Chromakey
From Fan Film Wiki
Chroma keying is a technique you may use to place actors in an area in which they aren't recorded. In most of the cases, this will be done when there is no time or money to build a set, or if such an environment cannot exist due to physical reasons.
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Basic principle
The basic principle of chroma keying is replacing a recorded piece of image with a specific color (often a color matte recorded in a studio, both green and blue are popular) with another piece of image that is in some way generated. In television production, a very common trick is cloning a character, and making it have a dialogue with itself. This is achieved by recording the character once in a studio, chroma key that image and 'paste' the resulting image on the recording with the character itself. When using a chroma keyer, all material that is designated as 'matte' (background) will be turned transparent.
Technique
Technically speaking, a good chroma keying result can be achieved with very little means. The idea is exactly the same as the "magic eraser" tool in Photoshop, with the only difference that a chroma keyer works on moving images (video) instead of photographs. The most important component in getting a good clean chroma key result is having a solid background which is evenly lit.
Buying the screen
Usually there are three colors used by keying: green, blue and red. Green and Blue are most common, since those colors aren't in the skin color. For a digital key, green often works better but blue will give adequate results, where blue is more common in the analog hardware keying. For most work, green should do very well, but avoid having shots where one of the characters is wearing a color similar to the screen (e.g. a person with all-green Orion Slave Girl make-up would become essentially invisible, the same goes for an Andorian in front of a blue background. This is also a common trick used in movies to create an "invisibilty cloak" type of garment.)
With digital keying, don't be afraid to go a little out of the regular keying routes when you have very elaborately colored characters. For example, if you have someone with green skin and a blue uniform, neither of the colors will work. But red, for example, isn't in that character's recording so it should give a good keying result. Don't be afraid to use either white or black as well and try out luma keying if things don't work as expected.
Often, keying backgrounds are made out of a somewhat heavier fabric or cardboard, one of the secondary requirements for a keying fabric is that it is durable. This is very important when your studio is mobile -- for fixed base installations a piece of plywood (or several pieces linked together) works a lot better since that always gives a straight, even background.
When using fabric, suspend it off a stand or truss bridge and let it drop down by its own weight. This should reduce creases in the fabric and keep it nice and level (remember, the most useful energy source for film makers that's completely free is called gravity.), and if the fabric doesn't drop completely vertical by itself, you could make a seam at the bottom end and insert a broomstick or scaffolding pole.
The best color for your keying fabric or paint is a somewhat darker shade of green, as a more apple green can easily result in yellow spots when using unfiltered incandescent or halogen lighting. Lighter greens will work well with cooler colored lights like projection (metal halide) or fluorescent, or any incandescent lights with a filter. The green used traditionally for card and pool table games is a good example, as it can accept a decent amount of light without creating bright spots, and the darker color makes it absorb shadows easier. For paint matching, RAL 6000 should be a good starting point.
Lighting
For keyed recordings, lighting is very important. One of the first errors beginning producers make is to improvise with lighting. Lighting makes the difference between a great shot, and something you can discard right away.
Character lighting
Three point lighting
Three point lighting is the most common lighting scheme used in movie and television production today. Three main light sources are used: Keyight, flood light and back light. For chroma keyed setups, the background has to be lit individually.
- Keylight is the main (key) source of light, pointed straight at the main character. Often the key light is offset to the studio corner, not to interfere with the camera path.
- Floodlight is used to give depth in your picture. It points at a straight angle off the keylight, and is placed on the other end of the studio. For example, in a straight close-up shot, the key light alone will result in a very shadowed out side on the other end of the picture. The floodlight is used to fill in this dark spot.
- Backlight is used to give a nice outline and get rid of nasty shadows. Backlight should be placed in the same line as the keylight, only behind the actor.
Screen lighting
Simple and free
Lighting the screen itself is a pretty tricky job, especially with artificial light alone.
The best light source used for lighting a green screen is simple and free: daylight. Avoid using direct sunlight as this has the same effect as a direct spotlight, and will only induce shadows on your screen. When recording outside, the best weather conditions are slightly overcast without chance of rain.
Number two on the list is any even source of ambient light you can provide -- in a large conference or classroom, use the overhead fluorescent lights to get a decent ambient light level. The main consideration is that the light should be diffuse instead of direct.
Artificial lighting
In terms of artificial lighting, the use of high powered studio lights (several kW's each) with soft boxes is the way to go if you have access to broadcast quality equipment. However, for most amateur filmers, these are impractical because they are bulky, power-hungry and expensive.
The next best thing for artificial lighting is to get a set of soft boxes and provide your own lighting. Dark Armada uses a set of 'improvised' studio lights with off-the-shelf soft boxes which work very well. If your budget is tight but your metalworking or carpentry skills are not, you could try building your own soft boxes. For small setups, reflective photo flash umbrellas work very well, and you could also turn some pieces of styrofoam insulation into a makeshift flecky.
The keying process
The keying process itself depends on your mode of recording. Most amateur filmers these days work on DV or HDV tapes for recording, but those of you on a budget could consider using an analog keying device. Most consumer-level video mixers are rebuild broadcast models (especially in the '90s, S-VHS was a very popular format among amateur film makers and some very good mixers were produced) and produce very decent results.
If your budget isn't an issue or you have access to sponsoring or rentals, you could go all the way and get a DV hardware keying unit (such as an Ultimatte DV) for your project. But for most of us, this is beyond reach. Going software is a good and more affordable solution. For a detailed explanation on software keying processes, see Post-production.
It is recommended to always do a test run of a few minutes with various characters, shot positions and lighting situations, to make sure your setup works out. If something is wrong, you can diagnose it and fine-tune it right away instead of needing a reshoot later.
Production
Camera and character setup
The most practical setup for most keying scenes is one camera, positioned several metres away from the background (5-7 m is a recommended distance). For ease of processing and if possible, zoom the camera so that the background fills the entire image.
A common beginner's mistake is to place the characters right in front of the screen, with little or no spacing in between. This is witnessed in the first seasons of Hidden Frontier, and results in very visible keying auras. Keep a few metres distance between the character and the screen (and focus the camera on the character!), the added advantage is that the screen in the back will become blurred and out of focus (and thus a more even color) due to depth of field.
Acting and filming
A character often interacts with his or her environment during the course of a shot, evidently this environment is nonexistent in the studio. Many projects use some 'mock up' set pieces in the global shape of the object that is interacted with (for example, a character sitting behind a console would have a green colored mock up in front with which he or she can interact), making the acting less difficult. Taking the console example, typing into thin air is very difficult to judge, whereas typing on a green colored object with the correct dimensions will often give very professional results. If you have the means, making small set pieces is not out of the question, real set pieces make the recording and interacting a lot more interesting. A corridor scene, for example, needn't contain the entire corridor but only the piece of wall or section where the character is, with the rest filled up by CG footage. Even in the professional world this is used a lot, both Battlestar Galactica (hangar deck) and Stargate Atlantis (sound stage) use this intensively.
Post Production
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