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The difference key creates transparency in areas which are similar between two frames. The Difference key effect must be applied to two tracks. One track contains the action in front of a constant background and another track contains the background with nothing in front of it. Apply the difference key to the track with the action and apply a shared effect of it to the track with the background. The track with the background should be muted and underneath the track with the action and the color model should have an alpha channel. It’s hard to get good results.
Pixels which are different between the background and action track are treated as opaque. Pixels which are similar are treated as transparent. Change threshold in the difference key window to make more pixels which are not the same color transparent. Change slope to change the rate at which the transparency tapers off as pixels get more different. The slope as defined here is the number of extra values flanking the threshold required to go from opaque to transparent. A high slope is more useful with a low threshold because slope fills in extra threshold. Use value causes the intensity of pixels (luma) to be compared instead of the color. Applying a blur to the top track with just the alpha channel blurred can soften the transparency border (figure 10.42).
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