The webm don't come from a camera, it's a file generated by a computer graphic software. The modeller have a very low connection and we need the alpha channel in the video. Here a rough example. Soon I will link the video file as asked by wwaag. Currently we are working with two files, one the video, another one the mask.
Here's a demo of its implementation in ImportAssist.
At the moment, no audio. If needed, that could be added. Since webm uses the Opus codec, it could be transcoded to either pcm or aac. From the demo, you can see that there are two steps. After the first step (lossless png creation), the user could import the image sequence directly into Vegas, thus avoiding the render to a lossless codec such as Apple Animation, MagicYUV or UtVideo.
The latest version of HOS can now import WebM files with alpha channel using AviDub with the lossless codec of your choice (Magic YUV, GrassValley HQX, Lagarith, UT, ProRes 4444), as long as it has alpha channel support. The HOS developer is doing a good job in this area of coding. Watch the video below.
If the "webm vp9 alpha" option is selected, the user will only be able to use PNG encoding with alpha channel, which is done in two steps and does not yet have audio import. That is the reason for choosing the "webm vp9" option. In this option, I can use AviDub to encode the file with alpha channel using the lossless codec of my preference and do everything at once, even importing the audio.