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  • Slow down a null exponentially

  • Chris Orcfilm

    September 12, 2011 at 7:14 pm

    I having a panning shot of a building from the sky down to the ground.

    I need to do a sky replacment.

    I can track a feature position on the front of the building and attach a null.

    Now I want the sky in the back to move with the pan but obviously a lot slower, exponentially than the null.

    What expression can I use to exponentially slow down the position of the null data when parenting the sky to it? Something on a sliding scale as it were.

    I’d prefer not to have to do a camera track given the brevity of the scene and am happy with “eyeballing” the speed of the sky.


    Thanks for your time!


    Orchard Film Studios, Canada

  • Dan Ebberts

    September 12, 2011 at 8:38 pm

    I think you’ll have to define a reference starting y position for the null and then use something like this:

    startY = 150;
    mult = .5;
    deltaY = thisComp.layer(“tracked null”).transform.position[1]-startY;
    value + [0,deltaY*mult]

    If the null is keyframed, you might define the reference y this way:

    startY = thisComp.layer(“tracked null”).transform.position.valueAtTime(0)[1];

    Adjust the value of mult to get the tracking you want for the background.


  • Chris Orcfilm

    September 13, 2011 at 12:50 am

    Thanks Dan, as always you save the day – and in record time!

    I also had a slight left shift so modified to affect the x, I know there would be a cleaner way but did this:

    startY = thisComp.layer(“Null 1”).transform.position.valueAtTime(0)[1];
    startX = thisComp.layer(“Null 1”).transform.position.valueAtTime(0)[0];
    mult = .7;
    deltaY = thisComp.layer(“Null 1”).transform.position[1]-startY;
    deltaX = thisComp.layer(“Null 1”).transform.position[0]-startX;
    value + [deltaX*mult,deltaY*mult]

    Works like a charm!

    This is for a submission to Sundance, and I know it won’t be your first, but thanks for contributing – your work indirectly appears in many great features I’m sure!



    Orchard Film Studios, Canada

  • Chris Buttacoli

    September 13, 2011 at 4:24 pm

    Let me point out, from a “physics” pov, that there would be no parallax effect on the sky during a panning shot. Just to be technically clear.

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