Welcome to the world of Blue Screen! Once the exclusive domain of Hollywood special effects artists, blue screen imaging has expanded to include video and computers. There are many mysteries to the succesful execution of a blue screen composite and considerable confusion as to what a blue screen composite is.
What is Blue Screen Imaging?
Creating a blue screen composite image starts with a subject that has been photographed in front of an evenly lit, bright, pure blue background. The compositing process, whether photographic or electronic, replaces all the blue in the picture with another image, known as the background plate.
Blue screen composites can be made optically for still photos or movies, electronically for live video, and digitally to computer images. Until very recently all blue screen compositing for films was done optically and all television composites were done using analog real time circuits.
In addition to blue, other colors can be used, green is the most common, although sometimes red has been used for special purposes.
Another term for Blue Screen is Chroma-Key. Chroma-Key is a television process only. A more sophisticated television process is Ultimatte; also the name of the company that manufactures Ultimatte equipment. Ultimatte has been the ultimate in video compositing for 20 years. With an Ultimatte unit it is possible to create composites that include smoke, transparent objects, different shades of blue, and shadows. Ultimatte now makes software that works with other programs to create digital mattes, either as a standalone program, or as a filter for programs such as Photoshop and Aftereffects.
How does Chroma Key work?
The Chroma Key process is based on the Luminance key. In a luminance key, everything in the image over (or under) a set brightness level is "keyed" out and replaced by either another image, or a color from a color generator. (Think of a keyhole or a cookie-cutter.) Primarily this is used in the creation of titles. A title card with white on black titles is prepared and placed in front of a camera. The camera signal is fed into the keyer's foreground input. The background video is fed into the keyer. The level control knob on the keyer is adjusted to cause all the black on the title card to be replaced by the background video. The white letters now appear over the background image.
Luminance keying works great with titles, but not so great for making live action composites. When we want to key people over a background image, problems arise because people and their clothing have a wide range of tones. Hair, shoes and shadow areas may be very dark, while eyes, skin highlights and shirt collars can approach 100% white. Those areas might key through along with the background.
Chroma Key creates keys on just one color channel. Broadcast cameras
use three independent sensors, one for each color, Red, Green and Blue. Most cameras can
output these RGB signals separately from the Composite video signal. So the original
chroma key was probably created by feeding the blue channel of a camera into a keyer. This
works, sort of, but soon manufacturers created dedicated chromakeyers that could accept
all 3 colors, plus the background composite signal and the foreground composite signal.
This made it possible to select any color for the key and fine tune the selection of the
color.
As keyers became more sophisticated, with finer control of the
transition between background and foreground, the effect became less obvious and jarring.
Today's high-end keyers can make a soft key that is basically invisible.
Recently I have been working with the Grass Valley Digital 4000 special effects switcher. This unit makes beautiful chromakeys from a good source. In combination with Hitachi SK2700w studio cameras the results are stunning. Fine hair detail is not lost and shadows and other variations in the backing can be maintained or dialed out. However it doesn't have the capability to remove blue spill, as an Ultimatte does.
Why Blue? Can't other colors be used?
Red, green and blue channels have all been used, but blue has been
favored for several reasons. Blue is the complementary color to flesh tone--since the most
common color in most scenes is flesh tone, the opposite color is the logical choice to
avoid conflicts. Historically, cameras and film have been most sensitive to blue light,
although this is less true today.
Green has it's own advantages, beyond the obvious one of greater
flexibility in matting with blue foreground objects. Green paint has greater
reflectance than blue paint which can make matting easier. Also, video cameras are usually
most sensitive in the green channel, and often have the best resolution and detail in that
channel. A disadvantage is that green spill is almost always objectionable and obvious
even in small amounts, wheras blue can sometimes slip by unnoticed.
Sometimes (usually) the background color reflects onto the foreground
talent creating a slight blue tinge around the edges. This is known as blue spill. It
doesn't look nearly as bad as green spill, which one would get from green.
Usually only one camera is used as the Chroma Key camera. This creates a problem on three camera sets; the other cameras can see the blue screen. The screen must be integrated into the set design, and it is easier to design around a bright sky blue than an intense green or red.
Lighting for Blue Screen
A considerable amount of mystery is usually attached to blue screen lighting design. Also, a number of myths have been nurtured through the years, most of which are only half true. Myth #1 is the flat lighting myth. While it is true that the blue screen must be lit evenly, this is not true for the talent or other foreground subjects. They may be lit as dramatically as you desire. The trick is in lighting the foreground without screwing up the background.
