This may be useful for the black background problem: starting from ImageMagick 6.7.5 you can remove transparency and replace it with a static color; you can read more about this command here
Hope this helps, unfortunately I have an older version of Imagemagick, so i can't try it myself
Example from ImageMagick documentation:
convert moon.png -background tan -alpha remove alpha_remove.png
The color "tan" replaces the transparent areas of the picture
Comment from mathematical.coffee
Using the above answer, I was able to generate the animations I wanted.
1: successive buildup of dots, all on a black background. Turned out to be as simple as creating a black background picture to put at the start of the animation, and using -coalesce
:
# where bg.png is a black png of the appropriate size:
convert bg.png min*.png -coalesce out.gif
# in the below the first line creates the black background
# image, same size as my first min00.png image, for me:
convert min00.png -alpha Opaque +level-colors black \
min*.png -coalesce out.gif
2. use the method mentioned above:
convert min000*.png -background black -alpha remove out.gif
In both I was using imagemagick 6.7.something.