• Adobe After Effects Expressions

  Changing amount of movement (Sliders, Nulls, Trackers)

  Posted by Dino Muhic on June 6, 2009 at 12:41 pm

  Hi guys,

  it’s hard to describe, but let’s try

  I have some tracked footage of some guys in front of a greenscreen. The camera is shaking really fast. I used tracking markers on the greenscreen to track the camera movement.
  Then I changed the greenscreen into a far distant sky background (you don’t see the floor in the shots).
  Now I of course have the problem of parallaxing because the sky background (with some airplane trails and so on) is moving exactly with the tracker and therefore looks like it’s just a wall a few yards away from the actors.

  So I thought about changing the amount of movement, or lets better say to reduce it so the sky doesn’t move that much with the tracking null. I don’t mean to smooth it out, because the camera movement is harsh and edgy but just to change the amount it’s moving. Let’s say the tracker moves from frame A to frame B with the amount of 100px along the x-axis, then I want to be able to change the amount using a percentual slider to let’s say 50% which would mean the sky only moves 50px from A to B.

  With this technique I also would be able to add more layers into the depth and using the percentual slider adjust it so the parallaxing looks good.

  I just don’t have the clue how to that…

  Is something like this possible?
  Would it even make sense or is my approach wrong?

  Thank you very much!

  Dino Muhic – Media Producer
  VFX – Motion Graphics – Web-Design – Or just ART
  http://www.dinomuhic.com

  Dino Muhic replied 16 years, 11 months ago 2 Members · 2 Replies
• 2 Replies

• 

  Darren Hardaway

  June 6, 2009 at 5:50 pm

  Well, I haven’t actually worked with markers before, but I’m pretty sure you just need the position of the markers to accomplish this.
  Come to think of it, I haven’t used a tracker either. 😛 But your basically saying move tracker from point A to point B 100 pixels only along x-axis. You also want a percentage slider. Start by holding the slider’s value with this

  temp = effect(“Slider Control”)(“Slider”)/10;
  if(temp<=0){0}; if(temp>100){100}

  If you have a set start and stop time on the timeline, this could work

  start = 1;
  end = 2;
  amount = thisComp.layer(“Null 1”).effect(“percent”).param(“Slider”);
  x = linear(time,1,2,value,value+amount);
  [x,value[1]]

  So thats saying as time approaches one sec, hold original value, and as time moves from 1 to 2 sec increase value by percentage null. Hope this helps!
• 

  Dino Muhic

  June 6, 2009 at 6:56 pm

  Thank you but it’s not as simple as that. What I need is a more intelligent script that uses every motion from the tracker and reduces it by the giving percentage of the slider.

  The motion tracker (which is basically just a NULL after the mocha part) has keyframes at every frame for position, rotation and scale and all of them have to be used.

  Dino Muhic – Media Producer
  VFX – Motion Graphics – Web-Design – Or just ART
  http://www.dinomuhic.com

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