@jhoepffner: merci beaucoup. I knew what an alpha channel is, I dealt with it at many occasions making use of plenty of applications. I know about codecs and containers and I found that Isadora is a bit picky when it comes to mainly movie content containing alpha. Since it took me a lot of trying to find out, it might be worth noting here that also png is a codec for movie containers and that the alpha channel of png encoded material is accepted by Isadora! What I still don't understand though, is what an alpha mask is doing …