• Adobe After Effects Expressions

  Calculating Change In Position(Delta) Using Motion Tracking Data

  Posted by Alex Macnair on January 6, 2013 at 4:22 am

  Hi there, I am fairly new to using expressions in After Effects and I am in need of some guidance. I am working on a project where I am using the X,Y position of a Null Object (Generated via a motion track) to calculate the velocity of the tracked object.

  This the expression I am using to pull the X,Y position and find it’s change in location every frame:
  ((effect("X")("Slider")-effect("X")("Slider").valueAtTime(time-(1/3)))^2)+((effect("X")("Slider")-effect("X")("Slider").valueAtTime(time-(1/3)))^2)
  I am storing the X,Y position as two slider values, as you can see from the expression.

  My problem is that even though the X and Y values are changing as they should be, the expression output is remaining at 4.00 and for a specific frame it will jump up to 6.00 and then go back to 4.00. I am baffled as to why, I assume that I am doing something wrong with using the valueAtTime…

  Here is a screenshot of all of my sliders and their values: https://i.imgur.com/soLIO.png
  And here are all of my expressions: https://i.imgur.com/bUPTv.png

  Thank you in advance for your help!

  View post on imgur.com

  Alex Macnair replied 13 years, 8 months ago 2 Members · 2 Replies
• 2 Replies

• 

  Darby Edelen

  January 6, 2013 at 6:46 am

  Well, one thing that jumps out at me is that you’re using the ^ operator, which doesn’t do what you want it to. The ^ is a bitwise XOR operation:

  https://en.wikipedia.org/wiki/Bitwise_operation#XOR

  What you want to do is probably closer to:

  x = effect(“X”)(“Slider”) – effect(“X”)(“Slider”).valueAtTime(time – framesToTime(1));
  Math.pow(x, 2);

  Also I’m not sure that all of your expressions are necessary for your stated goal. If you want the rate of change of the null’s position have you tried:

  thisComp.layer(“End of Shot”).transform.position.speed;

  The result is in pixels/second. No need to get Pythagoras involved 🙂

  Darby Edelen
• 

  Alex Macnair

  January 6, 2013 at 7:02 am

  Thank you so much, the XOR operation explains the strange results.

  [Darby Edelen] “Also I’m not sure that all of your expressions are necessary for your stated goal. If you want the rate of change of the null’s position have you tried:

  thisComp.layer(“End of Shot”).transform.position.speed;

  The result is in pixels/second…”

  No I did not know about this but I will defiantly use it instead! 🙂

  Thanks again for the help, I really appreciate it!

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