Discovery: Batch Convert with No Transforms Specified Works

[J Smith]

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.

[foxyshadis]

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.

[XnTriq]

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