Unable to get value at previous frame

[Dan Ebberts]

I’m not sure exactly what you’re trying to do, but this should get you the value of the property with the expression at the previous frame:

valueAtTime(time-thisComp.frameDuration)

Remember though, this will be the pre-expression value, not the value calculated by the expression at the previous frame. I apologize if you already knew that.

[Fabrice Leconte]

Hello,

valueAtTime(time-thisComp.frameDuration*time)-1;
// it doesn't return the value of the previous frame

// Try:
valueAtTime(framesToTime(timeToFrames(time)-1));

[Joss Gitlin]

Oops. I had somehow left that extra “*time” in there!

Is it not possible to get the value calculated by the expression at the previous frame? That’s exactly what I need to do…

[Dan Ebberts]

That’s correct, but there are usually work-arounds. The most drastic one (which may be what you need) is for your expression to loop through all previous frames to re-calculate everything the expression has done on previous frames. The obvious downside is that the expression will eventually bog down because it has to do more and more calculations at each frame.

[Joss Gitlin]

Gotcha. I saw you use a similar approach on your website for an audio threshold expression. I’ll look into that. I really only need to look back on the previous 3 frames

[Dan Ebberts]

Well that’s a lot easier, and very do-able. Come back if you get stuck.

[Joss Gitlin]

Well Dan, I’m ashamed to admit it, but I’m stuck.

x = thisComp.layer(“A_BASS”).effect(“Both Channels”)(“Slider”);
threshold = 50.0;

below = false;
frame = Math.round(time / thisComp.frameDuration);
oldFrame = Math.round(time / thisComp.frameDuration) – 3;

d = 3;
n = 0;
while (frame > oldFrame){
t = frame * thisComp.frameDuration;
if (below){
if (x.valueAtTime(t) >= threshold){
n=100+x;
below = false;
}
else d–;
}
else if (x.valueAtTime(t) < threshold && d < 1){
below = true;
n=102;
d = 3;
}
frame–
}

n

I need the ‘else if’ statement to only trigger when the x value is below the threshold for 3 consecutive frames, which is what I’m attempting to use the d variable for.

[Joss Gitlin]

I finally got it!

This expression checks the past 3 frames to see if they all fall below the threshold, and if so, executes a command.

x = thisComp.layer(“A_BASS”).effect(“Both Channels”)(“Slider”);
threshold = 50.0;

wentBelow = false;
isBelow = false;
frame = Math.round(time / thisComp.frameDuration);
oldFrame = Math.round(time / thisComp.frameDuration) – 3;

d = 3;
n = 0;
while (frame > oldFrame){
t = frame * thisComp.frameDuration;
if (x.valueAtTime(t) >= threshold){
n=100+x;
wentBelow = false;
break;
}
else wentBelow = true;
if (wentBelow){
d–;
if (d < 1) {
n = 102;
break;
}
}
else if (x.valueAtTime(t) < threshold){
wentBelow = true;
}
frame–
}

n