Hi again, and thanks for the answers to my previous questions. This one's a little different, as I'm now venturing into Houdini's shading, rendering and compositing features.

I've prepared a scene that renders a still image from a camera that is itself being animated through a tunnel, and achieved a nice motion blurring effect on the tunnel using the Mantra ROP's ‘Allow Motion Blur’ feature. However, I'd like to adjust my pipeline so that the motion blurring is performed as a post-render effect in COPs. The Velocity Blur COP seems as though it will do just this, provided that it has a velocity (V) plane to work with.

And this is where I'm stuck: I can't find a way of exporting the tunnel's relative pixel velocity as an image plane. I've tried using the Mantra ROP's ‘Extra Image Plane’ feature to add a ‘V’ plane, but the resulting rendered ‘V’ channel in MPlay is empty. The Velocity Blur COP help suggests that I need to apply a shader that exposes the velocity, but I'm struggling to figure out how I would so this, and even then I'm concerned it wouldn't work as the tunnel itself is not actually moving; only the camera in relation to the static tunnel geometry.

Any suggestions as to how I might best perform the velocity blur from the animated camera in COPs?