VFX 101: Greenscreen Basics, Part I

(Herman Wang) #1

This is the second of a semi-regular series of columns I’ll be presenting on how to do some basic special effects, for people who have little to no experience. We use these effects quite frequently in our Harry Potter-based web series The Spell Tutor.

In this column I’ll be answering some common questions about shooting with greenscreen. In the next column I’ll cover processing your greenscreen footage in post.

What’s “Chroma Key”?
That’s just the technical name for greenscreen. It’s the process of keying (singling out) a given chroma (colour) and turning it transparent, while leaving everything else alone.

What’s the difference between greenscreen and bluescreen?
Technologically, there is no difference. It’s exactly the same process, only with two different standardized colours. The decision of one over the other is based on the colours on your foreground actor - you want to avoid colour matching as much as possible. If you’re shooting a green Godzilla monster, you’ll want to go with bluescreen. If your actor is wearing a blue outfit, you’ll want greenscreen. For outfits without green or blue, either should do equally well. If you’re pre-producing a series that’s planning a lot of this type of effect, this is where your wardrobe decisions are crucial, and will save you headaches down the road.

When do I need a greenscreen?
Generally, greenscreening should be a last resort, not a first choice. If possible you should use methods that are easier to pull off, such as this one.
Some reasons that you would have to go with greenscreen are:

  1. The background you want is not possible. This comes in two flavours:
  • Setting. If the scene is set floating in outer space, you pretty much have to greenscreen that in. Or if you want a set that would be prohibitively expensive to rent or build, say, an ancient European castle interior.
  • CGI. If you want to add CGI elements that are behind an actor, greenscreen is the easiest way to make sure the actor’s body properly obscures those elements the right way.
  1. The actor’s actions are not possible. If you have Superman leaping over a tall building, a high-budget production might be able to achieve that with a wire rig and stunt team, but low-budget filmmakers will need greenscreen to make that work.

How do I make the most of a greenscreen shoot?

  1. White balance the camera before the shoot. (That’s the process of calibrating the colour capture mechanism. The camera’s instruction manual will explain how to do it.) Your VFX editing program most likely comes preset with the industry-standard greenscreen and bluescreen shades, and white balancing will ensure your takes will have something close to the standard colour. This will make your life much easier during post.
  2. Get the actor as far in front of the greenscreen as possible. This will minimize the actor’s shadows on the greenscreen and green reflections on the actor’s back, both of which can cause major headaches later on.
  3. Light the greenscreen and the actor separately - don’t try to do both with one light. Light the actor like you normally would and then separately light the greenscreen as evenly as possible. You’ll have to match the actor’s lighting to what you’re planning to substitute for the background. If you’re planning to put in a dark room, then your actor should be similarly darkly lit. A sunny exterior background will require bright lighting, and so on. In the beginning of this episode, we used greenscreen to allow our character to walk through a wall. The way we matched the lighting was simply to place the greenscreen in the same spot. (By the way, that’s me holding the greenscreen taut):
  4. In the same way you have to match lighting, you have to match sound. For example, if you’re replacing with an exterior background, but you’re recording the actors interior, the room echo will make the resulting scene feel weird to your audience. So you’d either have to move the shoot and greenscreen to exterior, re-record the dialog in post (ADR), or do your best to deaden the sound of the room with blankets.

What if I have to compromise?
You won’t always be able to do everything the proper way. This is a raw shot we did with my pet rabbit. (He’s a dwarf rabbit and his name is Tyrion.)
Tyrion sample
We didn’t use our greenscreen muslin for this because Tyrion has a tendency to nibble on fabrics and we didn’t want him to ruin a costly backdrop. Similarly, it wasn’t feasible to go to a rental green room and let him loose.
So we bought a cheap, green, plastic tablecloth and stapled it to a few pieces of wood. We put it down in his playroom and gave him treats to position him in the right spot. You can probably tell we had a few problems processing this:

  1. The green wasn’t the industry-standard greenscreen shade.
  2. The lighting is very uneven. There’s a bright spot near the floor and a dark shadow near the top. This makes everything look like different shades of green, even though the tablecloth was a uniform colour.
  3. There’s all kinds of wrinkles and creases, which only exacerbates the above problem.

In the end we got it all done, and I’ll cover how we accomplished that in the next column. It was helpful that it didn’t have to look particularly realistic, as you can see in the finished episode.

For now I’ll end on an earlier greenscreen technique I put together a while ago to help out a fellow Potter-based series (https://twitter.com/TheSpellTutor/status/713145388966936576)

VFX 101: Greenscreen Basics, Part II
Visual effects - Low Budget shooting
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