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Tracer

Tracer creates an outline around an object after a few points are designated, so it traces the object. Its operates in a similar manner to that of Gimp's magic wand. You can then Fill the alpha channel with a mask, like with a gradient or another object on another track or Invert that fill. Although it works best for still images or objects that stay in the same place on a video like a logo, you can also add keyframes. To limit the mis-tracking that occurs when working on a moving object, we can increase the number of points so that the tracking is more accurate and relies on a solid foundation. Rather than points as in a mask, tracer is based on edges to form an outline (figure 10.74). Frequently it will be desirable to use either RGB or YUV as the color model without the -A for Alpha.

Figure 10.74: Gui for Tracer plugin

New
  to create a new point.
Up/Dn
  to move highlighted point up or down.
Del
  to delete the highlighted point.
X, Y
  are the point’s x/y coordinate.
Invert
  to change the mask to outside.
Feather
  to sort of blur the edges (inward and outward). Feather influences the softness of the edge. The higher the number Feather is set to, the softer the edges become.
Radius
  for varying the size a little. Radius extends the area further in or out to cover more or less area.
Drag
  to enable or disable moving. If you need to clear the Drag enabled, you can easily do this with Allow keyframe spanning whose use is described in Allow Keyframe Spanning.
Draw
  to draw the outline for visual.
Fill
  to black out the object (no alpha); figure 10.75.
Mouse wheel
  rotate the outline, centered on cursor. Note that this rotates the outline around the nearest point. The slower you scroll, the more precise the the result. If you scroll faster, an accelerator kicks in that allows you to make complete rotations more quickly.
Mouse wheel + shift
  scale the outline, centered on cursor.

Figure 10.75: Rug in the top picture is traced in order to black it out or replace later

Be sure to uncheck Draw and Drag before rendering so that the lines do not show in the video output.