Composite interior and exterior separately

Question

I'm trying to make an interior scene where I want to composite the exterior separetely form the interior. So, before I go any further I want to set up the layers to accomplish this.

In Layer 1 I've got just the interior:

Layer 2, the Sun and the Earth without the atmosphere:

Layer 3, just the Atmosphere, which I need to blur.

How can I composite the interior and the exterior separately and the join them? I'm fairly new to compositing so I don't have a clear idea to do this, specially with the window in the way.

EDIT:

This is how I've set up the three different render layers:

Layer 1:

Layer 2:

Layer 3:

And this is the result using the Alpha Overlay in the Compositer:

Thanks,

The first thing you have to do is to enable film> transparent and get rid of the background (the stars and sky) on the first image, and use it as a separate element. Then use Alpha over to composite the layers in the order you need them. See this related links: https://blender.stackexchange.com/questions/57610/alpha-over-layers-but-maintain-environment-background/57612#57612 and https://blender.stackexchange.com/questions/38452/cycles-not-rendering-image-or-movie-clip-used-as-background-in-the-viewport/38464#38464 For the atmosphere layer use a color mix node as add or screen mode — , Feb 11 '18 at 16:47
Related: https://blender.stackexchange.com/questions/38333/background-image-shows-in-front-of-3d-objects-not-behind-them/38334#38334 — , Feb 11 '18 at 17:00
@cegaton I can't get it to work, there's no transparency in the first layer (because of the window?) — cuoka, Feb 11 '18 at 18:04
It's hard to guess how you set things up. All we know about your project is what you've posted so far. How did you create the background? — , Feb 11 '18 at 18:18
@cegaton It's just an HDR, please, tell me what else do you need and I'll update my question. — cuoka, Feb 11 '18 at 18:26
please read the links posted on my comments. The info you need is there. — , Feb 11 '18 at 20:09
@cegaton I've already tried the links but it's not working for me, or I'm doing something wrong, I've updated my question with the layers and the result in the compositor. — cuoka, Feb 11 '18 at 21:00

score 1 · Accepted Answer · answered Feb 12 '18 at 08:11

With the help of Cegaton I've found the solution. The main problem was that the window is in the way so it has to be rendered separately. This way there's a hole in the first layer without the background so later on the Alpha Over node can be used.

So, in this case, four render layers are necessary:

Layer 1:

In this layer is the foreground, just the spaceship but without the Window, masking the rest so that there's a transparent hole in the window.