RMagick 1.3.2
ImageMagick 6.2.0
Ruby 1.8.2
Gentoo GNU/Linux-2.6.9

#Example Script
good = Magick::ImageList.new("ruby.png")
good.background_color = "#000f"
good.rotate!(40)
good.write("good.png")
bad = Magick::ImageList.new("marble.png")
bad.background_color = "#000f"
bad.rotate!(40)
bad.write("bad.png")
#End of Script

Source Images:
http://badcherry.org/Files/ruby.png
http://badcherry.org/Files/marble.png

Resulting Images:
http://badcherry.org/Files/good.png
http://badcherry.org/Files/bad.png

As you can see in bad.png there are some nasty artifacts left form the
rotation transformation.  Shear also does the same thing.  Oddly enough
this happens to most of the images that I've tried, but some such as
ruby.png work just as you would expect.  How can I go about isolating the
cause of the problem (wether is it ImageMagick, RMagick, or something
else), and why are some images unaffected?

What I'm trying to do is create a script that will create a composite (a
mosaic of many images if you will) by reading several images and scaling
and rotating them first.  My goal is to make a script that create simple
animations and simulate movement by these transformations.  (Something like
south park.)  I can't do this with these artifacts being left in the
images.  Maybe I can't get rid of this problem, and if so what would be
some suggested hacks to work around it?

-- 
Barry Dmytro
badcherry / mailc.net
http://badcherry.org/