XnView Software

I discovered that if you execute batch convert with no transformations specified, it works.

I had some jpeg files that I though were uncompressed. Rather than open and save each image individually, I decided to try batch convert and see if it would simply rewrite the files and compress them more.

It did.

I also discovered that when I specified best for quality at options, read/write, write, jpeg, the files I was working all expanded by a factor of 3 to 5.

I also discovered than when I specified around 30 on the scale and reduced the images about 75 percent, the quality degradation on some close ups was dramatic.

This is actually one of the more common uses for batch convert, to recompress smaller without changing the size or anything else about the image. Particularly people who have a lot of 100% jpegs or lossless images. (100% jpeg isn't quite uncompressed though, it's still minimally lossy.)

That's pretty much the point of lossy compression: trade off quality to get a lower filesize. Below about 50-60% you lose tons of quality and barely gain any filesize, so it's important to be sure you absolutely need that reduction before saving. 75-90% is most common. Because of certain unavoidable cumulative roundoff errors, you can save jpeg to a larger file size while actually slightly degradating output quality, but for the most part it just saves a lot of useless extra information.

As always, great answer, foxyshadis

Some additional info from the “archives”:

• Smaller image has same file size
• JPEG: Big files - Bad quality
• Resizing image results in larger file