A great deal depends
on what matting process will be used. If you are using Ultimatte, then a great deal of
freedom is available. On the other hand, Chroma Key is not nearly so flexible and has more
restrictions. I am assuming that most of the readers are most interested in video or
computer uses, so I will not cover lighting for film mattes (perhaps someone with greater
experience in that area can create a page for film matting?)
Ultimatte units have controls that allow for "cleanup" of an uneven background and other adjustments to fine tune the matte. Ultimatte mattes can also maintain the background through shadows, veils, smoke, water, hair and other semi-transparent objects. Most Chroma Key units cannot even approach this level of subtlety.
One popular technique to minimize "the matte line" around the subject is backlighting. A straw, yellow, or CTO gel on the light helps to wash out blue spilling on the talent's shoulders and hair. (This technique is inappropriate for Ultimatte, as Ultimatte has a circuit that removes blue spill.)
If you are lighting a scene in which the subject does not need to be near the blue backing, then lighting is simpler because you can put distance between the subject and background. Generally you want the level of light on the backing to be the same as the level on the subject from the key light. In video terms, this would be between 60-75 IRE on a waveform monitor, although slightly lower levels will usually work. It is most important for the screen to be evenly lit. If the talent is standing or sitting on blue, then it is more difficult, almost impossible, to have separate lighting. With primitive chromakey systems, shadows can create a lot of difficulty, and so you must use a flat lighting scheme on the talent to minimize the shadows.
Many different lights work well for lighting the blue. Cyc lights are the old standard. A newer light rig called a "Space Light" also works well. This is a set of lights pointing up and down into a cylinder of white diffusing fabric. The new flourescent fixtures are ideal also. Some people use HMI's, on the theory that they will punch up the blue by using a blue light on the backing and warm tungsten light on the subject. Some special effects companies use transluscent blue screens that are back lit by dozens, even hundreds, of special blue flourescents.
An old favorite of pros and amateurs alike is a single thermonuclear fusion source,
placed 93 million miles away. This light source gives perfect corner to corner
illumination and makes a perfect match between the key level and backing level. Shadows
are easy as it makes only one set of shadows. If you place a water vapor diffusion screen several
thousand feet up, you get a great shadowless light. A thinner water vapor diffusion
softens the shadows nicely. Those who are inexperienced at controlling these types of
diffusion may want to use a large silk or other diffusion instead.
I'm serious--I've done some great mattes this way. If you're shooting
spacecraft models, this can be the best method. Plus the rental charge can't be beat. The
Death Star trench scene in Star Wars used this very same light source.
A waveform monitor is an essential accessory on a video blue screen shoot. Since it displays a graphic representation of the video level in the scene, small variations in brightness are very obvious. A screen that looks good to the eye may have considerable gradual falloff from top to bottom. I would recommend using one on film shoots, in combination with a cheap video camera. The graphic display is so much more useful in this case than a spotmeter.
Paints and Backings
The standard paints which almost everyone uses are from Rosco, the light gel manufacturer. They make ChromaKey Blue and Green, as well as Ultimatte Blue and Green. One of the reasons I dislike using green as a backing is that the green paint is difficult to apply and just looks hideous. There is nothing more unsettling than having to work on a stage that is completely covered in Ultimatte Green!
What is ULTIMATTE?
Ultimatte is a trademark of the Ultimatte Corporation, of Chatsworth CA. It is an outgrowth of work the company's founder, Petro Vlahos, did in the 1960s for the Motion Picture Research Council. The goal was to invent a better matting system for motion pictures. Electronic technology was not ready yet then for a film resolution system, but video could be achieved, and so the first Ultimatte units were created in the 70's.
It is useful to think of the Ultimatte process as a mixing process, not a keying process. This makes it possible to matte with shadows, hair, water etc. An Ultimatte uses the intensity and purity of the blue signal as a function to determine how much blending to perform between the foreground and background images. Another useful feature of the Ultimatte is the previously mentioned blue spill removal. Other circuits deal with glare, uneven or dirty blue backings, etc. Modern units from the Model V on up can independently adjust the color of the background and foreground plates. An Ultimatte used to have many knobs on its front panel, but the new digital units use a display screen and multifunction controls. The Current Model is the "8" and there are also models for High Definition work.
