Shadow Catcher is Lighter than Rest of Scene In Blender 2.79b, I'm trying to render this motion tracked scene with a simple cube then a plane as a shadow catcher. But when I render it, the shadow catcher is invisible but seems to be brightening the area it is in. You can clearly see it with the bright square under the cube. I'm pretty new to Blender so all help is welcome.

I cannot figure out what's causing this.



The scene in 3D viewport:



Node tree:
I came across this thread when searching for a solution to the same exact problem as the OP. I tried various fixes, included the "simplified" node set up mentioned above by cegaton. Nothing worked.

The node setup posted by bmanner is the same node setup I get when I use a shadow catcher plane in Blender 2.79b (Mac version). I'm a rank beginner with nodes, so reducing the three "inputs" down to the two shown in the fix was not working for me.

Through all the playing around, however, I did find a very simple fix. By checking the "clamp" checkbox in both of the "add" nodes, everything renders properly. The shadow catcher no longer brightens everything it touches. No other aspect of the scene seems to be affected in the render. No idea why or how this works, but it does. 

I realize this is an old thread. Since I haven't seen this solution anywhere else, I figured this would be a good place to mention it. The OP's screen shot is attached, with the boxes noted. I hope this helps someone.