I have a question for those who use Carrara to composite 3d and images or video.

Are there ways to catch reflections and lights as well as shadows?

I'd like to use the item in the image I have, but I would like to composite some light and add some 3D-object reflection onto that item. I have the 3D object and now I need to find a way to make it interact naturally with my 2D image.

As far as I understand, For the shadows to work properly I need to model something similar to the item I have and then drop the shadow-catcher onto the 3d model. Is that the only way? And what do I need to do to have reflections and liht too?

Also, are there any methods to simplify the modeling process for the image-based item I have to simulate?

Alternatively, I might have to texture the modeled item fully in Carrara. Are there ways to map the image I have onto the mock model I model in Carrara?

I have almost all DCG plug-ins and some inagoni ones too, if there is anything there to help.

Any compositing tutorials for Carrara out there?

Thank you all in advance!

Without seeing what you are trying to accomplish, I suspect this is a job for the G-Buffers and a couple of separate renders. Reflections are one of the G-Buffers that you can composite into your editor. 

Are G-Buffers the same as the multi-pass render passes or is that something else?

What I am trying to do is simple. I have a picture of a wall with windows, and I have a 3D glowing sign. What I am trying to do is to have the wall be a bit illuminated by the sign and for the windows to catch the reflection of it.

The wall is not the simple straight refular wall, it has a bit of scylpting and architectural embelishment. That's why I'd like to keep as much of the original image as possible, because I don't think I can adequately and accurately model it all. Plus, the image is of the place that I don't have access to (might not even exist anymore), so all I have to work with are just a few static images of it from a particular angle.

I'd like to make this into an animated video as the end result, with the sign elements flickering on and off, and for the light on the wall/windows to accurate reflect such interaction with the flickering lights.

Does this help to clarify the situation a bit?

Whywouldn't fully transparent objects work?  It's just the same as a shadow catcher but you would have to render the scene with just the lights and objects that should appear.

The objects would have transparency, reflection, highlight, shininess, no color.

Shouldn't that work?

No, full transparency does not work, unfortunately, as I am getting undesired effects with it.  Maybe it would work for extracting some individual passes.

I am getting a bit of the result that I want (not fully yet) with just the shadow catcher that is catching global illumination too. I am not using any lights at all and lighting the whole scene with just the shader glow and ambiance. No reflections, though :(.

But I am still getting 3D object that is way too sharp, saturated and clear compared to the image. Are there any compositing tricks for dealing with that?