Datenbank Glossar – Compositing

Advantages of digital mattes

Digital matting has replaced the traditional approach for two reasons. In the old system, the five separate strips of film (foreground and background originals, positive and negative mattes, and copy stock) could drift slightly out of registration, resulting in halos and other edge artifacts in the result. Done correctly, digital matting is perfect, down to the single-pixel level. Also, the final dupe negative was a “third generation” copy, and film loses quality each time it is copied. Digital images can be copied without quality loss.

This means that multi-layer digital composites can easily be made. For example, models of a space station, a space ship, and a second space ship could be shot separately against blue screen, each “moving” differently. (In such shots, it is the camera that moves, not the model). The individual shots could then be composited with one another, and finally with a star background. With pre-digital matting, the several extra passes through the optical printer would degrade the film quality and increase the probability of edge artifacts. Elements crossing behind or before one another would pose additional problems.

Colour correction:

Colour correction generally involves identifying reference colours. Of course the most obvious reference colours are black and white. In an RGB (Red-Green-Blue) colour space, black generally means minimum of all three colours and white means maximum of all three. However when it comes to printing, most printers are not RGB, but CMYK (Cyan-Magenta-Yellow-blacK). Because of the different ways in which shades of colour are created on paper and on screen, there are colours that can be represented on screen than cannot be represented on paper, and vice-versa. Also pure black can be represented in many different ways within the CMYK colour space because black can be printed by either using black ink, or by combining equal quantities of the other inks, or by using a little of all the inks, including black.

The brightest white thing in the image, i.e. the part of the image that most closely approximates to pure white

The darkest black thing in the image, i.e. the part of the image that most closely approximates to pure black

In both cases bear in mind that light sources (such as the sun) and reflections (e.g. sun bouncing off a window) do not count. Neither do shadows. The reason is that although these may well prove to be the brightest and darkest parts of your image, they are not necessary pure white or pure black and cannot therefore be used as reference colours. Instead you need to identify part of the image which you want to appear to be pure white or pure black, such as the white T-shirt of a person featured in the picture, a white window frame, etc. It is of course possible to have an image in which there are no reference whites or blacks, but this is quite unusual. For those images you will have to look for a reference grey, or colour correct by other means. (I will not describe other technique for colour correction here – there are whole books on the subject as I mentioned above.)

In the following examples I am using Adobe PhotoShop. Similar controls exist in other applications. From the curves dialogue, note the pipette colour picker icons on the bottom right (see below). If you select the one coloured black and then click on part of the image, the curves are automatically adjusted so as to make the chosen colour black. The rest of the image’s colours will shift accordingly. Likewise you can use the picker coloured white to select part of the image you want to be white. If you happen to know there is something in the image that is a pure grey (i.e. equal amounts of the colours) you can use the middle colour picker on it.

Luma

As applied to video signals, luma represents the brightness in an image (the “black and white” or achromatic portion of the image). Luma is typically paired with chroma. Luma represents the achromatic image without any color, while the chroma components represent the color information. Converting R’G'B’ sources (i.e. the output of a 3CCD camera) into luma and chroma allows for chroma subsampling, enabling video systems to optimize their performance for the human visual system. Since human vision is more sensitive to luminance (“black and white”) detail than color detail, video systems can optimize bandwidth for luminance over color.

Luma versus Luminance

Luma is the weighted sum of gamma-compressed R’G'B’ components of a color video. The word was proposed to prevent confusion between luma as implemented in video engineering and luminance as used in color science (i.e. as defined by CIE). Luminance is formed as a weighted sum of linear RGB components, not gamma-corrected ones[1]. SMPTE EG 28 recommends the symbol Y’ to denote luma and the symbol Y to denote luminance.[2]

Use of luminance

While luma is more often encountered, (photometric) luminance is sometimes used in video engineering when referring to the brightness of a monitor. The formula used to calculate luminance used coefficients based on the CIE color matching functions and the relevant standard chromaticities of red, green, and blue (i.e. the original NTSC primaries, SMPTE C, Rec. 709). For the Rec. 709 primaries the linear combination, based on pure colorimetric considerations and the definition of luminance (relative) is:

Y = 0.2126 R + 0.7152 G + 0.0722 B

The formula used to calculate luma in the Rec. 709 spec arbitrarily also uses these same coefficients, but with gamma-compressed components:

Y’ = 0.2126 R’ + 0.7152 G’ + 0.0722 B’, where the prime symbol ‘ denotes gamma correction.

Luminance

Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square metre (cd/m2). The CGS unit of luminance is the stilb, which is equal to one candela per square centimetre or 10 kcd/m2.

Luminance is often used to characterize emission or reflection from flat, diffuse surfaces. The luminance indicates how much luminous power will be perceived by an eye looking at the surface from a particular angle of view. Luminance is thus an indicator of how bright the surface will appear. In this case, the solid angle of interest is the solid angle subtended by the eye’s pupil. Luminance is used in the video industry to characterize the brightness of displays. In this industry, one candela per square metre is commonly called a “nit”. A typical computer display emits between 50 and 300 nits.

Luminance is invariant in geometric optics. This means that for an ideal optical system, the luminance at the output is the same as the input luminance. For real, passive, optical systems, the output luminance is at most equal to the input. As an example, if you form a demagnified image with a lens, the luminous power is concentrated into a smaller area, meaning that the illuminance is higher at the image. The light at the image plane, however, fills a larger solid angle so the luminance comes out to be the same assuming there is no loss at the lens. The image can never be “brighter” than the source.

Gamma correction

Film is carefully fixed display medium, but monitors are not. The entire film chain from film cameras to lab developing to film projecters is all very carefully calibrated and standerdized for consistent color replication.The same image displayed on three different monitors will have three different looks.The main factors that affect this are the monitor color tempaperature, the brightness contrast settings.and the gamma correction of each monitor.The color tempaperature, the brightness and contrast settings are addressed by normal monitor calibration, but because gamma correction is a matter of taste ,it varies from site to site. The purpose of the display gamma parameter is to compensate for the various gamma corrections used on different monitors.

Gamma compression, also known as gamma encoding, is used to encode linear luminance or RGB values into video signals or digital video file values; gamma expansion is the inverse, or decoding, process, and occurs largely in the nonlinearity of the electron-gun current–voltage curve in cathode ray tube (CRT) monitor systems, which acts as a kind of spontaneous decoder. Gamma encoding helps to map data (both analog and digital) into a more perceptually uniform domain.

The following figure shows the behavior of a typical display when image signals are sent linearly (γ = 1.0) and gamma-encoded (standard NTSC γ = 2.2). In the first case, the resulting image over the CRT is notably darker than the original, while it is shown with high fidelity in the second case. Digital cameras produce, and TV stations broadcast, signals in gamma-encoded form, anticipating the standardized gamma of the reproducing device, so that the overall system will be linear, as shown on the bottom; if cameras were linear, as on the top, the overall system would be nonlinear. Similarly, image files are almost always stored on computers and communicated across the Internet with gamma encoding.