I realize this is an old question, but I recently started experiencing a strange issue related to transparent backgrounds. I often use the -t flag when using manim, but just recently I am no longer getting a transparent background and can't figure out why. Manim is still producing a .mov file (instead of .mp4), but the file has a black background rather than a transparent background. I'm working on a mac and recently upgraded the operating system, so I suspect that might have something to do with it. Has anyone else experienced this issue and does anyone know a workaround?