Why did ImageMagick reduce my JPG quality severely when increasing saturation? - imagemagick

Question

My command was convert imagename.jpeg -quality 100 -modulate 100,200 imagename-ImageMagick.jpeg and if you look at the Imgur gallery here:
https://imgur.com/a/V8zr4Lu
you can see that it has tons of JPG artefacts, to the point where the text is almost illegible. All I wanted to do was increase colour saturation.
Version info:
Version: ImageMagick 6.9.11-21 Q16 x86_64 2020-07-25 https://imagemagick.org Copyright: © 1999-2020 ImageMagick Studio LLC License: https://imagemagick.org/script/license.php Features: Cipher DPC Modules Delegates (built-in): bzlib djvu fftw fontconfig freetype gslib heic jbig jng jp2 jpeg lcms ltdl lzma openexr png ps raw tiff webp x xml zlib

Answer 1

Because when you increase saturation that much you make the existing JPEG artifacts more visible (in fact you make all the image problems more visible...). Adding saturation is increasing the tone differences, while the JPEG compression is based on the idea that the tone differences it creates are still below your perception, so increased saturation makes the existing and barely noticeable artifacts suddenly very visible.
-modulate 100,200 is a very aggressive setting, try something lighter
-quality 100 is pointless, you are just saving existing artifacts with a high accuracy.
maybe try a very light blur before increasing saturation.

Answer 2

Using Imagemagick 6.9.12.77, it looks fine to me
convert galaxy.jpg -modulate 100,200,100 -quality 100 x.jpg
Perhaps you need to upgrade either or both Imagemagick or libjpg
convert -version
Version: ImageMagick 6.9.12-77 Q16 aarch64 17704 https://legacy.imagemagick.org
Copyright: (C) 1999 ImageMagick Studio LLC
License: https://imagemagick.org/script/license.php
Features: Cipher DPC Modules
Delegates (built-in): bzlib cairo djvu fftw fontconfig freetype gslib gvc heic jbig jng jp2 jpeg lcms lqr ltdl lzma openexr png ps raw rsvg tiff webp x xml zlib