more.

Jpeg-recompress is an open-source tool for Windows, Linux and Mac.