I'm not sure how the x265 patches with an alpha channel work. For my tests, I used an H.264 background without transparency (aereo.mp4). On top of that, I placed the four “puppet” files I downloaded from the Apple website Andrew mentioned. Here's the link to download the files. https://developer.apple.com/documentation/avfoundation/using-hevc-video-with... Or, to save time, here’s the link to my Dropbox folder where I’ve gathered the four files plus the render file: https://www.dropbox.com/scl/fi/ovavq5cqmtfvosz7ulhoy/Archivio.tar.gz?rlkey=5... Test: - puppets_with_alpha_hevc.mov It does not display transparency on the timeline or in the rendering created with the "h265_alpha.mp4" preset. - puppets_with_alpha_prores-60fps.mp4 It does not display transparency on the timeline or in the rendering created with the h265_alpha.mp4 preset. - puppets_with_alpha_prores.mov shows transparency on the timeline and in the final render - puppets_with_greenbg_hevc.mov Shows the transparency on the timeline and in the final render after removing the green background using chroma key. Other files with transparency don't work in either decoding or encoding. Given these results, I can't figure out what makes transparency work and what doesn't. Codecs? Formats? My system? NOTE: I tried the Kdenlive AppImage and got the same results. Here's a link to a video showing my tests: https://www.dropbox.com/scl/fi/lspehro82yvyqozdygq77/2026-05-18_10-22-41.mkv...