How to render out/record with an alpha channel?

Beginners
1

Hi all! I am trying to record a geometry moving around in empty space, occasionally flashing (phong material) with the background as an alpha channel so I can layer it on top of other things in post. I have tried every codec I can render (aside from NVIDIA H.264). How can this be done? Thank you!

2

Codec Animation / Pixel Format RGBA
Codec HAP / Pixel Format RGBA (Hap Alpha)
Codec HAP Q / Pixel Format RGBA (Hap-Q Alpha)

or you can choose Type Image Sequence / File Type TIFF or PNG

or else you can record a black and white video, tha you can use later as a mask to Multiply with the Color RGB video.