IM replace opaque color by transparency color.

[Fromen]

I work in Windows XP and use IM-6.8.0-Q16. And I want to reduce number of the image http://www.keeshond.com.ua/wp-content/u ... %D1%82.gif unique colors (to 4 colors). For this I use a next command: "convert input_file.gif +dither -colors 4 output_file.gif". After executing of this command the output file contains a dog with a white nose.
But when I installed IM-6.3.9-Q16 the output file has been good, without replacing opaque colors by transparency background.
What the difference between the versions and why they return different results?
Thanks!

[fmw42]

This works fine for me on Mac OSX Snow Leopard IM 6.8.0.4 Q16

convert 1dog.gif -alpha off +dither -colors 4 -alpha on 1dog_tmp1.gif


Many things have changed since your old 6.3.9 version of IM. That version is over 400 versions old.

[Fromen]

fmw42, thanks a lot for reply. It works fine. But I think that this solution is not a good practice (off the transparency, reduce colors, on the transparency. I am afraid the performance losing). In my opinion IM must to solve problems with tranparently images. Or am I wrong?

[Fromen]

I note that for png it can not work (see http://assets.tumblr.com/images/logo.png?alpha&6). May be it's depends on png multi-alpha-color?