• Adobe After Effects Expressions

  Change object size at certain time and make it stick

  Posted by Rasmus Hansen on August 20, 2019 at 7:48 pm

  Hi there,

  I’m making a lava lamp emulation, and since the end result should be about an hour long, it’s too time consuming to keyframe everything. Therefore, I decided to give expressions a go. Through a lot of studying (thanks, Dan!) and trial and error, I managed to come up with something that works okay.

  What I now want to do is to be able to change the size of the lava blobs, to a random size, whenever the blobs hit the bottom (posDown in the code below). And then KEEP that same size until the next time it reaches posDown. Alternatively, give a random x position when the blobs hit the bottom.

  Is that even possible? I played around with using random() on the size property, but it’s changing the size at random at every frame – and if I use seedRandom, it always changes to the same size at every loop. Moreover, I can’t get it to stay at that size.

  Any help is much appreciated.

  Thanks in advance.

  POSITION Y EXPRESSION
---------------------------------
seedRandom(index,true);
blobSize = content("Ellipse 1").content("Ellipse Path 1").size[0];
holdDur = random(2,8);
moveDur = linear(blobSize,100,800,10,40);
posUp = -blobSize*0.5;
posDown = thisComp.height+blobSize*0.5;

  t1 = holdDur;
  t2 = t1+moveDur;
  t3 = t2+holdDur;
  t4 = t3+moveDur;

  t = time % t4;

  if (t < t1){
  posDown
  }else if (t < t2){
  easeOut(t,t1,t2,posDown,posUp)
  }else if (t < t3){
  posUp
  }else{
  ease(t,t3,t4,posUp,posDown)
  }

  POSITION X POSITION
  ------------------------------
  seedRandom(index,true);
  xPosRand = random(thisComp.width);
  xPosWig = wiggle(0.2,200);
  xPos = xPosRand + xPosWig - thisComp.width*0.5;

  [xPos]

  Dan Ebberts replied 6 years, 11 months ago 2 Members · 3 Replies
• 3 Replies

• 

  Rasmus Hansen

  August 20, 2019 at 7:48 pm

  What I have now looks like this:
  

  drive . google . com / open?id=1RCBxJ0UQMhN7kbiFb_7vi_ZfNht1BbXI
• 

  Rasmus Hansen

  August 31, 2019 at 10:30 am

  Does anyone know if this is somehow possible? 🙂
• 

  Dan Ebberts

  August 31, 2019 at 8:43 pm

  It’s definitely possible, but somewhat complicated. The issue is that each cycle will have a different duration and since expressions have no memory, the expression will have to reconstruct everything that has occurred in the past. This is a rough attempt at the Y Position expression. It might not be quite right, but it should give you the idea of what you need to do to construct a solution:

  Dan

  blobSize = content("Ellipse 1").content("Ellipse Path 1").size[0];
moveDur = linear(blobSize,100,800,10,40);
posUp = -blobSize*0.5;
posDown = thisComp.height+blobSize*0.5;

  tCur = tPrev = 0;
  cycle = 0;
  while (tCur <= time){
  seedRandom(cycle,true);
  holdDur = random(2,8);

  t1 = holdDur;
  t2 = t1+moveDur;
  t3 = t2+holdDur;
  t4 = t3+moveDur;
  tPrev = tCur
  tCur += t4;
  cycle++;
  }
  if (time >= 0){
  t = time - tPrev;

  if (t < t1){
posDown
}else if (t < t2){
easeOut(t,t1,t2,posDown,posUp)
}else if (t < t3){
posUp
}else{
ease(t,t3,t4,posUp,posDown)
}
}else
value


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