Compositing : In visual effects post-production, compositing refers to creating new images or moving images by combining images from different sources – such as real-world digital video, film, synthetic 3-D imagery, 2-D animations, painted backdrops, digital still photographs, and text
Q. What are main concepts of compositing that one should learn regardless of what compositing app one choose to use?
You have to see things in black, white and grey, i mean seeing everything in its alpha channel, as that forms the basics of compositing. When you look at the image you shall look for its edge details and how it can be relieved from it background in terms of its alphas….it.
Compositing Mathematics/ concept:-
Alpha channel
32-bit graphics systems contain four channels — three 8-bit channels for red, green, and blue (RGB) and one 8-bit alpha channel. Simply, it’s the channel containing transparency information. In RGBA, A denotes alpha channel, alpha channel stores transparency information in black white and grey.
Black indicates 100 % transparency
white indicates 100 % opaque
Intermediate levels of grey indicate partial transparency/opaqueness
darker grey indicates more transparency… ie, as we move towards white, opacity increases.
Matte
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Mattes are used in photography and special effects filmmaking to combine two or more image elements into a single, final image. In film, the principle of a matte requires masking certain areas of the film emulsion to selectively control which areas are exposed. However, many complex special-effects scenes have included dozens of discrete image elements, requiring very complex use of mattes.
MASK
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As the name suggests…. masking out something. Suppose we have an image of a person standing in front of a building. If we want the person alone to be isolated we will resort to masking its like separating the person from the building using pen tool or suitable tools
Rotoscoping , keying techniques can be associated with masking.
Alpha compositing is the process of combining an image with a background to create the appearance of partial transparency. It is often useful to render image elements in separate passes, and then combine the resulting multiple 2D images into a single, final image in a process called compositing. For example, compositing is used extensively when combining computer rendered image elements with live footage. In order to correctly combine these image elements, It is necessary to keep an associated matte for each element. This matte contains the coverage information – the shape of the geometry being drawn – and allows us to distinguish between parts of the image where the geometry was actually drawn and other parts of the image which are empty.
To store this matte information, the concept of an alpha channel was introduced by A.R.Smith in the late 1970s, and fully developed in a 1984 paper by Thomas Porter and Tom Duff. In a 2D image element which stores a color for each pixel, an additional value is stored in the alpha channel containing a value ranging from 0 to 1. A value of 0 means that the pixel does not have any coverage information; i.e. there was no color contribution from any geometry because the geometry did not overlap this pixel. A value of 1 means that the pixel is fully opaque because the geometry completely overlapped the pixel.
If an alpha channel is used in an image, it is common to also multiply the color by the alpha value, in order to save on additional multiplications during the compositing process. This is usually referred to as pre multiplied alpha. Thus, assuming that the pixel color is expressed using RGB triples, a pixel value of (0.0, 0.5, 0.0, 0.5) implies a pixel which is fully green and has 50% coverage.
With the existence of an alpha channel, it is then easy to express useful compositing image operations, using a compositing algebra defined in the Duff and Porter paper. For example, given two image elements A and B, the most common compositing operation is to combine the images such that A appears in the foreground and B appears in the background; this can be expressed as A over B. In addition to over, Porter and Duff defined the compositing operators in, out, atop, and xor (and the reverse operators rover, rin, rout, and ratop) from a consideration of choices in blending the colors of two pixels when their coverage is, conceptually, overlaid orthogonally:
As an example, the over operator can be accomplished by applying the following formula to each pixel value:
where Co is the result of the operation, Ca is the color of the pixel in element A, Cb is the color of the pixel in element B, and αa and αb are the alpha of the pixels in elements A and B respectively.
Note that if it is assumed that all color values are pre multiplied by their alpha values (c = αC), we can write this as:
Alpha compositing on images can be done in most graphics programs.
Aspect Ratio: The ratio of width to height of a displayed image.
Academy Aperture
A film aspect ratio where the original image capture area is 1.37 (typical working image resolution 1828 x 1332) although it is often masked during projection to 1.85.
Anamorphic
A wide-screen film format that horizontally squeezes the image area captured on the negative by using an anamorphic lens. During projection the image is unsqueezed by the inverse amount to make it look “normal” again. Original image capture area is .838″ x .7″. Usually projected with a 2.35.1 aspect ratio. Cinemascope is the trade name of an anamorphic technique.
Vista Vision
A film format which runs standard 35mm film stock through the projector in a horizontal direction rather than vertically. Vista Vision frames are twice that of standard 35mm ones and they use eight film perforations per frame (sometimes it is called eight-perf). The aspect ratio of a captured Vista vision frame is 1.5 and a typical working resolution is 3072×2048.
