# Motion Tracking - Link track to empty and move camera instead of empty

I have some footage that I've added a single tracking point to and linked it to an empty. This successfully moves that empty in my scene in relation to the tracker and I can add objects into the scene and parent them to those empties and they stay in position. However, some of the objects I'm adding are in motion and have particle trails, and the trails do not behave correctly since the objects are moving around based on the movement of the empty, as well as their own keyframe animations. Is there a way to invert the "track to" constraint on the empty so that the translation of that empty is inverted and applied to the camera so that the camera is translating, and the empty is "pinned" to its initial position?

• maybe you have to change some options of the particles, such as the gravity, etc. – gladys Jul 8 '15 at 11:12
• Use the tracker for 2D stabilzation. – user1853 Sep 18 '15 at 14:25

I did something similar to this to avoid having to do full camera solving and achieved it by converting the track into an f-curve and then using the scale tool to invert the track (scaling by -1 in the y-direction) for each of the channels. This produced an empty that moved in the opposite direction to the original footage which I could then track the camera to. This can be achieved as follows :

Create a new Empty to act as the target and add a 'Track To' constraint. Uncheck the 'Active Clip' checkbox and select the relevant the Movie Clip containing the track. Set the Object, Track, and Camera. This should result in the Empty following the chosen point in the footage.

Click the 'Constraint to F-Curve' button in the Constraint panel. This will remove the constraint and convert it into an F-Curve on the empty so we can then manipulate the F-Curves.

Select the Empty and open a Graph Editor window and make sure the mode is set to 'F-Curve. You should see F-Curves for its X, Y, and Z locations.

Press 'A' until none of the graphs are selected (each press will toggle all or none). Right-Click on the 'X Location' curve to select a point on it. Press 'L' to select all linked points (ie, the whole curve). Press 'S', 'Y', '-1', Enter to scale it by a factor of -1 in the Y direction only. Repeat for the 'Y' and 'Z' curves. This will reverse the direction of motion for the empty in each direction.

Select the camera and add a constraint to Track To the empty and the camera should now mimic the motion in the original footage (rather than its inverse).

If you need to adjust the point at which the camera is pointing you can use the F-Curve editor to select one of the curves again and move the entire curve up or down (right-click curve, press 'L' to select the whole curve, 'G' then 'Y') to adjust the position.