White background for animation I'd like to create an animation (mp4 video) with a completely white background and use a hdri environment texture for lighting.

If it was just an image I'd have no problems, simply by setting Film to Transparent under Render options and then the PNG to RGBA, I could easily add a white background to the image subsequently using some photo editor.

But what about animations? If I do the same thing, so I set Film to Transparent, the video gets a black background.

Is it possible somehow?
To create a colored background with and HDR we want everything but the camera to see the world as the HDR, and the camera to see it as white. To do this we can use the Light Path node. use the Is Camera Ray attribute to see if the camera is what is looking at the sky, if it is, show white. If not, show HDR.