• Adobe After Effects Expressions

  Isolating X and Y tracking points

  Posted by Wr Sampson on January 5, 2006 at 5:25 am

  I have footage of an actor tracing a rectangle in space. I need to have a dot follow his hands in an idealized version of a rectangle. I can do keyframes, but I was wondering about using the tracker.

  It would be cool to be able to track the whole motion then selectively freeze Y changes while the hand is going horizontally and X changes while the hand is going vertically.

  I haven’t found a way to isolate X and Y values in the timeline so that I can hold one axis constant during a certain portion of time while the other continues tracking.

  Is this a job for a different type of tracker? ie a plugin or even a different app like Combustion.

  Wr Sampson replied 20 years, 4 months ago 3 Members · 4 Replies
• 4 Replies

• 

  Sam Moulton

  January 5, 2006 at 5:39 am

  i’m not sure exactly how to do this but tracker info is an array [x, y] so you should be able to write an expression that looked at timeline markers and then held the y value at the marker time when the hand changes direction. It would be somethin like if time< marker 1 then [tracker value x, y tracker value] then if time > marker 1 use the value of x at marker 1 for x and the tracker value of Y for y and then change for marker 2, 3, and 4 as the rectangle is drawn.

  I don’t know much about expressions but i’ll bet someone here could come up with the answer.
• 

  Steve Roberts

  January 5, 2006 at 1:55 pm

  Hmm … if you track the person’s motion perfectly, you might not get a nice rectangle. Have you considered this?

  1. draw a rectangular mask
  2. apply effect>render>stroke
  3. tweak the masks’s position to the picture
  4. animate the stroke’s end point from zero to 100
  5. add extra keyframes if necessary to follow the speed of the person’s hands

  … assuming the camera is static. 🙂

  If this doesn’t account for two-handed motion, try animating the start and end points, or adding vertices at the start and end points of the motion. Make one of those the “first vertex” of the mask (covered in the help).

  Hope it helps,
  Steve
• 

  Wr Sampson

  January 5, 2006 at 4:13 pm

  Thanks for the suggestions Steve. The stroke render idea is a good one. Keyframes to follow the hands are probably the way to go too. The camera is static.

  Tracking his hands precisely yields a very lopsided shape because human movement just isn’t exact. What I was hoping was that the X and Y values from a tracker would be available in a graph or table so that I could grab a whole region and hold them constant.

  Sam’s idea of writing an expression is something to try as well.

  Thanks again!
• 

  Wr Sampson

  January 5, 2006 at 4:16 pm

  Cool idea. I’m going to mess with it a bit. The movement goes with a beat, so I could even start with markers generated by SoundKeys and modify them.

  Thanks!

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