Shooting green screens for compositing

As a senior visual effects compositor who has worked in the industry for more than 20 years, I have composited hundreds of bluescreen and green screen shots.

By their very nature, they are difficult but there are also many common mistakes made during principle photography that makes them even more difficult.

These mistakes not only degrade the quality of the finished composite but they also add to both the schedule and cost in post. This article looks at the most common mistakes made when shooting bluescreens and green screens and offers tips and techniques to get better results.

We will be referring to green screens in most of this article, but of course all of these principles apply to bluescreens as well.

Before the photography even begins, the first question is always “which colour – green or blue?” There is a list of issues that affect this decision, but the one at the top of the list is always, “what colour is the target?” You must choose a non-competing colour for the green or blue backing.

Do not try to shoot blue objects on a bluescreen, for example. This often entails coordination with the costume department. Some believe that certain skin tones come out better on bluescreen or that blonde hair does not do well on green screens. Modern digital keyers largely render these issues obsolete.

Both green screens and bluescreens require a great deal of light, and lighting is expensive. One advantage of a green screen is that it is easier to light simply because tungsten lights put out far more green light than blue light.

One disadvantage of a bluescreen is that the blue record of both film and video has by far the most grain or noise.

This badly affects the matte in compositing, giving it “sizzling” edges. All other things being equal, this makes green the preferred backing colour.

During principle photography, the key thing to remember is that the overall objective is not to produce the best looking green screen shot, but to produce the best green screen composite.

Too often, the effort is in lighting and composing the talent with scant consideration given to the green screen itself.

For the compositor to pull a good key, the green screen must be evenly lit, have the correct exposure, and have high colour purity (chrominance or saturation) at the correct colour – namely, green. The talent can always be colour corrected during compositing, but the compositor is stuck with the green screen as it was shot.

The green screen needs to be lit totally separately from the talent. In fact, the lights for the talent should be turned off while lighting the green screen. It should be lit within half a stop of uniformity left, right, top and bottom, and about one stop lower than the key light.

If it is too bright, it looses saturation and throws too much spill light in the talent. Too dark and there is not enough luminance and chroma for a good key and it adds dark edges to the composite.

The exception is when shooting on a cyclorama (cove stage) since both the green backing and the talent are unavoidably lit by the same lighting.

The purity of the green screen colour is another critical issue. Many green screen shots that are delivered are too yellow (too much red) or cyan (too much blue).

The examples shown here were made by cropping samples out of the green screen area from 16 different feature films just to illustrate the wild variation in purity – and some of the worse ones were $100 million dollar blockbuster films. If the green screen lacks purity of colour, the resulting matte will loose fine-edge detail such as hair.

To avoid this, be sure to use fabrics and paints that are specifically designed for green screen shots. While it helps to use coloured lights or gels, be careful that they do not shine on the talent as this will add serious green spill problems during compositing.

Never use dimmers as they shift the colour of tungsten lights towards red. If shooting film, do film tests and give them to your compositor if budget and schedule permit.If shooting video, the on-set waveform monitor should show the green channel at around 70% with the red and blue channels about equal at around 20% in the green screen areas.

Once the green screen lighting is set correctly, its lights can be turned off and the lights for the talent turned on. The talent needs to be kept eight to ten feet in front of the green screen to minimise green spill (green bounce light) that will contaminate them.

The key points here are to make sure that the talent’s light does not contaminate the green screen and that the talent is lit for the intended environment they will be composited into.

Do not smoke the set, as this will totally degrade the green screen. Smoke can be added at compositing time. Also, any interactive lighting (flickering firelight, lightning flashes, etc.) must not hit the green screen.

Any shiny objects on the talent such as belt buckles or jewellery will reflect the green screen and put a hole in the matte. If the shining object cannot be replaced, then use dulling spray. Once the talent is properly lit, turn the green screen lighting back on and re-metre.

There is a great myth in cinematography that the spill light can be minimised by counter-lighting the talent with magenta (for green screen) or yellow (for bluescreen).

This must never be done. Modern digital keyers have excellent tools for removing spill light, and such counter-lighting can never correctly match the lighting from a wall of green screen.

The talent just ends up with ugly coloured lighting around the edges that is very difficult to correct at compositing time. However, it is a very good idea to add two small diffuse rim lights about mid-screen, one on either side of the talent and slightly behind to gently light their edges.

The usual practice of placing all of the lights out in front of the talent gives them an unnaturally dark rim at compositing time.

Many green screen shots are part of a match move sequence where the green screen layer has to be motion tracked during compositing to move it with the background. Tracking markers are placed on the green screen for the tracking department to lock onto.

The colour of the tracking markers is not critical as they can easily be keyed, painted, or rotoscoped out, but their shape and size is important. The best shape for the motion tracker is like a “plus” sign (“+”) as this provides both strong horizontal and vertical edges for the tracker to lock onto.

They also need to be large enough in frame to see their shape, not just small fuzzy dots.

A “plus” sign is also easily made with two short pieces of tape, one vertical and the other horizontal. The other key point is to make sure that there are always at least three tracking markers in frame at all times. Too few and the tracker cannot get a good track. If possible, avoid letting the tracking markers get out of focus.

When shooting green screens on film, there are a few points to keep in mind. The first is to use the finest grained film stock possible. Fast film stocks with large grain make for chattering mattes at compositing time. Also, never put filters on the camera lens.

Any filters would be subtracting light that the compositor needs to create a good matte. Filter effects can be added at compositing time. If you use 16mm film be advised that the grain will be dramatically larger than with 35mm.

When shooting green screens with video, the first rule is to turn off the video camera’s edge sharpening feature. All video cameras have this feature, called “sharpness” or “detail” or some other name, which is designed to make the picture look sharper than the camera actually captures.

The problem is that this sharpening adds edge artifacts that will seriously degrade the composite.

The picture can be sharpened during compositing. Also, since digital keyers use colour information to create the matte, shooting with a 4:1:1 or 4:2:0 video camera is a recipe for disaster as they are capturing less colour information than a 4:2:2 camera.

Combining these lower colour sampling cameras with the normal image compression that all modern digital cameras now use creates an image that produces very poor quality composites.

By following the guidelines in this article, you will be able to produce higher quality green screen or bluescreen shots resulting in much better composites. The finished effects shot will have higher quality with both a shorter schedule and lower cost in post.

Steve Wright is senior compositor, 2D technical director and industry veteran in visual effects compositing with more than 20 years of production experience. He conducts VFX compositing training courses for major visual effects studios using Shake and Nuke. He can be reached through www.swdfx.com.