Cycles: Shadeless "Shadow-catcher" Material for Rendering onto Footage

Question

I am superimposing CGI over video footage, and have run into a problem in making materials for objects that act as 'shadow catchers'.

Essentially I want a shader that is shadeless—like an emitter—but catches shadows and light from other objects. This is so I can reconstruct objects in footage so other objects in the scene interact convincingly with them.

Here's an overcomplicated diagram to illustrate my point:

(In this particular scenario, the problem could be glossed over by using RGB curves, but that is definitely not the case with more complicated scenes)

Isn't this a common situation? How to the real guys do it? Thank you.

(I am welcome suggestions of other ways of doing this.)

Answer 1

Note: this solution may not work for everyone...you give up some control because changing the appearance of the shadow can only be done in the compositor AFAIK. See the end of my answer for more details.

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There is a simpler way to build a basic shadow catcher now, thanks to Blender 2.78c. There is no longer even a need for a material, just enable "Shadow Catcher" in the Cycles Settings panel in the shadow catcher plane's object settings.

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For more advanced tweaking of the new shadow catcher, you can add a material to the plane as detailed in this article:

https://cgjam.net/how-to-use-the-shadow-catcher-in-cycles-2-79/

See here for more info, or the section in the Blender manual here.

This image literally took less than ten minutes to make thanks to this free model by MAC2001, this free model by hasapeta, Greg Zaal's amazing, free HDRIs, and this new Blender feature.

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This solution is not easily adjusted, however, as Jeroslav notes in the comments:

Unfortunately this method is very limited and will provide only black shadows, no color bleed...For someone it may work, but this does not produce correct result in general.

You can actually get somewhat more control over this shadow method by putting the shadow catcher on its own render layer and playing with it in the compositor. In this case, I put the shadow catcher on a different layer that had the layers that my cars were on as a mask layer. Enable the environment pass for one of the layers so you can get the HDRI in the compositor.

Here is a possible compositor node setup (note that I just made this so that it looks right to me, this is not technically "correct"). In this case, I gave the lefthand value of the compositor a slightly bluish tint as well as making it brighter (of course) to change the shadow color.

Result:

Answer 2

• Divide objects into 2 scene layers, move actors (Suzanne) onto layer 1, move backgrounds onto second layer, place lights into both of them.
• Setup 3 render layers - First for layer 1, second for layer 2 and third for layer 2 with layer 1 excluded:

• Render it with transparent bacgrounds, it will produce results like this:

• In compositor divide render layer 2 (BG_With_Shadow) with 3 (BG_Clean) - you now have a shadow to multiply over your backdrop and layer 1 to composite into backdrop:

Answer 3

One little idea: use built-in camera tracking. Then scene will be prepared for you by Blender. This is, I think, the simplest way. Maybe it won't work in your case, I don't know!

Camera tracking in blender has the ability to set up the scene. Then it writes everything you need into the composer, and just by creating the ground plane and extruding it, you should receive the result of shadow catcher. Basically, the ground plane in camera tracking is the shadow catcher.