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Threshold converts the image to pure luminance, and replaces pixels with one of three colors based on the luminance. Pixels with luminance values in the low range are replaced with black, pixels in the middle range are replaced with white, and pixels in the high range are replaced with black. Color and alpha (transparency) for each range are configurable with the buttons and interpolate according to keyframes.
The threshold window shows a histogram of luminance values for the current frame. Click dragging inside the histogram creates a range (blue) to convert to white. SHIFT-clicking extends one border of this range. Values for the threshold range can also be specified in the text boxes.
This effect is basically a primitive luminance key to produce a matte. A second track above the track with the threshold effect can be multiplied, resulting in only the parts of the second track within the threshold being displayed.
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