Embedded Files
Component video
A term gave to a recording system which does not require NTSC(National Television System Committee) encoding of RGB signals, but uses a means of recording and routing the luminance and color signals separately. This method does not reduce the bandwidth nor compromise the RGB components and results in images of higher resolution and better color quality than composite video. Though designed for high end television production, similar recording methods have recently become available to the consumer market.
Composite video
A composite video signal is one in which the luminance, chrominance and sync information have been combined into a single signal using one of the coding standards: NTSC, PAL, SECAM, and so forth. This is the form the signal must take before it can be broadcast or recorded by standard means. Until recently, most monitors and projectors have accepted only composite video signals, although many now available accept RGB.
The NTSC composite signal format is required for most video recording, manipulation, and display in the United States. The exception is the high-end post-production facilities that use component video for improved signal integrity.
NTSC Composite vs. Component
In a composite signal, the luminance signal and the color signals are encoded together into one signal. When the color components are kept as separate signals, the video is called component analog video (CAV), which requires three separate signals: the luminance signal (Y) and the color difference signals (R-Y and B-Y).
Because component video does not undergo the encoding process, the color quality is noticeably better than composite video.
What is Blue Screen Imaging?
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 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.
Page updated
Report abuse