[karim79]

> Years back I was concerned about the size of a large JPEG collection and recompressed them which was a big mistake because many of the images were compressed too hard.

Distortion metrics[0] such as MS-SSIM, MS-SSIM*, SIMM, MSE, and PSNR can be used to define a cut-off or threshold for deciding the point at which the image is "compressed enough" by using one or more of those algorithms and predefining the amount of acceptable/tolerable distortion or quality loss. Each of those algorithms has some trade-offs in terms of accuracy and processing time, but it can definitely work for a large set if you find the right settings for your use-case. It is certainly more productive than manually settings the Q-level per image.

Some SaaS such as https://kraken.io do this on JPG images.

[0] https://sourceforge.net/projects/iqa/

[alexflo]

Actually this isn't accurate, Kraken.io doesn't use any SSIM-related algorithms, it just blindly applies some standard compression regardless of the image's content.

If you're looking for a tool that really smartly optimizes the images (by using SSIM) that is https://shortpixel.com/online-image-compression

[JyrkiAlakuijala]

You can use jpegli for better jpeg compression heuristics. It uses custom heuristics that were originally used in JPEG XL, then copied over and further optimized using nelder-mead to minimize distortion metrics (butteraugli and simulacra 2)