[Bf-cycles] Shadow Catcher BSDF

[Lukas Stockner]

Hi,
I've been trying out how to implement a shadow catcher in Cycles, but 
apparently there are many different approaches.

My current idea is: At the point being shaded, evaluate the direct 
light, but also separately accumulate the contribution of lights without 
considering occlusion (the shadow_blocked check). Then, a shadow ratio 
can be calculated from "L_regular / L_no_occlusion". The shadow ratio is 
then used in a Holdout-like setup to set alpha and terminate the ray. 
For non-camera rays a diffuse BSDF is generated, but that could easily 
solved by adding a BSDF input to the shadow catcher node which allows to 
set a non-camera BSDF. By using an AlphaOver in the compositor, this can 
be easily combined with a suitable background.
There are two problems with this approach: First of all, it doesn't 
consider indirect lighting such as caustics or, which is more important, 
diffuse light. Therefore, the area around objects on the shadow catcher 
looks slightly darker than it should look. And secondly, the standard 
implementation "calculate both values, divide, set alpha" inside the 
shader doesn't work, because both values must be accumulated separately.
This could be implemented as an auxiliary internal pass, though, so that 
the division is performed in the buffer code. Still, not as nice and 
clean as I had hoped for.

Another approach is 
http://www.pauldebevec.com/Research/IBL/debevec-siggraph98.pdf, which 
basically works with a difference. However, as far as I see, that needs 
a really precise and accurate scene (for example, in the page 9 example, 
a wrong color for the plane would instantly give weird-colored shadows). 
Also, it produces negative colors in the difference image, which will 
probably cause havoc in the compositor.

So, my question would be: Are there any approaches that might work 
better? Am I overlooking something? If not, would one of these 
approaches be useful?
Any input is most appreciated :)