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robcat2075

some notes on Jumping

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This isn't a substantial tutorial but as it may be helpful I'll share these exchanges I had with several people on the 11Sec club forum who were having trouble doing what is a rather essential body mechanics basic, jumping.

 

They were trying to animate the "Pixar" lamp jumping with results like this:

 

PixoH.mov

 

(NOT done in A:M ;) )

 

my response was

 

Rule about jumping... you can't jump forward if you only jumped straight up. There has to be some forward motion when he's pushing against the ground or there certainly won't be any forward motion once he's coasting in the air.

 

Another rule... Jumping only works because you got some mass of your body moving so fast that it yanks everything connected to it with it into the air. The luxo lamp has two main masses, the head and the base. The base is at the bottom of the leg, which is pushing against the ground, so the base won't be the mass that is yanking the rest of the lamp into the air.

 

The head is at the top of the leg and is getting pushed up. This is the mass that will have to be moving fast enough to yank everything else into he air. If you slow the head down at the end of the leg's push (as you do) it will have no momentum to carry it and the rest of the lamp into the air.

 

The head's slowest motion will be when the leg is just beginning to push it up. It's fastest motion will be when the leg has extended its farthest and has reached the limit it's push. once the lamp has left the ground there can be no more acceleration as there is nothing more to push against and gravity will be trying to drag the lamp back down.

 

No ballistic object can travel up faster after it's launched than it was moving at the last instant that some actual power was behind it.

 

and later, with examples

 

How can they possibly move forward when they only jumped straight up? :/

 

To jump forward You HAVE to jump Forward...

 

http://www.brilliantisland.com/am/ForwardJump.mov

 

 

 

If you jump straight up you come back down where you started...

 

http://www.brilliantisland.com/am/lampjump.mov

 

 

The mechanics of jumping are this:

 

-the legs lower the body mass in anticipation of jumping.

-the legs shove the body into the air as fast as possible

- the body is moving so fast it yanks the legs up with it

-gravity brings everything back down

-once it has left the ground and until it lands again it's like any other unpowered object thrown or launched into the air, its path will be pretty much a parabola.

- to land, the character extends the leg and foot to reach for the ground

-the legs cushion and slow the fall of the rest of the body and raises it back up to a normal position.

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(NOT done in A:M wink.gif )
How is that even possible? ;)

 

The mechanics of jumping are this:

-once it has left the ground and until it lands again it's like any other unpowered object thrown or launched into the air, its path will be pretty much a parabola.

I think that specifically/technically the path of its center of gravity (not necessarily any one body part) is a parabola, but other than that, a big YES! to everything.

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The mechanics of jumping are this:

-once it has left the ground and until it lands again it's like any other unpowered object thrown or launched into the air, its path will be pretty much a parabola.

I think that specifically/technically the path of its center of gravity (not necessarily any one body part) is a parabola, but other than that, a big YES! to everything.

 

Well, I did qualify that with a "pretty much". B)

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i think we went over this before when i was learning the jump motion.

Jumping

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Here are some additional comments on jumping I made on a jump by an 11Sec Club person, this time for a straight up jump...

 

Here is his blocking...

 

https://dl.dropbox.com/u/59849243/Alt%20Jump.mov

 

I commented:

 

Where is she really moving that torso? It looks like she's winding up to do a back flip by flinging her torso backwards... but then she moves up? That's impossible.

 

This is a situation where blocking in held keys is unhelpful because the transitional motion is so significant. I'd change this to spline keys and work with it that way.

 

He put up a side view to show that the body wasn't moving backwards...

 

http://www.youtube.com/watch?feature=playe...p;v=S5XLqu3mMv8

 

So I composited the keyframes to show it was indeed moving backwards...

 

 

I shall make it painfully obvious...

 

jumpframes.jpg

 

that enormous head (red) and the shoulders (yellow) are definitely going backwards with only the slightest forward motion of the hips (green) to counter it.

 

The other thing i notice now is that the upward motion of everything is decelerating at the end of the push rather than accelerating. The motion from 37 to 42 is less than it was from 32 to 37. That won't get him (I guess it's a him) in the air.

 

Jumping is about getting mass moving faster into the takeoff moment, not slower. Jumping is about getting the upward inertia of the mass so great that it carries the body up even after the legs can no longer push against the ground.

 

And it's got to be predominately going UP... not back.

 

 

And I added...

 

 

Here's about the same jump done for real. Download that so you can see it larger and frame through it.

 

http://www.brilliantisland.com/am/File0062...umpDotsH450.mov

 

Notice...

 

1) how much farther back behind the feet the hips have to be to balance the mass of the head and torso in front

2) that the average of the hips, shoulders and head motion is pretty much straight up

3) that by the time the feet leave the ground all the motion is predominately up with very little sideways component

4) that the rotation of the torso from leaned-forward to upright is not quite complete at take-off time. It doesn't suddenly stop at takeoff time, it follows through a bit

5) that the upward motion of all three body parts is greatest at the moment before takeoff, they have been accelerating from the crouch pose, not slowing down.

6) that to get the greatest possible upward push, the shoulders and head start very low, down by the knees

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