How can you render out alpha channelled video files?

hi i was just wondering how or if it is even possible to do this without any external software. So that when you render it out you can use the compositor in blender to paste on top of other animated scenes/backgrounds, so you can effectively have a background with lots of different layered animations/simulations going at once over top.

Not possible, you need to render out a series of images with an alpha channel to have transparency but unless you are rendering out your final movie you should do that anyways. Never render out video files to use with compositing.

NinthJake is correct you should ideally use image sequences such as .png files for rendering animations. Also ensure you enable RGBA in the render output options as well otherwise you won’t get the alpha channel. If you really need to render to a movie format with an alpha channel you would have to use a format that supports it. One such format is the Quicktime Animation codec. Whether you actually have that option in your blender version depends on your operating system, otherwise you could try a suitable build from that includes quicktime.

It is possible, in theory, haven’t tried this myself, to render out a ProRes 4444 Quicktime. This supports alpha. This would only work on a mac and only if you have the pro codecs installed (they come with final cut pro). The files would be smaller than the animation codec and it keeps the same level of detail (some compression is used, but it’s lossless).

I’d agree with Rich and NinthJake though, just render out an image sequence that supports alpha, such as png, tiff or EXR. Blender can read them in as an image sequence and some editing software, like Premiere Pro, can do so too (final cut 7 couldn’t, but x might be able to).