At the risk of academically beating this topic to death, I want to ask a series of very short and annoying questions about green screen compositing.
Why shoot on a green screen?
First, the simple answer: To place a person or object onto another background.
Fair enough. Green screen footage can be keyed, usually making the “cutting out” process much simpler than rotoscoping.
Here begins my recursive questioning style of a three-year-old…
Should I shoot on a green screen?
The first rule of green screen is to avoid it whenever possible. It is time-consuming and tedious, and minor mistakes along the way can result in poor, unbelievable results in the end.
Hollywood studios can afford to pay enough to create pretty much any idea a writer can write. They get to throw money at problems until the problems go away. However, you cannot “out-Hollywood” Hollywood.
If you can actually shoot the person or object in the actual location, do it. Or if you can figure out how to avoid green screen shots through creative framing or editing during the storyboarding process, do it. You’ll save yourself so much time and trouble in the lighting, keying and color correction, and the footage can typically be shot with more freedom in cinematic camera angles, movement and depth of field.
Additionally, many problems that you might assume require a green screen solution may be solved through creative stylization (see Michel Godry’s work), a change in medium (perhaps animation or motion graphics), or a simple re-write or reframing of a story’s core concept.
That said, there are still plenty of reasons to plan a green screen shoot…
Why should I shoot on green screen?
The background location may not be available to you for a variety of reasons.
The location is booked, populated or otherwise occupied in such a way that you cannot complete shooting in the time you have allotted.
The location is only available for a portion of your schedule — enough to shoot wide shots and mediums, but not close-ups, inserts or pick-ups.
The location is not available at the same time/day/week/month as your actor(s).
The concept calls for specific conditions — meteorological, seasonal or time-of-day — that are incompatible or unsustainable with your schedule.
Ex: A romantic dialog scene takes place at sunset… while it’s snowing… and you’re shooting in the summer.
You cannot afford to rent or pay for permits to shoot in the desired location.
You cannot afford to rent the location for the amount of time needed (see also Schedule).
You cannot find or afford to build and/or set dress the entire location as called for in the script.
Ex: A city street scene needs to be set dressed for winter with 6" of snow. A heat wave hits Chicago the 1st week of February, the week before shooting, and the 50ºF weather melts most of the snow. Use portable green screens to isolate the block, then extend/enhance the existing snow with matte painting and CG. See also: Stargate Studio
You cannot afford to travel to the desired location.
Ex: The scene takes place in New York, the Himalayas or on the surface of Mars (see also: Schedule and likely Permission, unless you’re John Carter).
The owners of the location will not give you permission to shoot there. (See budget)
The owners of the location cannot give you permission to shoot there due to legal restrictions, security, etc.
Ex: A blind man and his guide dog encounter TSA agents at O’Hare International Airport.
The location as called for in the script does not exist, for example in a period piece or docu-drama. Perhaps archival footage of the location will be used, an existing location will be enhanced through set extension and matte painting, or the location will be entirely created in CG.
Ex: Sci-fi or fantasy-based sets are frequently enhanced in post.
The location is a virtual set that has never and will never physically exist.
Ex: A commercial or news/sports/information piece may use a virtual set, a weather map or infographic, a patterned/abstract background or simply a plain white cyc.
The location, its occupants or the activity potentially endangers the cast or crew.
Ex: A person runs through a street of quickly moving traffic.
Ex: A couple have a dialog scene while riding a motorcycle.
Ex: A child plays in the lion exhibit of a zoo.
Ex: A person stands on the ledge of a tall building.
The green screen subject endangers the background environment or its occupants.
Ex: An explosion erupts in a crowded area.
Ex: Actual lions guard the steps of the Art Institute of Chicago.
Some concepts or budgets call for shots that involve a convincing illusion of the impossible, not merely the improbable.
The green screen subject must be enlarged or reduced in size compared to its normal existence.
Ex: Attack of the 50 Foot Woman… again…
Ex: Honey, I Shrunk the Kids… again… and again…
Ex: The Hobbits in Lord of the Rings appeared small in stature next to their human co-stars (although some of that illusion was created practically in-camera).
A scale model must stand in for the real thing. The choice to use a scale model is fundamentally budgetary, but of course actually enlarging it is physically impossible.
Ex: A model jet appears to be parked on a suburban street.
The project may involve compositing an actor into pre-existing footage that is, for whatever reason, impossible or impractical to recreate.
Ex: Tom Hanks appears in historical footage in Forrest Gump.
Enhanced physical abilities (ahem)
The actor needs to jump incredibly high, fly, etc.
Ex: I can’t think of any. Nope. None.
Altered physical appearance
An actor’s role may call for a body part replacement or removal. Dressing that part of the actor in green will allow it to be easily identified for removal. However, actually removing it is not simply a matter of keying out the green. You would also need to manually paint in the missing background information that the green body part was covering up and track and paint in a stump for the limb… for every frame.
Ex: Gary Sinise plays a double amputee in Forrest Gump.
Ex: Tom Cruise’s character in Valkyrie is missing the ring and pinkie fingers on his left hand.
Ex: Machine-gun-leg-girl in Grind House?
A project may call for multiple instances of the same, unique object or person to appear on-screen simultaneously.
Ex: Moon, Being John Malkovich, Multiplicity, etc.
Elements for effects
Some elements can be recorded against blue/green/black screen to be comped over other footage.
Ex: Smoke, fire, explosions, dust, etc.
Background object extraction
Portable green screens may be used on-set in order to create easier isolation and separation of complex, on-location objects in post.
Ex: A blue screen may be placed behind an on-location tree, allowing compositors to place a CG castle behind the tree.
If the project calls for particularly poor acting from even incredibly talented actors, consider placing them on a green screen set with poor direction, little to react to and inconsistent eyelines.
Ex: Episode I: The Phantom Menace, Sky Captain and the World of Tomorrow, etc.
What should my concept be for my green screen shoot?
Bassackwards, your thinking is.