I have simple scene set up. One object sitting on a plane. No lights, just an HDR sky and GI on. I'm trying to get the exact same result as using the Shadowcathcer plugin, which gives you a render of the object with it's shadow (transparent) and no floor.
However, it doesn't seem to work with GI, just plain ol'lights. Is there a work around this?
There's no easy solution, unfortunately, because there's no way for the program to distinguish GI shadows. One thing you can do, though, is use a separate render to create your own GI shadow multipass:
* place a floor object under your object, give it a white material, and add a compositing tag with "compositing background" checked;
* add a object buffer in the comp tag;
* add a background object and give it the same white material;
* render out the image and the object buffer;
* in post, use the object buffer to knock out the object, leaving just the GI shadow, and multiply that over the full the render.