Linear Keyers and Hi-Con Mattes
There are cases in which edge detail isn't a factor because an element doesn't need to be completely isolated; for example, you might create a holdout area of an image using a high-contrast (hi-con) matte. To specify one area of a clip for adjustment, a linear key often will do the trick.
Linear keyers are relatively simple and define a selection range based on a single channel only—red, green, or blue, or even just overall luminance. They're useful in a wide variety of cases outside the scope of bluescreen and greenscreen shots, although the underlying concepts are also operative when working with such high-level tools as Keylight.
The most useful linear keyers:
- Linear Color Key
Keyers to be avoided at all times:
- Luma Key
- Color Key
Extract and Linear Color Key
Extract is useful for luminance (luma) keying, using the black and white points of an image or any of its individual channels. Linear Color Key is a more appropriate tool to isolate a particular color (or color range).
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. For example, you duplicate a layer and matte its highlights to bloom them.
Extract employs a histogram to help isolate thresholds of black and white; these thresholds are then graded with black and white softness settings. You can work with averaged RGB luminance, or access histogram controls for each of the color channels.
One of the three color channels nearly always has better defined contrast than overall luminance, which is merely an average of the three. Either green or red is typically the brightest and most contrasty channel, whereas blue generally has lower contrast and higher noise (see Figure 5).
Figure 5 The red channel contains the strongest foreground contrast (left); the green channel (center) shows that the matte contains some green. 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.
Extract is interactive and easy to use, a cousin to Levels. Its histogram shows the likely white or black thresholds on each channel. 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). It's intuitive and relatively easy to use.
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. However, they're not weighted evenly, because that's not how the eye sees them (see Figure 6). If you want to use a particular channel as a luma matte, use Shift Channels (see Figure 7).
Figure 6 This grayscale conversion (using Tint) of the channels shown in Figure 5 weights the three channels according to how the eye sees them. The result appears closest to—but not identical to—green.
Figure 7 Shift Channels shifts all channels to red. Alpha is set to Full On as a precaution against transparency data, which shouldn't be used for a luma matte.
Linear Color Key
Linear Color Key offers direct selection of a key color with an eyedropper tool. The default color is blue—ironic, since bluescreen keying is exactly the wrong thing to do with this tool, given the alternatives. The default 10% Matching Softness setting is arbitrary and defines a rather loose range. I often end up with settings closer to 1%.
There are three eyedropper tools in the Linear Color Key effect. The top one defines Key Color, and the other two add and subtract Matching Tolerance. The eyedroppers work directly on either of the adjacent thumbnail images, as well as in the Layer panel. Double-click the layer and choose Linear Color Key in the View pull-down (see Figure 8). For whatever reason, the eyedropper tools don't work consistently in the Composition viewer.
Figure 8 Linear Color Key can matte a non-primary color range; in this case, the magenta sign on the bus is selected. (Source footage courtesy of Pixel Corps.)
You can use RGB, Hue, or Chroma values to match color values. RGB usually will do the trick; in unusual situations, sample each option before you start fine-tuning. You can adjust Tolerance, which specifies how close the colors have to be to the chosen value to be fully matted; and Softness, which then grades the threshold, softening the edges according to how close the color values are to the target range.
The default setting for the Key Operation is Key Colors, which is straightforward enough. You might guess that the other option, Keep Colors, would simply invert the result. In fact, Keep Colors was designed for cases in which the first instance of the effect eliminates something you want to bring back. That's right—you can recover foreground colors despite the fact that they've already been keyed out. Add a second copy of Linear Color Key targeting a second range, set to Keep Colors.
A difference matte is simple in principle: Frame two shots identically, the first containing the foreground subject, the other without it (commonly called a clean plate). Now, have the computer compare the two images and remove everything that matches identically, leaving only the foreground subject. Great idea.
To set up a second instance of Linear Color Key to Keep Colors, you should temporarily turn off the first instance (set to Key Colors), in order to set the color to keep using the eyedropper (see Figure 9). Alternatively, work in Layer view.
Figure 9 Two instances of Linear Color Key. The first is set to Key Colors, the second to Keep Colors.
In practice, of course, all sorts of criteria preclude this technique from working very well:
- Both shots must be locked off or motion-stabilized to match. Even then, any offset—even by a fraction of a pixel—can kill a clean key.
- The foreground element may be entirely unique from the background. Low luminance areas, in particular, tend to be difficult for the Difference Matte effect to discern.
- Grain, slight changes of lighting, and other real-world variables can cause a mismatch in two otherwise identical shots. Raising the Blur Before Difference setting helps correct for this problem, but only by introducing inaccuracy.
To try it for yourself, begin with a locked-off shot containing foreground action, ideally one in which a character enters the frame. Duplicate the layer, and lock off an empty frame of the background by using Layer > Time > Freeze Frame. Apply Difference Matte to the other layer. Adjust Tolerance and Softness. If the result is noisy, try raising the Blur Before Difference value.
Figure 10 shows the likely result of attempting to key some footage by using only Difference Matte. It's not a terrible way to isolate something when clean edges are not critical, but it can't compare with more sophisticated methods for removing a solid color background.
Figure 10 The effects shot (left) includes a clean plate (center); and the alpha channel of a Difference Matte effect (right). The result demonstrates that this isn't an effective way to key a foreground. Unwanted features such as the tracking markers disappear, however, so this matte could be combined with a regular greenscreen key.
The hand-to-hand greenscreen shots in this article (such as those in Figure 10) include tracking markers on the background. These markers are intended to provide dimensional perspective for 3D camera tracking (a.k.a. match moving), which otherwise might not be possible.