Position the Reveal Source

To get this retimed source into position, I'll start with a standard MatchMove node, trackType set to 4 point, and the four track points linked to the corresponding track data parameters in the original Tracker node. I could have tracked directly in the MatchMove node, but it's limited to four channels of track data. To give myself as much flexibility as possible I generally do the tracking in a Tracker node where I can add as many track data channels as I want. This gives me the ability to average several of the track data channels or do any kind of preprocessing or manipulation needed to create four usable points.

Ordinarily, I'd now simply copy the values from the first frame of track data to the source X/Y position parameters. This would give me that first frame lined up nicely with whatever frame of the original I happen to be working on. However, I need the lineup to be defined by whichever frame is coming into the MatchMove's foreground, not by one fixed frame. I'll get this by setting an expression on the source position parameters that references the track data values at the same time as the input frame, which is defined by the value of newTime in my TimeX node "sourcetime". For example, I set source1XPosition to track1X@@(sourcetime.newTime). Notice the "@@" notation, which tells Shake to evaluate the parameter name preceding it at the time defined by the expression following it. The parentheses aren't essential here, but use of them is a good habit to get into to avoid unexpected results with a more complex expression.



I can A/B the MatchMove against the original footage to check the lineup. (Don't forget to make the MatchMove's applyTransform active and set the outputType to Foreground.) Be aware that if your offset is set to take frames from before the current frame you'll see a mismatch on initial frames until the offset frame comes into the tracked data. A similar situation occurs at the end if your offset source is after the current frame. Before and after the keyed track data, values will be an extrapolation of the existing track data. Fortunately this is easy enough to avoid by starting out with your offset keyed to frames ahead, then move to frames behind. A Step curve is a good choice for the offset keys as it insures that you'll get only the offsets you've keyed, never an interpolation of them.

This whole setup is designed specifically to make it possible to use any frame in the sequence as a source -- take advantage of it! But in general, you'll want to key the offset to the closest frame that provides a clear area to reveal. The farther away in time you go, the more likely it is that the simple transform to match the plate into place will not look quite right due to perspective changes baked into the image.