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RE: Problem attaching motion path in MAYA

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Chris McCulloch
RE: Problem attaching motion path in MAYA
on Jun 23, 2012 at 3:08:39 pm

Hi,

I'm pretty new to animating in MAYA. I've got to animate a tractor that drives in a smooth circle at a couple of points. I've rigged the wheels so they turn realistically in relation to the ground, so it animates fine driving in a straight line. I now need it to move in a circle but when I try to attach it to a motion path an error appears translate x already has an input connection, if I break the connections the wheels no longer turn so I need the wheels to turn and attach it to a motion path...I've tried just keyframing it but the results are very jerky. Any other way of doing this?

Any help would be really appreciated.


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Eric Santiago
Re: RE: Problem attaching motion path in MAYA
on Jun 25, 2012 at 1:55:20 am

Group the tractor or use a null as parent and attach that to your motion path.
The error is due to the fact that they node you are trying to attach to path already has transform keys.


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Steve Sayer
Re: RE: Problem attaching motion path in MAYA
on Jun 28, 2012 at 1:39:10 pm

When you say you've rigged the wheels, what do you mean? Do you use an expression to rotate them? The solution to the issue you're describing will depend on how you've achieved the first part of the effect.

There are a few tutorials on realistic wheel rotation out there; maybe try Googling it.


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Chris McCulloch
Re: RE: Problem attaching motion path in MAYA
on Jun 30, 2012 at 12:25:04 pm

Hi Steve,

yes I initially used an expression to get the wheels to rotate and found that they only rotate on one axis so I downloaded a mel script called WheelRigger which rotates the wheels on all needed axis but when I try to parent them to the car rig they just wont turn at all, so I have sussed the motion path and the wheel rotation but not combining them :/ Any suggestions would be really appreciated as it's getting to tugging hair out levels.

Chris


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Steve Sayer
Re: RE: Problem attaching motion path in MAYA
on Jul 5, 2012 at 2:51:43 pm

Sorry, it's very difficult to offer suggestions in a case like this because we have no idea how that script works or what your hierarchy is like. For example, the script might be tracking the wheels' world position, or it might be reading their translate values directly. Those two behaviours will work completely differently depending on whether or not the wheels are part of a hierarchy, constrained to something else, etc.

Starting from scratch, I would try this:

1. Assemble your tractor hierarchy. This means do all your parenting FIRST. No expressions, motion paths, or animation.
2. Make sure you have an extra transform above each wheel. You can do this just by grouping each wheel to itself.
3. Put another empty transform above the entire tractor (i.e. above the top node in its hierarchy).
4. Attach this new transform to the motion path, using whatever settings you want. This should bring the tractor around on the path while still leaving you the freedom to animate the tractor (and the wheels themselves) separately.
5. Now apply your script to the wheels. Depending on what the script actually does, as I mentioned above, this may or may not work. But if it doesn't work you should be free to try other scripts or just keyframe the wheels by hand.

The thing to remember is that attributes can only be controlled by one input at a time. That means if something is controlled by a motion path, it can't also be controlled by an expression, for example, or if something is controlled by a constraint, it can't also be controlled by keyframes. However, hierarchies allow child nodes to inherit the transformation of their parents, so you use extra transforms to have multiple levels of control over an object.

The classic example of this is a hovering droid that flies along a motion path, while still being free to bob up and down and rotate as it goes. In that case you create a transform above the droid's top node, and attach that transform to the path--this will create the smooth flight. But it also leaves free the droid's original top node, which you can animate with keyframes or expressions to create the bobbing motion. The two patterns of motion will be layered together to produce the final animation.

Hope that helps!


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