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By default, the planar tracker will look at only the luminance data of the image as it does its matching. This works well on many common shots with good contrast, and saves processing time versus full RGB processing, which predictably takes about 3 times as long for the image-related processing.
You can select any image channel for processing, using the channel selector flyout immediately above the tracker mini-view. It is the rectangular patch with a top to bottom gradient. You can select among full 3-channel RGB processing; red, green, or blue channels; (the default) luminance; or the alpha channel.
Notice that the tracker mini-view shows the full RGB image, even when the channel selector is set to a monochrome selection such as luma or green. That RGB display means you can't see what the tracker is seeing. To see the channel being tracked, click the button immediately to the right of the channel selector, with 3 overlapping RGB circles, or a single gray one. This button selects whether either the full RGB image or the selected channel is shown in the tracker mini-view. This is a temporary user-interface control, not a permanent property of any tracker.
When an image has a single strong color, for example a green or blue screen, you can often choose one of the other channels and get very good contrast. (The blue channel is usually noisiest for video cameras.)
If the image has little luminance contrast but a variety of colors, you should switch to the full RGB processing.
If you want to track a chroma-keyed shape, supplying an alpha channel and tracking it may be a good choice. (The alpha channel can be part of the source images, or a separate set of images, see Separate Alpha Channels in the main SynthEyes user manual.) Tracking a chroma-key shape with no interior detail does not work because the pattern can shrink continually, while still exactly matching the reference image. It is the border that conveys useful information. You have to track some of the surrounding background, to prevent the tracker from being able to shrink, and then changes in the background moving behind the keyed object will affect tracking. If you track a keyed alpha channel, then the background is always black and the keyed object white, and the larger tracker receives no interference from the background.
Note : when preparing to use alpha-channel tracking, be sure to check the Keep Alpha checkbox when opening the shot.
Important : Do not use alpha-channel masking when tracking the alpha channel, as this will allow trackers to shrink arbitrarily.
Additional channels could be added in the future: an edge channel is likely for another approach to lighting invariance (in the interim, use the image preprocessor!). Other possibilities include chrominance or saturation or the color differences R-Y and B- Y (these differences are efficient to generate). Your suggestions are welcome.
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