mid0nz:

Sherlock: The Empty Hearse Behind the Scenes –
Computer Graphics

How to fake a bungee cord

So Ben’s really dangling on a safety wire and not a home-made bungee cord (surprise!) So how did they swap out the wires on Ben’s safety rig for the computer generated (totally fake) bungee cord?

Matthew McDougal (Compositor): The way that we did this was [to] take still frames of the building when there was actually no Sherlock there, paint out the wires, and then it was just a simple matter of…basically creating a layer of Benedict Cumberbatch and putting that back over the image with the CG bungee cord cable. The bungee cord is totally CG, totally fake, and it’s lit and textured in such a way that it looks real. (x)

OK— that’s how you get rid of the wires and add the cord. But how do you light the cord so it a) looks real and b) looks like it was actually at Barts? That’s more slightly complex than you might think.

What’s up with the silver ball?

The VFX crew did a light probe of the scene with a chrome ball. (It’s chrome on one side and matte on the other.) 

A light probe is an omni-directional (360° panoramic) high-dynamic range image. Because they ‘see’ in all directions and can record actual light levels, light probes are useful for providing measurements of [a scene’s] illumination. As such they can be used to provide interesting and realistic lighting environments and backgrounds for rendered graphics [such as the bungee]. (x)

The chrome ball is placed on the scene where the bungee cord will be added. The ball is photographed from several viewpoints, each using a range of exposure times. These images are made into a radiance map (a map of light intensities) of the environment (the spot where the bungee cord will be in the scene) as it appears at the center of the sphere. A model of the environment is constructed and the CG bungee cord is added to the model and rendered using the radiance map as the source of illumination— thus fake shadows that look like they’re made by the lighting conditions at Barts. (x)

Phew! Get all that?! Me either.