A Blender trick to create better blurred backdrops when simulating movement for automotive applications.

Further to the projection technique to render backplate motion blur in Blender, I’ve been using a neat trick to mirror the backplates on the backdrop geometry when out of frame.

In case you haven’t read the previous article about this blurring technique, I suggest you read it first as this article will make more sense.

First off, let’s define some terminology:

  • The backplate is the image that the CG car will be composited into, and that needs to be blurred to give the illusion of movement.
  • The backdrop is the geometry that will receive the backplate texture. You can think of it as a simple geometry that simulates the terrain of the backplate.

The main problem with projecting UVs from the camera view — using the Project UV modifier, or unwrapping using “project from view” while viewing the scene through the camera — onto the backdrop is that when animating the car to simulate a motion path the area outside the field of view is by default tiled, which means that as the camera moves away from the projector, the repeated backdrop becomes visible. An illustration will make this point easier to understand:

While it’s not complicated to retouch or crop this in post, there’s a more elegant solution. Because the projected UV coordinates can be visualized as colors, you can split them as red and green respectively and apply some logic in the node editor (a new vector splitter is coming in the 2.72 release, if you want to be semantic):

So what’s going on here? It has to do with how the projection onto the backplate works: the U and V values range from 0 to 1 in the horizontal and vertical directions respectively for the portion of the backplate which is in the frame, and extend in range beyond that. So what I’m doing with the nodes is to invert the U and V coordinates when it’s outside of the 0 to 1 range, that is to say when it’s out frame. The result is to have a mirrored image at the framing edge borders, which is more likely to be what’s physically located just around the edges of the frame when I took the original photograph.

It’s not a foolproof solution for every backplate, but it has been useful enough for me that I thought I’d share it.

The Aston Martin DB5 is based on this model by natman.