Creating a Temporary Offset

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‌Creating a Temporary Offset

Suppose you are tracking a sign on a wall, and an actor suddenly walks in front of it, blocking it during frames 10 to 20. There's a light fixture a bit higher on the wall that

is never occluded by the actor. You'd like to use offset tracking to create one continuous track for the sign. Here's what to do:

1. Go to frame 9, the frame BEFORE the sign gets blocked, when both sign and light fixture are visible.

2. Turn on the Offset button on the Tracker control panel.

3. In the camera view, hold down shift and drag the tracker from the sign to the light fixture. This will leave the offset marker (final tracker location) at the original location (sign) and put the tracker on the feature to be tracked (light).

4. Track forwards normally (on the light), ie by hitting frame forward, the period key, Play, or scroll wheel in the mini-tracker view. If necessary, adjust the tracker location, but do not move the offset marker or change the offset spinners.

5. Track until you reach frame 21 stopping on frame 21when the sign and light are both visible again.

6. Adjust the position of the offset marker to re-position it exactly on the sign.

7. Turn off the Offset button. The tracker snaps back onto the sign.

8. Continue tracking the sign.

The process looks more complicated than it is in practice. Due to changes in the camera viewing angle, the required offset (from light to sign) won't be the same on at the beginning and end of the offset. The point of the steps above is to set exact keys on the offset channel at both ends of the offset section; the offsets interpolate linearly in between.

You can change to a different reference pattern at any time during offset tracking, simply by shift-dragging the tracker to a new feature. (This sets a new key on the offset channels at the prior frame, in addition to the current frame, to make the offsets behave properly; see the graph editor for details.)

You can also use offset tracking when you are tracking backwards (from large frame numbers to small frame numbers); the procedure is the same, though you track in the other direction.

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