Q:

Imagemagick - replace transparent pixel by black ones

I have an image which contains some transparent pixels. I want to change the color of these pixels to black using imagemagick (or any line command tool). 
The transparent pixels are not all of the same color so I can't just switch their color to black.
Do you have any ideas?
Thanks in advance.

A:

Ok I just found my answer...
I did not know that transparent pixels are considered as background so this works :
convert sourceimage.png -background black destinationimage.png

