How do I replace a moving object?

[Graham Quince]

You need to use After Effect’s tracker on the original video. Select the video in your timeline, and go to the top menu, Animation>Track Motion.

Place the cross hairs within the falling object and make sure the inner box is large enough to see all of some feature. For instance, if the falling object has a red dot on it, have the tracker box be larger than the dot. If the falling object is small enough, have the box be just larger than it.

The outer box is the area After Effects will look to see if the image pattern in the smaller box is anywhere to be found, so the bigger you make the outer box, the harder time After Effects will have of making the right decision.

OK, now that I’ve talked about the tracker, you could avoid all that hassle simply by keyframing by hand the object to fall. 🙂 Because gravity is one directional, and falling things tend to be short sequences, you could simply go to the point in the timeline where the object to be replaced falls into view and set a position keyframe for your new object, so it’s on top of the original.

Move a few frames forward and reposition the object over the orginal. Move back between the frames and you might need to adjust it again, but basically, the new object will follow the path of the orginal.

Hope that essay helps 🙂

Graham

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