However, they're not weighted evenly, because that's not how the eye sees them ( see Figure 6). If you set an RGB image as a luma matte, the red, green, and blue channels are averaged to determine the luminance of the overall image.
It's intuitive and relatively easy to use. You bring in the White Point or Black Point (the upper of the small square controls below the histogram) and then threshold (soften) that adjustment with the White Softness or Black Softness controls (the lower of the small squares). Its histogram shows the likely white or black thresholds on each channel. Although it may not be clear at this size, the blue channel (right) is noisier than the other two, typical of digital images, and one reason why greenscreens became popular in the digital era.Įxtract is interactive and easy to use, a cousin to Levels. Either green or red is typically the brightest and most contrasty channel, whereas blue generally has lower contrast and higher noise ( see Figure 5).įigure 5 The red channel contains the strongest foreground contrast (left) the green channel (center) shows that the matte contains some green. One of the three color channels nearly always has better defined contrast than overall luminance, which is merely an average of the three. You can work with averaged RGB luminance, or access histogram controls for each of the color channels. ExtractĮxtract employs a histogram to help isolate thresholds of black and white these thresholds are then graded with black and white softness settings.
For example, you duplicate a layer and matte its highlights to bloom them. When is linear keying useful? Keying using a single channel (or the average of multiple channels) is useful to matte an element using its own luminance data, in order to hold out specific portions of the element for enhancement.
Linear Color Key is a more appropriate tool to isolate a particular color (or color range). Seriously, there's no reason ever to use them, so it's unfortunate that they have the more memorable names.Įxtract is useful for luminance (luma) keying, using the black and white points of an image or any of its individual channels. Color Key and Luma Key cannot calculate semi-opaque threshold areas of a matte Edge Thin and Edge Feather controls instead crudely choke, spread, and blur a pure bitmap.
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