The Blue / Green Screen Page, Pt. 2

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 should 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.

    Much depends on which matting process will be used. If you are creating the mattes live, with real time hardware processing, using Ultimatte, then a great deal of freedom is available. On the other hand, Chroma Key with a Special Effect Generator is not nearly so flexible and has more restrictions. I'm assuming that most readers are interested in video or computer uses, so I will not cover lighting for film mattes. Also, traditional photochemical film matting has pretty much completely dropped out of use, as the cost of creating digital composites from film masters has plummeted.

    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.) A good but not excessive backlight does often help to make a line less noticeable.

    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 (as viewed hitting an 18% grey card, or caucasian face, not the actual screen itself), 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 and very soft lighting scheme on the talent to minimize the shadows.

    Many different lights work well for lighting the backing. Cyc lights are the old standard. A newer light rig called a "Space Light" also works well, particularly for the floor. 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 specially fabricated blue or green 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 lighting 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 lumninance levels 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 can be so much more useful in this case than a spotmeter alone.

Paints and Backings

    The standard paints which almost everyone uses are from Rosco, the lighting 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!

You can buy Rosco blue or green paint, fabrics, tapes and Photoflex backdrops directly from my links to Studio Depot.  

Stewart Filmscreen of Torrance CA makes a backlit screen.

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 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. It is possible to 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. There are models for both Standard and High Definition work. These all work in real time directly on the video sources.

  In addition, there are very useful Ultimatte plugin filters for Adobe Photoshop, AfterEffects and Premiere Pro, Apple Final Cut Pro, and Avid. Although After Effects 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 a 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, which often 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 its 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 hardware or software removes this blue spill, the subject effectively has no side fill light, and thus 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 matte-ing. 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.

I've set up links to buy paint, tape, and fabric for blue and green screen shooting through StudioDepot.com.

Bob Kertesz is the Grandmaster of Ultimatte. His page is www.bluescreen.com. He's also developed a refinement of Ultimatte techniques to create very useful on set alignment mattes for film shoots. And, he now sells Ultimatte software online.
Ultimatte  specializes in  matting hardware and software for motion pictures, stills and video.
Primatte sells a chroma key plugin for Photoshop, Aftereffects, Combustion and other imaging programs.

Books About Special Effects

    Most books about compositing have been about the post production side, not the on set side, but there is finally a book that covers both: Bluescreen Compositing by John Jackman.

 

Digital Compositing for Film and Video by Steve Wright
This book seems very useful for anyone making digital composites from either film or electronic masters. It is not specific to any one software package, but rather focuses on the tools that all have in common, and more importantly really explains well how they all work.

The Art and Science of Digital Compositing by Ron Brinkmann

Another interesting book is Digital Compositing in Depth by Doug Kelly. It includes a CD-Rom and is specific to Digital Fusion and other software programs.

The following is a good general text on Special Effects:
    The Technique of Special Effects Cinematography  Raymond Fielding's classic. It is about film optical/mechanical effects; not video or digital effects.
    Here's a page of Useful Books on Film and Video topics.

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