[Magick-bugs] Quality degradation in JPEG conversions

[Tuure Laurinolli]

I have noticed that various conversions to and from JPEG express strange
quality degradations. JPEG -> JPEG and JPEG -> PNG ->JPEG conversions
compared to JPEG -> PNM -> JPEG often appear blocky, especially on the
red channel.

When comparing jpeg-jpeg and jpeg-png-jpeg conversions, the result look
very similar - especially the blockiness of the text on the backdrop. In
comparison, the text is crisp and clear in jpeg-pnm-jpeg conversion. I
wonder what causes the difference.

Files can be found in:
http://fop3.kyla.fi/~tazle/imagemagick-bug/

Commands used and convert version below:

Original (large) image is original.jpg
Following commands were applied:

convert -unsharp 3x2 -resize 1000x1000 -quality 90 original.jpg
jpeg-jpeg.jpg

convert -unsharp 3x2 -resize 1000x1000 original.jpg intermediate.png

convert -unsharp 3x2 -resize 1000x1000 original.jpg intermediate.pnm

convert -quality 90 intermediate.pnm jpeg-pnm-jpeg.jpg

convert -quality 90 intermediate.png jpeg-png-jpeg.jpg

convert -version
Version: ImageMagick 6.2.4 03/30/06 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2005 ImageMagick Studio LLC