Compositing to create crowds

Green screen technology assists us in cutting out the key action and removing it from the green background to place it in an artificial, CG or other wise impossible or impractical space. The key aim of shooting against a green screen is separation. The green screen is a means to an end, the means by which you separate your subject correctly for compositing purposes.

Light your green screen flat and smooth and make sure that all the action occurs in front of this screen. People often make the mistake of throwing too much light onto the chroma key background. The ideal chroma key is one that is even and smooth in colour. Throwing too much light on will create hotspots and troublesome ‘green spill’ (objects reflecting the green light) on the subject. Remember, you’re lighting for colour, not light. Try not to light your green as bright or brighter than your subject. A good guide is to try and keep your screen about a stop under the subject.

Next, always light your scene with the green screen off. This way you will obviously get a much better idea of how your intended action will look; you’re lighting for what you will see in the composited scene. Try and shoot in a space that’s big enough to allow you to light your action separately from your chroma key background.

Always talk to the person who will be either doing the compositing work or supervising it and, ideally, have them on set when shooting. Sometimes on a green screen set, directors fret about light stands or booms in shot. As long as the action doesn’t cross over these, these are fine. Questions can be answered so quickly with an effects supervisor or artist on set and sometimes even rough composites can be prepped live on set, helping actors and directors get the full picture.

Wrinkles are the enemy of green screen. Very often a makeshift green screen is required and usually comes in the form of a mobile pop-out portable green screen or a larger sheet that is usually held in place with poles. Wrinkles mar the desired smooth colour around your talent and should be avoided at all costs.

Only put tracking marks in if they are required. There’s usually no point in placing several large Xs on the wall three metres behind your subject if it’s going to end up as mountains that appear miles away in the background of the final composite. More importantly, if your camera doesn’t move then keep the trackers off the wall – they will only increase the workload later.

Try and keep factors such as loose hair, reflective elements and motion blur to a minimum: these are also enemies of green screen. Try and shoot progressively – interlaced footage can cause keying problems. Pay attention to your shutter speed as fast moving motion-blurred action can be tricky to key.

Everybody’s talking about shooting digitally and compositors tend to love working with the higher end of digital footage: HDCAM SR, RED, etc. Shooting digitally for compositing, especially in formats that give you greater colour bit depth than standard video, is great not only for its colour information but also for the lack of film weave and grain – the enemies of compositing! Super 16 and 35 mm are great and loved formats. However, you have to deal with the very, very slight ‘bounce’ in your footage due the mechanical process of running celluloid through a sprocket-driven system.

Crowd Duplication

One little trick that compositors get asked to do more and more these days is crowd duplication. In theory it’s pretty simple. You take fifty or so extras in a group, shoot them in different positions around your shot and layer them all up to give the illusion of hundreds of people.

It could be any open space: a football stadium, a train station, a concert or a rally. The key thing is to divide your shot into imaginary ‘blocks’ that you populate with each ‘instance’ of your crowd. You then get them to remove/change jackets, clothing, caps, tie hair up, let hair hang loose, etc. and place them in the next block or layer. And so on, until all the blocks that make up your shot have been filled.

The compositor takes each layer and places them over each other. Obviously, the empty space around each action needs to be cut out to allow us to see through to the action behind or around it. The entire picture is built up and you’ve got a crowd of perhaps several hundred people.