Background Plate
The primary plate over which all foreground elements are composited. Usually live action.
Blue/Greenscreen
Blue/Green Screen Elements are often filmed in front of a uniformly lit blue, green or red screen. This forms a clear, bright color from which a matte can be extracted. This process separates into layers elements from the original image
Chroma Key
A matte extraction technique that separates a subject from its background based on a color that is unique to either the foreground or the background image. A pure color which has a strong “chroma” yields the best results.
Calibration
The adjustment of a display device to show colors as closely as possible to the way they will appear in their final viewing format. At Cinesite this usually refers to workstation monitors. Color Lookup Tables (CLUTs) are created which show the values of colors on the monitor and how they will look on projected film.
Cineon File Format
The most common file format, developed by Kodak, used to store and represent images scanned from original film for visual effects work.
Optical printer
An optical printer is a device consisting of one or more film projectors mechanically linked to a movie camera. It allows filmmakers to re-photograph one or more strips of film. The optical printer is used for making special effects for motion pictures, or for copying and restoring old film material.
Common optical effects include fade outs and fade ins, dissolves, slow motion, fast motion, and matte work. More complicated work can involve dozens of elements, all combined into a single scene. Ideally, the audience in a theater should not be able to notice any optical printers work, but this is not always the case. For economical reasons, especially in the 1950s, and later in TV series produced on film, printer work was limited to only the actual parts of a scene needing the effect, so there is a clear change in the image quality when the transition occurs.
The first, simple optical printers were constructed early in the 1920s. Linwood G. Dunn expanded the concept in the 1930s, and the development continued well into the 1980s, when the printers were controlled with minicomputers.
In the late 1980s, digital compositing began to supplant optical effects. Since the mid nineties the conversion to digital effects has been virtually total.
Aerial image
An aerial image is a projected image which is “floating in air”, and cannot be viewed normally. It can only be seen from one position in space, often focused by another lens.
Aerial image technology was used in optical printers and movie special effects photography before the advent of computer graphics in movie production, and also for combining animation and live action footage onto one piece of film.
Schufftan process
The Schüfftan process is a movie special effect named after its inventor, Eugen Schüfftan (1893–1977). It was widely used in the first half of the 20th century before it was replaced by the travelling matte and bluescreen effects.
The process was designed by the German cinematographer Eugen Schüfftan while working on the movie Metropolis (1927). The movie’s director, Fritz Lang, wanted to insert the actors into miniatures of skyscrapers and other buildings, so Schüfftan used a specially-made mirror to create the illusion of actors interacting with huge, realistic-looking sets.
Schüfftan placed a plate of glass at a forty-five-degree angle between the camera and the miniature buildings. He used the camera’s viewfinder to trace an outline of the area into which the actors would later be inserted onto the glass. This outline was transferred onto a mirror and the entire reflective surface that fell outside the outline was removed, leaving transparent glass. When the mirror was placed in the same position as the original plate of glass, the reflective part blocked a portion of the miniature building behind it and also reflected the stage behind the camera. The actors were placed several metres away from the mirror so that when they were reflected in the mirror, they would appear at the right size.
In the same movie, Schüfftan used a variation of this process so that the miniature set (or drawing) was shown on the reflective part of the mirror and the actors were filmed through the transparent part, as shown in the illustration.
Over the following years, the Schüfftan process was used by many other film-makers, including Alfred Hitchcock, in his film Blackmail (1929), and as recently as The Return of the King. The Schüfftan process was later replaced by matte shots, which were easier and more efficient to create.
Multiple exposures
In photography, a multiple exposure is an exposure in which the sensitivity to light is reduced and then increased at least once during the total exposure time.
Ordinarily cameras have a sensitivity to light that is a function of time. For example, a one second exposure is an exposure in which the camera image is equally responsive to light over the exposure time of one second. The criterion for determining that something is a double exposure is that the sensitivity goes up and then back down. The simplest example of a multiple exposure is a double exposure without flash, i.e. the camera image is responsive to light twice during the complete exposure.
Some single exposures, such as “flash and blur” use a combination of electronic flash and ambient exposure. This effect can be approximated by a Dirac delta measure (flash) and a constant finite rectangular window, in combination. For example, a sensitivity window comprising a Dirac comb combined with a rectangular pulse is considered a multiple exposure, even though the sensitivity never goes to zero during the exposure.
In film and photography, double exposure is a technique in which a piece of film is exposed twice, to two different images. The resulting photographic image shows the second image superimposed over the first. The technique can be used to create ghostly images or to add people and objects to a scene that were not originally there. It is frequently used in photographic hoaxes. It also is sometimes used as an artistic visual effect, especially when filming singers or musicians.
