Check out my Online Store for Blue and Green screen materials.
Welcome to the world of Blue Screen / Green Screen compositing! Once the exclusive domain of Hollywood special effects artists, blue screen compositing expanded to include video and computer imaging. 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?
(First a note about terminology: when I first wrote this page in 1995, the common term in use was "blue screen" compositing. Since then the vogue has shifted to calling it "green screen". The broader term from the days of film optical effects is "traveling matte composite", but that has fallen out of favor. For now, this page will mainly refer to the process as blue screen, but almost everything here applies to green screen effects too, except where noted.)
Creating a blue screen composite image starts by photographing a subject in front of an evenly lit, bright, pure blue (or green) background. The compositing process, whether photo-chemical or digital, 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, with dedicated real time hardware for live video, and digitally using software to composite still and motion images. Until the the 1990s most blue screen compositing for films was done optically, and all television composites were done using analog real time hardware.
In addition to blue, other colors can be used. While green has become the most common; 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 After Effects from Adobe.
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.) For example, 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 luminance 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 backdrop.
Chroma Key creates keys on just one color channel. Broadcast and high end consumer cameras cameras use three independent sensors, one for each primary color-- Red, Green and Blue. Many 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 luminance 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 tint, chroma level and luminance level.
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 essentially undetectable. Some of the best modern Special Effects Generator Switchers from Grass Valley Group, Sony, and others can create composites rivaling the performance of a dedicated Ultimatte unit. (Though they are not as good at removing blue spill, working through water or fabric, etc.)
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 its 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 least noise in that channel. A disadvantage is that green spill is almost always objectionable and obvious even in small amounts, while 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.
Traditionally, a single camera was 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. However, modern Special Effects Generators (Usually just called "Switchers" in the US, more accurately called "Vision Mixers" in the UK and elsewhere) can accommodate multiple camera sources, whether as RGB analog, or SDI video, inputs.
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Forward to Page 2 of the Blue / Green Screen Page:
Lighting and, Ultimatte
Forward to Page 3 of the Blue / Green Screen Page:
Film Composites, and Camera Recommendations
I've set up a page of links to buy paint, tape, and fabric for blue and green screen shooting.
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.
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 is a page of Useful Books on Film and Video topics.
Other pages by me that may be of interest:
My Home Page, with more links, and a resume.
Copyright 1995-2008 by Steven Bradford, All Rights Reserved
Mail: Steven Bradford