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Green Screen Tutorial – by John Kennedy

Visual effects artist John Kennedy gives a practical guide to the commonly used post technique, green screen.
Resources_Post_127 - VFX Green Screen_3

Your green screen is not your set.

Take a typical person shot against green screen. Now imagine this person in silhouette. Essentially, this is what green screen technology does. It assists us in cutting out our key action and removing it from the green environment to place it in an artificial, CG or other wise impossible or impractical space. The key goal in shooting green screen is separation. Always think of the screen as a means to an end, the means by which you separate your subject correctly for compositing purposes.

Light your screen first – then your scene.

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. One of the best ways to light a green screen is using the green Kino Flos. 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.

Involve your post-production people.

Always talk to the person who will be either doing the compositing work or supervising it and, ideally, have them on set when shooting. For example, I’ve often been on a green screen set and directors have fretted about light stands or booms in shot. As long as the action doesn’t cross over these, they’re actually 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.

The enemies of green screen.

Keep the wrinkles out. 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. Either way – avoid wrinkles. Wrinkles are the opposite of the desired smooth, even 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 obvious enemies of green screen. Also, 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.

Shooting formats.

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. You need to make sure your composited end result either eliminates or matches this subtle, random and (if treated incorrectly) very visible film weave. Combine this with matching the noise and grain signature of film with pristine CG or, worse again, footage shot on a different stock – then you have a whole world of extra pain that was probably never anticipated. Experience will allow most filmmakers to factor this in as part of the compositing process. Digital is the way of the future and, for the purposes of green screen work, is welcomed.

At the lower end of things: if you shoot on a lower or domestic format digital camera don’t expect your green screen work to win an Oscar® for technical achievement just yet. You can achieve very satisfactory results on lower end cameras with poorly-lit green screens but don’t expect a miracle.

And stay away from HDV – it’s the worst format ever made. Ever!

A few last-minute tips.

A good habit to form is to always shoot a few seconds of your set empty before or after your key action is shot. There’s a little compositing technique called difference keying that can often be helpful to compositors.

This little tip applies not only to green screen shoots but to any shoot to which you may be adding CG elements. It’s very helpful to shoot your set lit and empty before rolling on action with a matte white ball in the space that your CG element will occupy. This gives 3D artists and compositors a good idea of your lighting model and lets them know what colour value and direction the hotspots and shadows have.

And finally, to state the obvious: please make sure your subject isn’t wearing green! Unless you actually want the ‘invisible man’ effect, that is…

 
This article first appeared in Issue 127 of Film Ireland Magazine.