Not sure, but i think there is no video format/codec for transparency like in PNG for example. You can use masking in video editing, but you need some background that shines through, so that you can deterime what background/color you want.
Just googled for you (video codec with transparency / alpha channel).
You could try export formats like Apple ProRes 4444 / Apple ProRes 4444 XQ
or DNxHD or DNxHR for example.
Not sure if all are offered in SC, but i think the DNxHD/R are available.
I have used the Quicktime Animation format in Shotcut export to produce a transparent video in the past. You can see it is transparent if you put it on a V2 track in Shotcut and see that the video on V1 shows through.
Below is a snapshot of this in action. The Heartbeat.mov video is transparent and overlaid on top of another. Of course if you open this transparent video in the VLC video player it will have a black background, since that is the underlying layer of VLC. I haven’t tried in in an HTML file, but I’m reasobly sure it will work correctly.
Read this thread starting at this post to the end:
It provides options for Animated PNG with transparency. It also explains the trick of putting a Chroma Key on the Output track to create transparency that the compositor will preserve through the export process.
Most likely, your videos don’t have transparency because there are multiple tracks, causing the compositor to merge tracks against a hidden black background layer. It needs to be keyed out whenever multiple tracks are involved.
As for video formats that support transparency, ProRes 4444 as well as the options under Export panel > stock > alpha will work. DNxHD/DNxHR will not.
I have gone through all of the steps you’ve written.
But When I add the chroma key to the source video, it doesn’t do anything. The color that I select From the V1 clip doesn’t get keyed out. As a result I don’t get a transparent background.
I had even selected “from source” in the export dropdown options.
For example here I want to key the green color in the background and make it transparent and I did that in the source and exported the video and put it over another background but as you can see The green background is still visible and it wasn’t keyed