Composite two renders into one image


I am looking to produce an image like this:

Where there are two objects, with one object casting a shadow on the second object. The catch is that I do not want to create this by rendering two objects together in the same scene. I want to create this by combining separate images of each object, rendered separately.

Basically, I want to take this image:

And combine it with this image:

To get this:

Any ideas how to do this? To reiterate, I want to get this result through compositing or other post-processing means, not by rendering the images together. So far, I’ve messed around with the compositor a lot with various render passes, but haven’t figured it out. If you have an idea or method of how to do this, I would love to hear it!

zip file with renders and blend file:


You have two things to render, and then you have a “shadow-catcher,” which is a geometric stand-in for the object that is to receive the shadow. For this you use only the “shadow-location” channel to tell you exactly where the shadow is and how intense it is. Be sure that you render to MultiLayer OpenEXR files so that you have all of those different channels of data available to you. “Alpha Over” is the usual way to put one object on top of another, but Z-distance (distance from the camera lens) can be used too. After you have the two objects, you inject the shadow. (Then tint it, and so forth. For instance, real-world shadows are often slightly blue – from the sky.)

Basic tutorials on this subject are literally everywhere, and even tutorials made for different CG products can provide applicable concepts.

I should clarify that, in a render including the “shadow catcher,” the “catcher” might be “in plain sight,” but when you use it in your compositing you select only the “shadow” channel of data, which tells you where the shadow is, and how dark it is. (MultiLayer files give you all of these numeric-data streams separately … exactly as they were originally calculated.)

“Compositing itself” is done through nodes which allow you to “pipe” the data any way you wish in order to come up with the desired final result. For instance, sometimes you tint the shadow, often with a slight blue cast, because shadows are lit by blue light from the sky.

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