What the heck is a Holdout Shader? [video]

The first thing we need to do, is switch to Cycles, and create a holdout material.

At this point, our scene is looking really, really, boring… To see what the holdout shader can really do, we’ll need to enable transparent in the render settings. Note that this will also make your sky transparent (hence the green cube).

If your goal is to use this in the compositor, hit Render (F12), and you’re good to go. If you want the transparency in the final image (e.g. for Photoshop/GIMP), there’s one more step. Change the color mode (Render Settings -> Output), to RGBA, instead of the default RGB.

http://i.imgur.com/XLktl0B.jpg

Let me know if you have any questions, or tutorial requests. If it’s a compositing problem, please post a screenshot of your node setup.

Can you use the holdout node to make the background/floor transparent, but the shadow or reflection visible?

Safetyman, kinda. I’d create two floors that exist in the same place, but are different objects. give one a holdout and the other just a plain diffuse.

The diffuse is your shadow object. Put it on a separate render layer and enable the shadow pass. In composting use Alpha Over to combine the shadow pass with the color pass of your main render layer.

Most likely, you won’t need the holdout object at all.

And here’s my node set up: