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AE6 Overview Part 4: Multipoint Tracking

Tracking CG elements to real world footage By Stephen Schleicher
Not only can After Effects 6.0 track a single point in a motion clip, it can also track multiple points. These multi-points are tracked and the resulting data can be used to distort a layer you are applying the data to. In this segment, well look at multipoint tracking and how you can use it to create a believable effect.

Before you begin, you might want to familiarize yourself with the first three parts of our multi-part overview of Tracking and Stabilizing Motion in After Effects 6.0.

Part 1: Tracking and Stabilizing Motion
Part 2: Stabilization
Part 3: Single Point Motion Tracking

There are two types of multi-point tracking in After Effects 6.0. The first is Parallel Corner Pinning, which tracks screw and rotation using only three points. Why only three points? This type of multi-point tracking does not track perspective, instead it ensures that parallel lines stay parallel and the relative distances between Anchor Points is preserved. The calculations between the three points create the position for the fourth corner pin point. You would use this type of tracking to simulate skewing and scaling of a layer.

For example, lets say you have a shot of a building and you want to replace one of the windows in the shot. Using Parallel Corner Pinning ensures that the lines of the window pane remain parallel even if the shot zooms in or out.

That is all well and good if you are shooting at an angle that makes the window frame parallel, but what happens if you have a shot that causes the pane to recede (where only two lines are parallel and the other two taper to the vanishing point)? To simulate perspective distortion, you would use the second multipoint track option Perspective Corner Pinning. When applied to a target layer, Perspective Corner Pinning tracks four points in the source footage, and distorts the layer to simulate changes in perspective. Perspective Corner Pinning is excellent for putting a sign on a moving bus, a cover for a book, or in the case of the next exercise, tracking a billboard on the side of a building.

Tracking a billboard

In this exercise you will use Perspective Corner Pinning to place a billboard of a corporation to the side of a building.

Our Final product.

A couple of things about the footage used. The Mindsprung logo was originally referenced in the Radio Waves exercise.
It can be found HERE. The reason the shot of the building was not done on a tripod was to really demonstrate how Perspective Corner Pinning holds up. If I had shot the building with a tripod, I wouldnt have had to use Motion Tracking. I could have used the Corner Pinning effect and locked the billboard down.

You can download the project files for this exercise here. The project file includes the the full size QuickTime movie of the building. Be aware that the file is in .sit format and is 31MB in size.

You can download the Mindsprung project here. You will need to render the movie in order to use it with this project.

Step 1: Create a new project (Option+command+N or Control+Alt+N) and import (Command+I or Alt+I) the building.mov and your rendered Mindsprung file.

Step 2: Create a new composition by clicking and dragging the building.mov clip to the Create new Composition icon.

Step 3: Rename the new composition to Mindsprung Corporate HQ.

Step 4: Place the Mindsprung clip in the Timeline above the Building layer. For now turn off visibility for the layer by clicking on the Eye icon.

Step 5: Return to the beginning of the Timeline by pressing the Home key on the keyboard. With the Building Layer selected, Right+Click and select Track Motion from the pop up menu. The Tracker Controls Palette will open and the Building layer will open in its own Layer Window.

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Related Keywords:After Effects 6.0, corner pin, tracking, multi-point tracking, motion tracking, mindsprung, tutorial, schleicher, AE6


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