How does quality setting work?

[brad.baker]

We're running the following command on our images to reduce file size:
.\mogrify.exe -quality 80 -strip -interlace Plane.

The image quality seems ok after running the command so we want to try a lower quality setting.

If I re-run mogrify against the same images who's quality has already been set to 80%, with -quality 75 does that mean the image is 75% of the original or 75% of the resized image? I'm assuming its 75% of the resized image since the original is now effectively gone but want to confirm.

Thanks
Brad

[snibgo]

"-quality" isn't a percentage but a setting from 0 to 100. If affects png and jpeg compression. It isn't relative to any setting that might have been present in the input image.

[fmw42]

see http://www.imagemagick.org/script/comma ... hp#quality. Quality values mean different things for different formats. JPG 0 to 100 is straightforward with 0 being lowest and 100 being highest. But values for PNG are not as simple.