10 - Capturing HDR lighting

Ensuring the lighting in a scene looks as realistic as possible.

Lighting is a major factor in determining how realistic 3D film effects appear to be, especially when you mix virtual objects and characters with real actors and sets in the same shot. When you shoot either of the latter with just a green screen as the background, you need to use the correct method if you don't want the virtual elements to look completely unnatural.

The first step is effectively capturing the lighting of the real footage in a virtual format before you start on any CG; usually, this means a visit to the set with an HDRi (high dynamic range imaging) device. This enables the CG team to record the full range of lighting in the scene, however bright it is, so that you can light the CG scenery and characters appropriately.

Sue Rowe, Cinesite's visual effects supervisor on 'The Golden Compass', explained how this was achieved on set for the movie. 'We use a specifically designed camera, which we call The Bomb,' said Rowe. 'It's like a Rubik's Cube, but with six cameras, one on each side.' Interestingly, the cameras attached to the device aren't particularly high-tech, and Rowe describes them as being similar to those found in mobile phones.

According to Rowe, the effects team will usually be pressed for time on the set, as there will be hundreds of people waiting for them to finish so that they can start shooting. 'They'd moan if you took more than a minute to do it,' explained Rowe, 'so I'd literally goin and place The Bomb in the centre of the room. It takes seconds to shoot, and you then have 360 degrees of light information - up, down and all around - which we then immediately download onto a laptop.'

The HDRi information will then be sent back to the team at the Cinesite offices, where it's imported into Maya. They then know that there was a huge 20K light at the back, a variety of light sources elsewhere and a big silk on the top for soft lighting, explained Rowe. 'The HDRi image provides us with a digital representation of where those lights are,' she added. 'It's almost as simple as plugging in our image, and then saying here's my virtual 20K and here's my virtual silk.'

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