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easygoing_idea1549
easygoing_idea1549
Inspiring
June 4, 2017
Answered

Expression: Snapshot of current value

  • June 4, 2017
  • 3 replies
  • 3054 views

Hi, how would you guys solve the following problem using expressions?:

I have custom Slider Control "xPos" , it is keyframed and its value changes constantly;

I have a custom Checkbox Control "Check". It is keyframed too. When Check is checked, i want to store xPos's Current Value in a Variable "a".

While xPos is changing further down the timeline, i want to be able to access "a", which should always hold the Value xPos had, when Check last changed to value 1.

When i uncheck Check, and check it again, i want a to update to the new current value of xPos.

If you want to see, what i tried so far, check this thread of mine : Expression: I am stuck

Problem is, that when i set a = xPos, it updates constantly, instead of just taking the current value and holding it...

Thanks and kind regards

    This topic has been closed for replies.
    Correct answer Horshack

    Mutable global variables in AE (ie, variables whose value can change) are tricky because there are no global variables in AE and you're not allowed to change the value of another expression control (that you could use as a global variable) from within an expression control. But I came up with a hack for you. Create a slider named "a" (for your "global" variable 'a') and use this expression in it:

    if (effect("Check")("Checkbox") == 1) {
        // walk backwards in time to find last time checkbox was Off
        var prevTime = time;
        while (prevTime > 0 && effect("Check")("Checkbox").valueAtTime(prevTime) == 1)
            prevTime -= 0.0001;
        // set value of this slider equal to the value that xPos slider had at point this checkbox transition to On
        effect("xPos")("Slider").valueAtTime(prevTime);
    } else
        // checkbox off - set current value to -1 (or anything else you want it to be)
        -1;

    Note that this expression will noticeably slow down AE because it has to iterate a lot of values for each frame.

    3 replies

    jasonchow
    jasonchow
    Participant
    May 24, 2020

    Thanks Horshack, your code really helped me out! I adjusted it to check by frames instead of by .0001 of time, which decreases the amount of calculations it has to run. Worked a treat for me, hope it does for someone else out there. Here's my adjusted code, which assumes a constantly changing slider called 'pos', and a checkbox called 'status':

    if (effect("status")("Checkbox") == 1) {
// walk backwards in time to find last time checkbox was Off
var prevTime = timeToFrames(time);
while (prevTime > 0 && effect("status")("Checkbox").valueAtTime(framesToTime(prevTime)) == 1)
prevTime --;

// set value of this slider equal to the value that pos slider had at point this checkbox transitions to On
effect("pos")("Slider").valueAtTime(framesToTime(prevTime));

} else
// set value of this slider to 0, or whatever you want
0;

     

    easygoing_idea1549
    easygoing_idea1549Author
    Inspiring
    June 13, 2017

    Just saw your replies, guys, thanks so much for contributing!! I love AE Expressions!

    Horshack
    HorshackCorrect answer
    Legend
    June 4, 2017

    Mutable global variables in AE (ie, variables whose value can change) are tricky because there are no global variables in AE and you're not allowed to change the value of another expression control (that you could use as a global variable) from within an expression control. But I came up with a hack for you. Create a slider named "a" (for your "global" variable 'a') and use this expression in it:

    if (effect("Check")("Checkbox") == 1) {
        // walk backwards in time to find last time checkbox was Off
        var prevTime = time;
        while (prevTime > 0 && effect("Check")("Checkbox").valueAtTime(prevTime) == 1)
            prevTime -= 0.0001;
        // set value of this slider equal to the value that xPos slider had at point this checkbox transition to On
        effect("xPos")("Slider").valueAtTime(prevTime);
    } else
        // checkbox off - set current value to -1 (or anything else you want it to be)
        -1;

    Note that this expression will noticeably slow down AE because it has to iterate a lot of values for each frame.

      easygoing_idea1549
      easygoing_idea1549Author
      Inspiring
      June 4, 2017

      Hey, thank you so much, that seems to work. You are right though, it does slow playback down quite drastically, once the checkbox is checked... Nonetheless - Great work!!

      Horshack
      Horshack
      Legend
      June 4, 2017

      Here's a refined version that doesn't slow down AE (at least not enough to be noticeable for most comps):

      if (effect("Check")("Checkbox") == 1) {
          // walk backwards in time to find last time checkbox was Off
          var minTimeIncrement = thisComp.frameDuration;
          var prevTime = time - minTimeIncrement;
          while (prevTime >= 0 && effect("Check")("Checkbox").valueAtTime(prevTime) == 1)
              prevTime -= minTimeIncrement;
          // set value of this slider equal to the value that xPos slider had at point this checkbox transition to On   
          effect("xPos")("Slider").valueAtTime(prevTime >= 0 ? prevTime+minTimeIncrement : 0);
      } else
          // checkbox off - set current value to -1 (or anything else you want it to be)
        -1;
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