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Expression for constant speed?
Hi all
I have over a hundred paths being carved out with multiple instances of the Write-on effect with the 2D Brush Position being animated. Some of these paths are quite long, others pretty short, and I’m trying to figure out an expression that could cause each to move at the same speed.
I know the expression would be applied to the “Brush Position” attribute, and I extract speed from that, and I’m thinking it has something to do with modifying the (time) in value_at_time(time) for “Brush Position”, but my attempts so far keep failing. Does anyone have any advice? Thanks!
Ben Unguren
Motion Graphics & Editing
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6 Replies
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Hmmm… I think it’s possible. It would be easier if your keyframes are roving, so the actual speed is constant and the expression would only have to sample it once. One question though is when to start the movement. First keyframe? In point? Time = 0?
Dan
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Those are good points, Dan, thanks.
Yes, so far I’ve been using roving keyframes. I figured I can and ease the expression if I needed to.
I didn’t even think about when to start the expression. I was thinking I would have multiple effects on a single layer, so it would probably need to be at the first keyframe for each expression.
So… I could sample the speed at the frame after the first keyframe — that makes sense. But how could I reverse engineer the speed to influence the resulting position?
Ben Unguren
Motion Graphics & Editing
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The speed at the first keyframe multiplied by the time elapsed between the first and last keyframes gives you the total distance traveled (in pixels). Divide that by the control speed to get the corresponding time after the first keyframe where you want to sample the position with valueAtTime(). It should work.
Dan
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Thank you sir!
Ben Unguren
Motion Graphics & Editing
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I think I left a piece out. This seems to work:
if (numKeys > 1){
spd = thisComp.layer("control").effect("Slider Control")("Slider");
t1 = key(1).time;
t2 = key(numKeys).time;
tTot = t2-t1;
dTot = speedAtTime(t1)*tTot;
deltaT = time - t1;
valueAtTime(linear(deltaT*spd,0,dTot,t1,t2))
}else
value
Dan
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Works perfectly! You are singularly awesome, Mr. Ebberts.
Your use of linear in there is great – and I can just swap in “ease” instead to soften the motion. Very helpful; that taught me a lot!Ben Unguren
Motion Graphics & Editing
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