There are also very useful Ultimatte plugin filters for Adobe Photoshop and AfterEffects. Although the After Effects production bundle has an excellent matting filter of its own, it requires considerable manual tweaking of the controls to perfect the composite. The Ultimatte plug-in automates these functions, making the work of compositing much faster. Highly recommended and worth the cost if you have a lot of mattes to do.
A handy feature is Screen Correction, which allows the operator to create perfect mattes from really mediocre backings. With Screen Correction, a still is first recorded of the backing alone, with no talent or other non blue pieces. This recorded still is then fed into the screen correction input. The circuit cancels out all the uneveness of the backing before any foreground elements enter the scene.
Lighting for Ultimatte
Ultimatte Lighting is not so much difficult as it is misunderstood. Ultimattes can retain shadows onto the background plate. (As can other advanced compositing software programs, such as Aftereffects.) Yet camerapeople often run into trouble trying to create a shadow! This happens because they first light the blue and the subject with an overall flat light and then add a light on the subject to "cast" a shadow. They see a "shadow" on the background, but it doesn't show on the matte. The shadow is still lit by the overall key. The new light is pointlessly creating brighter area around the shadow.
The backing should be lit to the same intensity as the key light. So to retain shadows, in which the shadow is actually darker than the rest of the backing, the same light should be used to light both. Also the light must be even. If there are darker corners, then the composited background will be darkened in the corners also! You can use this effect to improve the look or even relight a background plate. Since a shadow on the backing becomes a shadow on the background image, the background can be "touched up". Very useful for backgrounds created in computer modeling programs, almost all of which have very bad and artificial appearing lighting tools.
Blue gels can't be used on the backing, if they will also light the talent. Another big problem (with all blue screen work actually) is blue floors. They invariably have a slightly different shade of blue. This is because the light is glancing off them at a different angle from the wall. (This glare effect can be removed with a polarizing filter. The downside is the two stop loss through the filter. The camera will need to open up two stops or the set will need 4 times more light.) Try to position lights so they are pointing in the same direction as the lens, and not straight down into the floor. This will reduce most glare to a minimum. Where this becomes a bigger problem is set pieces such as blue desks and props that pick up glare from side lights and back lights.
Also, never use dimmers on the lights lighting the background blue or green. If you are in a facility with dimmers, only use the lights at a full 100% This is because lowering a light's intensity with a dimmer also lowers it's color temperature, making it more orange, and therefore making the backing more orange, and less pure of a blue.
Another difficulty that causes beginning Ultimatte artists to tear their hair out is a lack of sidelighting. To the naked eye on the set, there may appear to be sufficient illumination on the sides of the subject. But the subject is in what amounts to a brightly lit blue bowl, and is bathed in blue bounce light. When the Ultimatte removes this blue spill, the subject suddenly has no side light, and very dark shadows. If the background plate is bright, say a beach scene, the subject looks very out of place. In fact the effect will almost look as if there is a brown matte line around the subject. So you need to provide the same fill lighting as the scene you are matting into would provide. This effect is easy to see if you are doing on set matting. If the matte is to be done in post, try to turn off as many lights as possible that only light the backing, while setting the subject's lighting. Generally it is best to start lighting the subject first, then adding fill light to the backing to even it out.
Film Compositing
Creating composites on film, for final display on motion picture film
projection, is another specialty onto itself. While the lighting concepts don't really
differ, the matting process is done optically and very different from electronic
processes. In the last ten years, most feature films have stopped using the optical
processes, in favor of scanning film negatives into digital form, creating the composite
on a computer, and scanning them back out to film. I had hoped that someone would create a
page detailing the fascinating steps involved in the optical creation of color-difference
mattes for film but as of yet, no one has. One advance is from Eastman Kodak. Their motion
picture film division has created a new stock specifically for shooting foreground film
elements. The name of the stock is SFX200 and is
only available in 35mm. I've used it once, and it seems to perform quite well.
It is generally not recommended to shoot any elements destined for
compositing in 16mm. This is because 16mm is more prone to image weave and unsteadiness
than 35. Even if the camera has perfect registration, the transfer to video is
problematic, as most transfer machines are not pin-registered. So the image can weave.
Still, it is possible to get good results, but you should be in consultation with both
your lab and your video transfer facility before shooting.