ImageMagick 8:6.8.9.9-7ubuntu5.13 quality regression

[Jeroen]

I would like to report a regression in ImageMagick 8:6.8.9.9-7ubuntu5.13. This report is related to https://bugs.launchpad.net/bugs/1796563, but involves image quality instead of permissions.

What happens: In the new version (= security patch) of ImageMagick, the quality of converted images is in some cases lower compared to the old version. It is not visible on all converted images, some seem to suffer more than others. In our case, converting postal barcodes from PDF to PNG results in unscannable codes. The release information for the new version does not mention any impact on quality.

Versions involved:

- OS: Ubuntu 16.04.4 LTS
- Imagemagick: 8:6.8.9.9-7ubuntu5.13 (suffering this issue)
- Imagemagick: 8:6.8.9.9-7ubuntu5.12 (working properly)

# convert --version
Version: ImageMagick 6.8.9-9 Q16 x86_64 2018-09-28 http://www.imagemagick.org
# convert demo.pdf demo_new.png

# convert --version
Version: ImageMagick 6.8.9-9 Q16 x86_64 2018-07-10 http://www.imagemagick.org
# convert demo.pdf demo_old.png

Attached a few sample images;
- On the demo_[old|new].png, note the text in the yellow area
- The eventbrite images are screenshots of a part of the converted
PNG, due to privacy protection.