It is considered easiest to have a manual winding camera for double exposures. On automatic winding cameras, as soon as a picture is taken the film is typically wound to the next frame. Some more advanced automatic winding cameras have the option for multiple exposures but it must be set before making each exposure. Manual winding cameras with a multiple exposure feature can be set to double-expose after making the first exposure.
Since shooting multiple exposures will expose the same frame multiple times, negative exposure compensation must first be set to avoid overexposure. For example, to expose the frame twice with correct exposure, a -1 EV compensation have to be done, and -2 EV for exposing four times.
Medium to low light is ideal for double exposures. You don’t have to use a tripod if you combine different scenes in one shot. However, in some conditions, for example, recoding the whole progress of a lunar eclipse, a stable tripod is almost a must-have. Double exposures are not the limit, as multiple exposures are possible, but the photographer should try not to overexpose the film. Another way to superimpose pictures is digitally, using a software photo editor like Adobe Photoshop or GIMP. Simply alter the opacity of the two images and line them up over each other.
With electronic imaging, long duration sensitivity is not practical, so multiple exposures are usually made using CEMENT (Computer Enhanced Multiple Exposure Numerical Technique). This simulates the effect of multiple exposures numerically, by using a computer, or computation.
The use of CEMENT as a medium of artistic expression dates back to the 1970s and early 1980s with the invention of the wearable computer as a tool for visual artists. Using CEMENT for the production of visual art, especially when walking around with a flash lamp, is called light-vectoring. A more colloquial term for light-vectoring is dusting.
With traditional film cameras, a long exposure is a single exposure, whereas with electronic cameras a long exposure is best attained by integrating together many exposures. This averaging also permits there to be a time-windowing function, such as a Gaussian, that weights time periods near the center of the exposure time more strongly. Another possibility for synthesizing long exposure from multiple-exposure is to use an exponential decay in which the current frame has the strongest weight, and previous frames are faded out with a sliding exponential window.
Cineon Lightning Recorder
The Cineon Lightning laser recorder uses proprietary optics for optimal digital picture to film image transfer. The red, green and blue lasers expose the Eastman EXR color intermediate 5242 film without adding more grain to the film and matching the quality of the original negative.
Cineon Lightning Scanner
The Cineon scanner uses proprietary CCD (image capture chips) sensor technology for optimal film to digital picture image transfer. This technology is designed to scan 35mm film frames in all film aspect ratios and formats (including Vista Vision) at one-quarter, one-half or full film resolution.
Cine speed
A program offered in Cineon compositing software that modifies the length of sequences by subtracting or adding frames i.e. it is often used for lengthening and shortening shots.
Clean Plate
A clean plate (a plate with no subject in frame) is used to replace some portion of a shot. I.e. a clean plate might be used in a wire-removal to provide the image area that was obscured in original live action photography.
Color Correction
Changing the degree of contrast, highlights, color saturation and shadows in a shot. Color correction allows for the delicate adjustment of aspects that a cinematographer controls when shooting a film.
Crowd Replication
Small groups of actors or extras can be replicated digitally in a scene, creating on illusion of many people; often used in stadium, crowd or audience shots.
DataCine Master
This term is commonly used by people who work with a Spirit DataCine to complete a film transfer to data. It refers specifically to the use of a DataCine device for scanning of film.
Chalice
Cinema compositing software maker Silicon Grail has been acquired by Apple Computer, according to a Web page that replaces their existing Web site.
Apple has acquired technologies from Silicon Grail, including the RAYZ and Chalice product lines, reads the statement in its entirety. Founded in 1995, Silicon Grail was started by Academy Award winner Ray Feeney, who founded Hollywood digital effects pioneer RFX. The company’s first product was Chalice — the first commercially available high-end 2D film compositing solution. Developed to run on Irix, Linux, Windows and OS X, RAYZ is a compositor based on Chalice, which provides effects and color correction tools for cinema. The software has been used in feature films including Deep Blue Sea, Titanic, and Star Trek: Insurrection, Men In Black and others.
RAYZ sports support for drag and drop functionality, customizable menus and toolbars, and a reconfigurable layout that can be set up to suit the user. It implements features like Composite Decision List (CDL) technology, and implements Kodak’s CINEON image file format technology as well.
The news of Silicon Grail’s technology acquisition follows other similar moves from Apple as the company continues to expand its influence into the professional film production industry, including the company’s acquisition of compositing software maker Nothing Real.
An Apple spokesperson confirmed for MacCentral that they had acquired technologies from Silicon Grail and said only that “Apple intends to use the acquired technologies in future products.
Jayant Mehra
Faculty Member
Picasso Animation College, Lucknow