After Effects 7 Graph Editor - Separate Position Axis Velocity?

Discussion in 'Amateur Video Production' started by gmccreight, Apr 27, 2006.

  1. gmccreight

    gmccreight Guest

    Perhaps I'm spoiled from using Blender (which is really really
    amazing), but it seems that you ought to be able to have separate X, Y,
    (and what the heck, Z) position velocity information in the Graph

    For example, let's say you want to simulate the throwing of a ball off
    a tall building. X velocity decreases over time and Y velocity
    increases. This would be extremely easy to do with two seperate
    "channels" of velocity information, but a real pain otherwise.

    I mean, do I really need to bust out expressions code for this, or nest

    Call me crazy. Please!

    gmccreight, Apr 27, 2006
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  2. gmccreight

    gmccreight Guest

    You're crazy. Oh wait. I'm crazy. Turns out, you can do this by
    using a null object. Put one of the axes of position change in the
    null object (say the x axis change). Now use that as a parent, and put
    the y axis change in the child, which will now have both the x and y
    change because of inheriting the x change from the parent.

    This seems a little bit odd to me, but it's easy and it works.

    --Gordon McCreight
    gmccreight, May 1, 2006
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  3. But to be physically accurate, the x-axis speed should not change (or
    not much, anyway).

    You should end up with a parabola expressed as x = v*t and y =
    0.5*g*t**2, where v is the original x velocity and g is the
    acceleration of gravity, 32 feet (or 9.8 meters) per second squared.

    If you don't like my choice of the y-coordinate (and who could blame
    you!), we could rewrite the second equation as y = h - 0.5*g*t**2,
    where h is the starting height.

    Exception: if the object is not very dense (like a chunk of
    light-weight plastic foam or a sky diver with an open parachute), both
    formulas break down, because of air resistance. Actually, they always
    break down for that reason, but it's only noticeable for lower density
    objects or for long fall times.

    Totally OT: The day of my first sky diving, I watched a sky diver fall
    most of the way to the ground with an unopened chute. She got her
    auxiliary chute open at the last minute and had a rough landing, but
    managed not to be injured. Another diver that day bounced off some high
    voltage power lines near the airport, also without injury. I went home
    without jumping and never went back.

    Gene E. Bloch, May 1, 2006
  4. gmccreight

    gmccreight Guest

    But to be physically accurate, the x-axis speed should not change (or
    not much, anyway).

    As you say, it all depends on what the initial speed of the object is,
    whether you're factoring in drag, etc. In my case I just wanted to
    make something which looked semi-real. I can see how using Null
    Objects and the expressions you provided could make a nice powerful

    Cheers - G
    gmccreight, May 2, 2006
  5. At the very least, it looks to me like you're having a lot of fun!

    Of course, in your extensive experimentation, you might discover the
    perfect fall trajectory in perceptual terms, which might not relate
    that closely to the physical perfection I'm obsessing over :)

    I was thinking that if your editing software doesn't automate this
    process enough, you could make up a spreadsheet using the appropriate
    formulas. Each row in the spread sheet would end up with a frame
    number, an x position, and a y position. That might help a lot if you
    end up editing the image manually. All you need to do is figure out the
    most convenient units to use, such as pixels or inches or mm.

    Gene E. Bloch, May 2, 2006
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