Compositing Particles/Smoke Around 2D Video Object - Blender

Just three quick ways regarding this problem. I couldn't find the solution anywhere so decided to share the method. This is a general problem of compositing 3D effect or object that lies both in front and behind an object in 2D footage.

Problem:

Having a footage of a person, wanting to add 3D particles to circle around this person. Where is the catch? We can render particles rotating around certain point in 3D scene but how do we add them to the footage in compositor? Part of the rendered particles need to be behind the person and part in front of it. Simply using a mask won't do if the particles at the back are right behind the ones in the front.

Solution:

In fact we can use a mask, but we need to tell the compositor that there are some objects that are going to appear in front of the mask. So we take the mask node with the mask around our person, and we multiply it with Z-depth value from the render input node. Also, we need to put a math node with greater than function in the depth connection to choose from which depth the object should appear in front of the mask. This might however not work with volumetric effects such as smoke.

Second, more versatile way is to create a grid or Bezier surface. Animate it around the person in 3D in a similar way as when creating a mask (you can use Background Images in camera attributes to see the video in the 3D scene) and then assign a Holdout shader to this mask object. Anything behind it will be invisible.

And that's all. Quick and simple!

Keywords: blender,compositor,masking
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