animation and render layers

Hello all,

I’m trying to render a scene which is basically a coin spinning in the air in a cave;
I’ve figured out that, since the camera is not moving, I’d render the background once (takes forever), and then render all frames for the coin animation.
Works fine for a still image, I can render quickly any frame, but I absolutly can’t figure out how to render the animation and it’s driving me MAD.
When i render “animation”, it renders either the coin spinning but with a black background, either the coin and the background, but it renders the background every frame, which is exactly what I want to avoid.
I’d replace the background layer with a rendered image, if not for the glow and depth of field effect which doesn’t work properly then…
Some renders to make it clear (there’s a layer for sparkles too, hope that doesn’t confuse it too much):
here is what I want:

here is what I get with a rendered image instead of a background render layer:

So I want to render the animation like the first image up there, but without re-rendering the background every frame… where do I got it wrong?

Thanks in advance for the help!

Okay, I’m gonna reply to myself since I have the solution for my problem now.
Dear noob from 2 hours ago, all you have to do is save the background render as an EXR Multilayer file, and not a jpeg image.

Hopefully this can be useful to someone

Yeah, Jpegs are probably the poorest choice for any compositing work. AFAIK they can’t hold an alpha channel, for one thing. Glad you figured it out.