Com Positing Notes Ok

What is compositing Composting may be define as combining multimedia images to create a single image. In visual effects post-production , compositing refers to creating complex images or moving images by combining images from different source such a real world digital video, digitized film, 3-D imagery, 2-D animations, painted backdrops digital still photograph and text. This is normally performed in movies to enable a computer graphic generated character appear on a previously filmed background. •

Video compositing is the technique of combining layers of video and still images, sometimes hundreds of layers are combined to form stunning visual compositions. It is also a way to combine hand drawing with computer generated imagery or live video.

The elements involved in composting The main elements in composting are selection, copy and paste operations and positioning of image elements. The five aspects require blending, colour matching and general attention to detail. Compositing today Today compositing generally involves the use of computers. In future film, movies may be shot on 35 mm film. For compositing, the film has to be digitized with a film scanner. It is then transfer to a computer where it can be edited. The computers gather all material they need for a scene and then combine the different clips to active the result that they need. The professional filmmaker use software of compositing is combustion and after effects. 1. Ever been at the movies and saw an actor do something that seemed impossible? 2. Have you wondered how Spider-man could hang from a building one second and be sailing hundreds of feet through the air the next second?

3. They were making use of a film technique known as ……….. Video Chroma Keying Blue Screen Imaging

The color of the background that can be used for keying is normally either blue (above) or green (below) The color can be electronically removed and another video source or virtual set inserted (keyed in) to replace it.

Introduction of Chromakey Chromakey compositing is a technique that has been used for many years in the TV and film industry for separating actors and presenters from a background, and then adding (compositing) a different

background electronically, it is sometimes referred to as matting. This technique is just an automated process of making cut-outs, a process we do every day in Photoshop.

A model in a studio sitting in front of a blue background can be made to appear within a computer-generated fantasy scene. Although this is a fantasy scene, very realistic settings can now be computer generated And keyed in behind talent.

Computer-generated set Virtual Reality Sets Within the last few years computergenerated graphics have moved into a whole new arena simulating complete settings. These are referred to as virtual reality sets. On the left a model with a fan in front of her is made to look as if she is bouncing along on a jet-ski in the ocean. A video recording of the ocean and surrounding scenery taken from the back of a speeding boat is keyed into the green area behind her. This results in a realistic scene, and the model doesn't even have to get her hair wet!

The woman standing against the blue background on the right. Now look in the two monitors at the top of the photo and notice that a complete setting has been inserted (keyed into) this live scene. Although the woman may not be able to actually sit down in the chairs seen in the setting, she can convincing walk behind them and they will pass in front of her body as she moves.

However, she can sit down on the blue box shown behind her in this photo, which, if things are positioned correctly, can make it appear that she is sitting down on one of the virtual reality chairs. The camera can pan, tilt, and zoom on her, and the keyed-in setting will shift appropriately.

Matching the Sources When video sources are combined the effect should be "seamless," that is, the combined result should blend together unnoticed. Six characteristics of the video sources should be consistent. 1. The brightness and contrast of the video sources must be the same. 2. The lighting must match, especially the direction of the key light. 3. The color balance of the video sources must match, especially if skin tones are involved. 4. The sharpness of the video sources should be the same. As a last resort you may have to slightly soften one source so that it will not "stand out" from a companion source. 5. Video sources as well as film have a subtle but inherent, "grain" or "noise" pattern. If one video source is "cleaner" than another, the difference will tip off views that something is amiss. 6. The relative perspectives must match. This is primarily established by the original lens focal length and camera-to-subject distance.

Blue Screen Imaging Video Chroma Keying is also known as "blue screening" in the movie industry. You may have seen it in action before, where an actor stands in front of a large blue screen and acts out the scene by himself.

Next, the camera operators slip a background in behind him, making it appear that the actor is bounding from building to building or climbing the side of a volcano in an attempt to destroy an evil ring!

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 colours 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.

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 colour from a colour generator. A Luminance Key is really only useful when creating grayscale images, such as in black and white imagery. 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 colour channel. Broadcast video cameras use three independent sensors (CCD) one for each color, Red, Green and Blue. Most cameras can output these RGB signals separately. 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. This made it possible to select any color for the key and fine tune the selection of the color. 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 opposite color to flesh tone (Skin) …since the most common color in most scenes is flesh tone, the opposite color is the logical choice to avoid conflict. Blue or Green is generally chosen as a key color because there is no blue or green

in a flesh color (Skin), you want to use the a chromakey color that is very saturated and that it is not present in the foreground subject that we want to cut-out. The most effective colors are Blue and Green. 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, whereas blue can sometimes slip by unnoticed. Lighting for Blue Screen Flat lighting Lighting is the most important part of Chroma Keying. You can never be too careful with it. Take extra care properly lighting your background, and use large white cards to bounce light so that you can achieve even illumination across the entire screen. Remember, different degrees of illumination can create color variations that can confuse digital camera. 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.

As you can see, it's too bright (background blue in the bottom). Normally this would mean a bad key.

This is a screen corrected version of the same shot. The background is now evenly lit.

Once the subject has been properly lit and the background color successfully keyed, it is time to add the new background scene. 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. One popular technique to minimize "the matte line" (blue tinge) 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. 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. 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. 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. Camera will need to open up two stops or the set will need 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. • (Colour temperature is a way to characterize the spectral properties of light source. Low colour temperature implies warmer (more yellow /red) light, while high colour temperature implies a colour (more blue) light. Presets for a number of different light source and conditions are provided in degrees Kelvin, the standard unit of measure for colour temperature.)

Another difficulty that causes beginning artists to tear their hair out is a lack of side lighting. 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. 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. Video Camera and Format Recommendations Good mattes are affected by several factors, from lens quality to resolution to signal to noise ratio. The background might come from any source, and doesn't affect the technical quality of the matte. But the foreground video must be as high in resolution and low in noise as you can afford. The best option is to Use a Studio or EFP Camera feeding an RGB (Component signals) signal directly into the inputs of an the Chromakeyer. Today's high end cameras can achieve amazing resolution, well in excess of most recorders to record it. Next best is to directly record the camera component output through a SDI connection (Serial Digital Interface) directly to a digital format such as DigiBeta, or DVCPro50. After recording, the foreground material can be played back as the foreground source into the keyer or it can be captured through

component inputs to a digital file for use in one of the many compositing programs. DigiBeta, DVCPro 50, though compressed slightly, sample the luminance and chrominance at the same rate as the non-compressed Digital Formats.