Why is my Shadow Catcher layer shadow-pass entirely black? Using the standard layers / compositor setup created with the Setup Tracking Scene button (in Cycles), I've used the second layer 'Background' to build a rough mockup of the street in order to act as a shadow catcher. However:

1) The shadow-pass for this layer is always entirely black. Is that correct? If not, how do I correct it?

2) With default settings, the shadowcatcher model darkens EVERYTHING it covers in the render, whether a shadow is cast there or not. I imagined that shadowcatcher layers should ONLY darken the movieclip in areas where the foreground layer (the ship model (please ignore the cube!)) is casting a shadow - that's not happening... so how can I achieve that effect?

3) Is bothering with Ambient Occlusion more trouble than it's worth for the shadowcatcher model? Given that the ship rarely touches the roofs, etc, but that the shadowcatcher model has lots of places with planes meet each other, etc...

(Still quite new to this! Please explain like I'm five!)

All the best,

--RoB
As Sanix, Marasoftware and Sebastian Kónig point out (https://twitter.com/s_koenig/status/656693360955150336), the Cycles shadow pass does not work with environment texture lighting (nor does it work with mesh lighting), hence my shadow pass was entirely black.

A number of solutions were suggested, usually to identify the direction of the environment texture sun light ("Is there anyway to match a sun lamp to the direction of the sun in an environment map?") and then to mimic this light with a sun lamp, excluding it from the main layer. This made a lot of sense - and then I happened across the following:

Cycles: Shadeless &quot;Shadow-catcher&quot; Material for Rendering onto Footage

What a brilliant, simple idea! Render the shadow catcher twice, once with shadows from your main object and once without - then subtract the clean shadowcatcher from the shadowed layer, leaving only the shadow behind! (Then use RBG curves or what have you to tweak the darkness of the shadow).

Barring any future mishaps or objections from the community, I'll be using this in from now on.

Two side notes:

1) This seems like an insanely useful material for this kind of task in future, though I haven't done all my research on it yet. Could be handy for someone doing a little compositing...

2) Had some trouble with backfaces in my Cycles shadowcaster, this solved my problem. Hope it's of use.