The Replace Footage node can be used to swap the background clip with another. The node has two inputs and one output. The clip from the first input will be replaced with the clip from the second input. The second clip may have a different resolution or number of frames.
After connecting a second clip, the trackers and camera provided by the first input will be adjusted to fit the new clip. A frame offset can be applied to adjust the in/out points of each tracker or camera in cases where frame numbering has changed.
Typically, this node will be used when the source footage has been changed or extended, and can help avoid having to track features or solve the camera motion again. The following screenshot illustrates how to use the Replace Footage node when the original footage has been extended:
In this case, the old clip (PFTChurch1) contains 100 frames, and was tracked using a User Track node (named "User Track"). The new clip (PFTChurch1A) contains 110 frames and is attached to the second input of the Replace Footage node. This associates each tracker with the new clip and adjusts the frame numbers of the tracker keyframes according to the frame offset in the Replace Footage node.
The old clip contained frame numbers from 1 to 100, and the new clip has 10 extra frames attached to the head of the clip but they are now numbered 1 to 110. This means that frame 1 from the old clip is now frame 11 in the new clip, so a frame offset of +10 is set in the Replace Footage node.
Once the Replace Footage node has been used to adjust the tracker keyframes to match the new clip, the trackers can then be extended to fill the additional frames that have been introduced at the head of the new clip. This is done by attaching a second User Track node (named "User Track 1") and using the Fetch button to fetch trackers from up-stream for editing in the node. Because of the frame offset applied in the Replace Footage node, these trackers will already be tracked between frames 11 and 110 in the new clip, so each tracker only needs to be tracked backwards from frame 11 to frame 1 to complete the track.
Frame offset: The offset added to keyframes and in/out points of the trackers and camera. This is useful in situations where the frame numbers of the old and new clips do not match, so the trackers and camera can still be aligned with the new clip.