Help outputting for compositing

[caspian]

Hi — I've never had to render anything out that needed any post effects in AE, until now.
I have a scene with a tree in the middle-ground. I'd like to be able to add something behind the tree in AE but in from of the background. What are my options specifically for Corona, or will it be exactly the same as if I was doing it in Standard renderer in C4D? (which I also have't done but I'm sure I could google it)

[jojorender]

Is this for animation? Asking because lots of (32bit) still comping is done in AE too.
You are basically asking to mask out the tree? Are you familiar with setting up masks?
If possible try to use similar colors behind tree to avoid fringing around leaves or clamp mask.
We probably need a bit more info to give helpful advise.

[caspian]

I need to compost a flock of birds flying behind the tree, so I will need to create a layer/mask to achieve this I think. Is this simple a job for Corona Compositing Tag with an object buffer enabled + MultiPass Mask?
 
If my tree is animated does this change anything?

[jojorender]

Correct, it’s as simple as that.
Maybe not noticeable in this distance, but you might get a light blue 1-2 pixel halo between leaves and birds. Just clamp the mask if necessary.

[BigAl3D]

Here is the basic mask setup for your tree. One thing I noticed when I add a Mask to the Multi-Pass, it also adds an Alpha too. The Alpha, whether this one or if I add one manually, always includes all the objects as there are no settings. If you look at my images, notice the Alpha mask for the tree looks a lot smoother and more detailed. I have no idea why that is.

Also, the small hill the tree sits on may also need a mask if the birds start that low. If it's a separate object, also drag it to the Mask list. Worst case you need to cut it off from the background mountain.

[caspian]

Thank you