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"Is CHILDHOOD always the best time ?”
15 years ago
Compositing is a technique by which one shot is super-imposed on another, resulting in a composite shot. A common example is our everyday weather forecast on TV. The weather map is a separate computer generated shot onto which the announcer is super-imposed, making it look as if he/she is standing in front of a giant TV screen flashing different weather images.
By separating the foreground and the background into distinct layers, we can manage each layer with much more control. As you will see, this technique alone has given rise to enormous possibilities in the special effects realm. Let us study this technique with the help of an example. The following shot involves flying a plane through a congested city, between tall skyscrapers. Obviously, this is a very risky shot, and would not be permitted by any sane mayor of any large city. The only alternative is to resort to special effects.
Sources : Internet
Look at a bright light for a few seconds and then abruptly close your eyes. The image of the light seems to stay in your eyes a little longer even though your eyes are closed.
This phenomenon is termed as Persistence of Vision because the vision seems to persist for a brief moment of time.
When the retina of the eyes are excited by light, they send impulses to the brain which are then interpreted as an image by the visual cortex in the brain. The cells in the retina continue to send impulses even after the incident light is removed. This continues for a few fractions of a second till the retinal cells return back to normal. Until that time, the brain continues to receive impulses from the retina, and hence seems to perceive an image of the source of light, giving rise to the phenomenon called Persistence of Vision.
This phenomenon is termed as Persistence of Vision because the vision seems to persist for a brief moment of time.
When the retina of the eyes are excited by light, they send impulses to the brain which are then interpreted as an image by the visual cortex in the brain. The cells in the retina continue to send impulses even after the incident light is removed. This continues for a few fractions of a second till the retinal cells return back to normal. Until that time, the brain continues to receive impulses from the retina, and hence seems to perceive an image of the source of light, giving rise to the phenomenon called Persistence of Vision.
The Principle of Motion Picture is totally based on the phenomenon of Persistence of Vision. Without it, motion picture as we know it simply would'nt exist. Our eyes can retain a picture for a fraction of a second after seeing one. Before this time frame expires, if a another similar picture is shown in its place, the eyes see it as a continuation of the first picture, and don't perceive the gap between the two.
If a series of still pictures depicting progressively incrementing action is flashed before the eyes in rapid succession, the eyes see it as a scene depicting smooth, flowing action. All visual media (Movies, TV, Electronic Displays, Laser Light Shows, etc) exploit this phenomenon.
Thanks to Persistence of Vision, our entertainment industry could make a transition from perpetual live shows like dance and dramas, to recordable entertainment like movies.
If a series of still pictures depicting progressively incrementing action is flashed before the eyes in rapid succession, the eyes see it as a scene depicting smooth, flowing action. All visual media (Movies, TV, Electronic Displays, Laser Light Shows, etc) exploit this phenomenon.
Thanks to Persistence of Vision, our entertainment industry could make a transition from perpetual live shows like dance and dramas, to recordable entertainment like movies.
Motion of an object is the continuous displacement of the object in space with reference to another object. In the absence of a reference object, motion ceases to be apparent. What this means is that motion is always measured in relation to another object, which is used as a reference point.
When we drive, the road & the surroundings move past us. Thus we get the sensation of motion. So the road & surroundings are our reference points. When we fly, the earth beneath us is our reference point. But as you can see, the closer the reference point, the more acute the sense of motion. That's why astronauts in orbit seldom sense speed (though they are moving at thousands of miles an hour ) because earth, their only reference point is quite far away.
OK, but what has this got to do with Special Effects ?! A Sfx technique called Compositing totally relies on the way our mind perceives motion. Compositing is one of the most useful tools in a Sfx technician's bag of tricks. Keep these two in mind; the object, and its reference point(s); both of these are necessary to perceive motion in a scene.
When we drive, the road & the surroundings move past us. Thus we get the sensation of motion. So the road & surroundings are our reference points. When we fly, the earth beneath us is our reference point. But as you can see, the closer the reference point, the more acute the sense of motion. That's why astronauts in orbit seldom sense speed (though they are moving at thousands of miles an hour ) because earth, their only reference point is quite far away.
OK, but what has this got to do with Special Effects ?! A Sfx technique called Compositing totally relies on the way our mind perceives motion. Compositing is one of the most useful tools in a Sfx technician's bag of tricks. Keep these two in mind; the object, and its reference point(s); both of these are necessary to perceive motion in a scene.
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.
Sources : Internet
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.
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