3 Replies Latest reply on Jul 28, 2017 10:59 AM by adamtlv

    Expressions 101 - rotation and postion


      I just don't understand this basic expression and why it's causing this result. I wanted to make a circle rotate properly along a path, like a ball rolling. I started to play around and when I pick whip my rotation to the separated position dimension of X and start adjusting the X position, it works, but why? All that is in the expression is "transform.xPosition" so I am not understanding where in that tiny expression that if the position moves the object should rotate equally. I feel like to do that would entail a much more extensive expression. Any help explaining this is greatly appreciated!

        • 1. Re: Expressions 101 - rotation and postion
          Mike_Abbott Adobe Community Professional

          You got lucky ; )
          I bet your ball was just over 100px in diameter?  Ideally about 115?


          Your expression says: make my rotation equal my x position movement.

          ball moves 360 pixels to the right = ball rotates 360 degrees clockwise.


          For the ball to rotate without slipping or skidding it's circumference would need to equal the distance it moved.

          Diameter = circumerefence / Pi    = 360 / 3.14    = 114.6 pixels.


          Your homework, should you choose to accept it... 

          make an expression that rolls without slipping or skidding for a ball of any size : )

          1 person found this helpful
          • 2. Re: Expressions 101 - rotation and postion
            Mike_Abbott Adobe Community Professional

            OK... to answer my own question : )


            The key thing to note is that a pick-whipped expression basically  just copies the values from one property to another - and those values are unitless. They are interpreted in terms of the units of the property to which they are applied. So in your case X pixels of movement is interpreted as X degrees of rotation.


            You need to work out what proportion of a circle's circumference is the distance moved, which is:

            distance / (Pi x diameter) . Having got the proportion, you can x 360 to calculate the degrees.


            In expression speak:


            dia = 300;

            transform.position[0] / (Math.PI * dia) * 360



            1. Change the number 300 to whatever your circle diameter is.

            2. No need to separate the dimensions. The '[0]' at the end of transform.position says 'use the first ('X') value'.


            Here is the site to learn all about expressions from the master:


            2 people found this helpful
            • 3. Re: Expressions 101 - rotation and postion
              adamtlv Level 1

              Ha! You are correct, I got lucky... about 120px on the circle, now this is making sense!

              And I will do my homework!


              Thanks Mike!