This is what you have to do when saving the video:
ani_.save(‘output.avi’, fps=args_[‘fps’], savefig_kwargs = { ‘transparent’ : True , ‘facecolor’ : ‘black’ }
Argument savefig_kwargs is the key parameter. It is applied to savefig which is called every time a frame is saved into video file. Ref: http://matplotlib.org/api/animation_api.html