GIF output file with noise

Hi,
I've been tracing a bug that I saw using JMagick where random scaling
executions on GIFs produce corrupted output images (random noise)
This problem seems to occur only for GIFs and when using the same image more
than once in the application lifetime.

I was able to reproduce the same noise using ImageMagick 6.5.8-6 and
command:
rm 1.gif 2.gif; cp beach.gif 1.gif; convert -scale 177x177 beach.gif
/tmp/1.gif -scale 120x120 /tmp/1.gif /tmp/2.gif

Image beach.gif can be found here :
http://img215.imageshack.us/img215/7875/beachg.gif
My result image (just the first frame) :
http://img98.imageshack.us/img98/6262/93072755.gif

Concatenation of operations is just to force ImageMagick to use twice a GIF
image (maybe there is a better way if so I'll gladly learn something new).
As a result of mentioned manipulation a corrupted image is created. Created
noise is relative to target dimension. In my case acquired noise was higher
for different target dimensions but it wasn't always reproduce-able. With
that 177x177 dimension I am always able to reproduce this problem.


I was not able to reproduce that issue with just one image manipulation.
If you split those tasks to :
convert -scale 170x170 beach.gif 2.gif
convert -scale 120x120 2.gif 2.gif
then the output image is pretty nice


-- 
Best Regards
Ryszard Perkowski