I used to play transparent videos (.webm format & libvpx-vp9 codec) perfectly fine in Psychtoolbox. Last weekend, after I upgraded GStreamer from 1.20.6 to 1.22.7 on Linux (Mint 21.2), the transparency no longer works.
I could certainly roll GStreamer back to the earlier version. But I wonder if there is anything I could do to “calibrate” Psychtoolbox with the newer version of GStreamer.
There ain’t enough info in here to know what specifically you did or how it failed.
There is one color-masking approach implemented in PlayMoviesDemo.m. which could be used on movies without proper alpha channel.
But also further investigation would require paid support, and this is my last work week this year, and it is packed with work, and support will probably be suspended throughout January 2024.