Apple Shake 4 - Manual (Page 732)

732 Chapter 25 Image Tracking, Stabilization, and SmoothCam
Increasing Contrast and Preprocessing the Image
It is often helpful to apply a Monochrome node to an image, and drop the blue channel
out if you have particularly grainy footage. Another good strategy is to activate the
preProcess flag in the tracker. This applies a small blur to the footage to reduce
irregularities due to video or film grain.
In some cases, you may want to modify your images to increase the contrast in the
reference pattern, either with a ContrastLum node or ContrastRGB node. Since you’re
only using this image to generate tracks, you can disable or remove these nodes after
you’ve finished the track.
Tracking Images With Perspective, Scale, or Rotational Shifts
For images with significant change in size and angle, you can try two different
referenceBehaviors, “update if below reference tolerance” or “update every frame.” The
second choice is the more drastic, because you get an inherent accumulation of tiny
errors when you update every frame. Therefore, try “update if below reference
tolerance” first.
Another strategy is to jump to the midpoint frame of the clip and track forward to the
end frame of the clip. Then return to the midpoint frame and track backward to the
beginning of the clip.
A second strategy is to apply two Stabilize nodes. The first Stabilize node can be
considered a rough stabilize. The second Stabilize then works off of the first, and
therefore has a much better chance of finding acceptable patterns. Since the Stabilize
nodes concatenate, no quality is lost.
Tracking Obscured or Off-Frame Points
There are two basic techniques to correct track points that are obscured by moving off
screen or an object passing in front of them.
The first strategy is to use a different failureBehavior, either “predict location and create
key” or “predict location and don’t create key.” The first setting is good for predictable,
linear motion behavior—it continues to lay down keyframes following the vector of the
last two valid keyframes, so the two frames prior to the pattern become obscured. It is
excellent for points that go off screen and never reappear. The second setting is a
better choice for patterns that reappear because it continues to search on a vector, but
only creates a keyframe if it finds another acceptable pattern. You have an even
interpolation between the frame before the pattern was obscured, and the frame after
it is revealed again. These strategies only work when the clip contains nice linear
movement.