Chroma key and rotoscoping

Why keys go wrong at the edges

Removing the bulk of a green screen is easy. Every editing program does it in seconds. The difference between an amateur and a professional key is at the edges: fine hair, motion blur on a moving hand, a glass of water, a semi-transparent scarf. Push the key too hard and those details vanish into a hard outline. Too soft and a green fringe remains.

The other common problem is spill: green light bouncing off the screen onto the subject's skin, hair and clothes. It makes people look slightly ill once placed on a new background. We key in layers, treating the core, the edges and the fine detail separately, then remove spill and add a subtle wrap of the new background's light so the subject sits in the scene.

Rotoscoping, and how to avoid needing it

When footage was shot without a green screen, the subject has to be separated from the background by hand: drawing a shape round them and adjusting it as they move, often frame by frame. Modern tools speed this up, but hair, fast motion and complex overlaps still need close manual work. It is slow and priced accordingly.

The cheapest rotoscoping is the rotoscoping you avoid. If you plan to replace a background, shoot on a green or blue screen, light the screen evenly and separately from the subject, keep the subject at least two metres from the screen, and avoid green clothing. We are happy to review a shoot plan before the day; ten minutes of advice can save days of work.

• Light the screen evenly, with no hot spots or shadows
• Keep two metres or more between subject and screen
• Shoot at a high shutter speed to reduce motion blur
• Record in the least compressed format your camera allows