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Tangent on model not working properly? Bug?

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Marcin KozielTangent on model not working properly? Bug?
by on Jul 7, 2013 at 11:14:02 pm

So for a while now I've ignored this problem and tried to go with but now it's beyond letting go.

I've animated the head of my model to look from left to right and when I attempt to play that animation there is a random motion in between. For a moment the head even looks up when I did not keyframe that.
This is utterly annoying and it happens with a lot of the joints I animate with on my model.

Now, the deal is I've searched this and figured it must be this auto tangents feature. When I try to highlight the keyframes, go to timeline, under "key" I would click "zero angle" & "zero length", it did NOT solve the issue!

Can anyone help me at all? This has really become irritating!

YouTube preview:

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Adam TrachtenbergRe: Tangent on model not working properly? Bug?
by on Jul 8, 2013 at 4:12:59 am

The problem has to do with the rotation order rather than key tangents. It's a bit hard to explain, but internally the heading and pitch axes don't behave quite how you'd expect once the object's rotation has been animated. You can get a better understanding of what's going on by activating the rotation tool and then selecting "gimbaling rotation" in the tool options. Then watch your animation play forward and you'll see what's going on.

Notice that the B (or Z) axis DOES behave the way you'd expect. That's because by default it is first in the order of rotation. The practical implication of that is that you should try to design your rig so that joints can be rotated into whatever position you want by keying only the Z axis. Generally you accomplish that by nesting the joint in nulls.

The neck, for example, might have the joint itself with the Z axis facing right or left. You would key the joint's Z axis to make the head nod up and down. To move the head from side to side, you would place the neck joint below a null that has it's Z axis facing forward or back, and you would animated the null's rotation rather than the neck joint's rotation to make the head rotate from side to side. Then, for twisting from side to side you'd place that null inside another null that has its Z axis pointing straight up or down. Etc.

I suggest you search in the help docs for "rotation order" and read up on it a bit.

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Marcin KozielRe: Tangent on model not working properly? Bug?
by on Jul 8, 2013 at 4:48:27 am

That was very much a high quality post and I appreciate the reply.

I understand what you told me but either way I still find the rotation order strange because I was capable of rotating the head and "simulating" that axis moving just fine where when playing forward the keys, it's not the same. Anyway maybe I'm just not understanding the concept right just yet but again, thanks!


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