In this tutorial, besides watching Walter and The Devil fight it out, you will learn all about the concepts and the practical application of Motion Tracking!
Motion tracking in itself is not a visual effect, but an essential tool to help you create visual effects. It allows you to track the motion of moving elements in your footage and extract information about their position, rotation and scaling.You can then use this tracking data to control other layers in your composition and, for example, create visual effect elements and have them follow your tracked object.
This allows you to do things like have blood stains stick to walls during a moving shot, add a halo or other effect to follow your actor, erase objects from your scene and much much more.
If you want to do motion tracking in a scene, make sure the element you want to track stands out cleanly against the background while shooting your footage. A person dressed in black walking in front of a dark sky will likely be hard to track. You can make motion tracking easier by adding tape markers or other helpers to your actors or objects to make them easier to track.It is MUCH easier to remove a tracking marker than it is to track an indistinct object!
For this tutorial, we are going to track Walter's head as he walks through the shot and attach a halo to it
To get started with motion tracking in After Effects, open the Tracker window by going toWindow -> Tracker.
A small tool window containing all the controls necessary for tracking your footage will appear on the screen.
Scroll to a time position in your clip where Walter's head is fully in frame, make sure the footage layer is selected and click on the 'Track Motion' button in the Tracker window. Since we also want to track the rotation of Walter's head, make sure the 'Rotation' checkbox is ticked. You should now see 2 track points overlayed onto your footage, connected by a white line.
If you zoom into your track points, you will notice that each of them consists of 2 rectangles!
The inner rectangle defines the part of the frame we want to track. Make sure you position it on a very distinct and high contrast element that is easy to follow visually from frame to frame.The outer rectangle defines the area that will be searched each frame to locate the contents defined by the inner one. Make sure it is big enough so that the element you are tracking remains within its borders across consecutive frames.
Notice that the larger you make the rectangles, the more processing After Effects will have to do and the slower the motion tracking calculations will become. Also note the small arrows on the white line connecting the 2 track points.Track Point 1 will define the position data we are recording and Track Point 2 is used to record the angle i relation to Track Point 1 and store it as a rotation value.
Before we can get started tracking the footage, we need to understand what Null objects are and how we can use them to store our tracking data!