Green Screen Done Right – Fix it on the Set (Part 1)

Posted by on Nov 6, 2011 in Tech Talk and Trends | 0 comments

Alot of DP’s think that shooting green or blue screen shots are a “no brainer” these days. After all, the screen doesn’t even have to be perfectly lit anymore. While it’s true that much of the “heavy lifting” is still done in post, for great results, compositing shots require careful planning in pre-production between the various departments. Although a lot of sloppy work done during shooting can SOMETIMES be fixed in post, it may only take a few extra minutes in pre-production or production to save hours of time in post-production. If a proprietary system like Inferno or Smoke is used, the hourly rate can easily exceed $500 per hour. Even when using a program like After Effects, with a standard editing system, an editor can spend a lot of time tweaking elements like green spill, which could be eliminated or reduced on set.

Preparing to shoot "virtual extras."

Communication Critical

There are the obvious dos and don’ts like keeping the talent as far away from the green screen as possible to avoid shadows and minimize green reflection. But there are lots of other little things that can cause problems later. Many of these issues can be eliminated or reduced with clear communication between the following people: director, DP, art director or set decorator, wardrobe, make-up & hair, editor or effects person. Often, the wardrobe person is ONLY told to avoid green clothing. But I have also seen issues with shiny fabric or lacey material like scarves. Any sheer material that is partially translucent may allow green screen “bleed through.” Hairstyles with teased fine hair can also be difficult to key cleanly. Blonde hair, in particular, is very susceptible to reflecting green spill. If your only talent has blonde hair, it’s worth discussing with the effects person the idea of using blue screen instead of green screen.

If set dressing is used, it’s also important to get the art department involved prior to buying or renting furniture. I’m still surprised at how often chrome and glass furniture arrives on a green screen set. Even wood furniture, like a desktop, can reflect lots of green. Some of these issues can be mitigated on set using strategically placed flags that will be garbage matted out later. And yes, there are other ways to fix these issues in post – but the set aesthetics vs. the post budget should be thought out and considered ahead of time.

As much as possible, I try to first discuss green screen projects with the director, as well as the person executing the effects, prior to shooting. I want to make sure we’re on the same page and that I make the job as easy as possible for post production – within the time limits of the shooting schedule. From the director, I want to know if the background (replacing the green screen) has already been shot or if it will be shot after the green screen shoot. Sometimes the background might be a virtual background – either pre-existing or to be created with CG tools. In these cases, I don’t usually have to match any lighting.

Matching Foreground and Background

However, if the project intends to composite green screen foreground elements over a background plate that was shot with live action, I have to study the background plate carefully (assuming there is no technical information available). If realism is a goal, I first study the lighting (angle, intensity, soft or hard quality) so that I can match the foreground lighting with the background plate. Nothing screams “process shot” like grossly mismatched lighting. I also pay close attention to the focal length perspective of the lens (wide, normal, or telephoto), depth of field, and camera height perspective. In the reverse scenario, it can be more challenging to match lighting if the green screen elements are shot prior to shooting background plates – particularly if the background is an exterior. In this case, the production team has to anticipate what kind of shooting conditions are likely (or possible) when the background plate is to be shot. What time of day will it be? What direction will the camera face and where is the sun? Or, is it likely to be overcast?

One advantage to shooting the green screen action first, is that I always record additional matching data such as the lens focal length (bearing in mind the camera format and sensor size), camera height, camera to subject distance, and focus point. The f-stop could also be a factor, as it relates to depth of field. The ideal green screen scenario is shooting the background plates first and recording all of the important data for matching later in the studio. However, this is not always possible for a variety of reasons.

In future posts, I’ll be including videos with project examples that demonstrate specific visual effects including both basic and advanced green screen techniques.

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