Animation help with foot sliding
polycounter lvl 19
This is probably a very simple question for an animator, but I have never really done much animation before.
I am getting sliding on the inward hop. I cannot figure out what I need to do to anchor the foot between these frames so it does not move. Trying to clone the position of the toe joint from the first frame to the next frame does not seem to work, nor does making the keys linear between the keyframes. Is this something simple I am overlooking?
Replies
I personally think this slide is a non-issue. If you look at the knees you can barely see the slide unless you know about it. I think the animation looks great and the slide is not distracting, to me at least.
If I copy the posture of the foot and toe and paste it, the foot keeps that pose just higher up (essentially obeying the knee and upper leg bend). It is hard for me to explain because so much of this is new for me, but I feel like I should be able to copy the keyframe from the first frame in some way, and paste it on the 2nd keyframe, with a linear curve between, and it should not move. In Maya at least, this is what I would expect, but I cannot get it to do that in max.
To do that on any rig you use the built in IK system. In biped they call it "Planted Keys", but all rigs run on the same basic 2 system principle.
To lock a foot into world space...
You move the time slider to the beginning of where you want to lock it down (IK always takes the position from an earlier IK key).
Select the foot, click the motion tab in the command panel (little wheel) to access the biped menu, expand "Key Info", then click "set planted key".
On each planted key you can select what pivot point you want to stick to the world.
It's under "IK". You can click "Select Pivot" and then click on the pivot in the viewport, or click the box next to it and get a top down diagram of the pivot points, top is the tip of the toes, bottom is the heal.
Then move the time slider to the end of where it should be planted, and set another planted key, then right after it a free key.
This will lock that foot to the world between those two keys. If you want the position to change but it to "slide" between those two points, use a "sliding key".
If you're trying to copy a toe pose.
Use the copy/paste pose system in the biped menu. Select a biped piece probably a foot & toes, click the motion tab, expand copy/paste and copy the pose from the key you want, and paste it onto the key you want to be similar. This will not force the previous position, but just the rotational relationship between the joints.
The problem with the copy/paste system is that it only copied and pasted the rotation of the toe, not it's position in world space. If I copied the entire leg chain, it sort of worked, but because the biped01 node was so much higher in world space, the leg could not "reach" that far down.
The way I got it to work finally was to shift+drag to clone the movement key on just the toe. Like I said before, I am almost positive I tried this before, but maybe I was trying so many things I got confused and was not actually doing this on the toe joint.
But I'm glad you got it working. If you're happy to leave it at that, cool.
If you want to know why what you did, worked in this case, I'll explain in a little more detail for you or anyone else that finds this thread.
Forward Kinematics: Dog wags tail. Moving the parent moves the child.
Inverse Kinematics (planted keys): Tail wags dog. Child stays in place, parent bones rotate and distort to keep it in place.
By default Biped treats each system (denoted by the color of the pieces, green leg, blue arm, purple toes ect) as one whole unit with one key. If you grab a foot and move its keys you are also moving the thigh and shin keys, but not the toes. Copying just the toe keys won't wag the dog. Copying the keys for the leg system (even just the foot) and the toe would get it all in the right rotation but not lock it to world space, for that you need Planted Keys/IK.
You can force separate keys for each individual piece in the "keyframing tools" section under "separate FK tracks" but it doesn't default this way.
If you want the foot to wag the dog, you need to use planted keys to stick the foot in world space.
You can kind of fake it like you did by copying the keys for the two systems but that will only work in specific cases and will break easily if the COM is also moving.
It's best to lock the foot down in world space using Planted Keys/IK, that way you can move the COM around and it won't effect the position of the foot in world space. Unless of course you move the COM farther away than the leg is long, then the foot slips off but still points to that spot, which is actually helpful in some animations. It slips off because to keep it there the leg would have to stretch/scale, which is something biped doesn't do by default.
A note about the curve editor...
Depending on what mode you have biped set to, Quaternion or Euler, it will depend on what curve editor you can use. Biped was originally built to use quaternion (points in space) and has it's own curve editor called "Workbench" it provides easier access so some of the harder to locate tracks in biped as well as quick tools for things like reducing keys or filtering out knee wobble. You edit those curves using Tension, Continuity and Bias which is set of values instead of curve handles.
Euler is the method used by all other animation systems, and uses the curve handles people are typically used to seeing. Quaternion handles IK blending much more efficiently and avoids "gimbal lock". Maya added quaternion equations to their animation system recently because it really is a good way to work around certain problems and a much more efficient way to handle some things, at least on the back end.
The Eular system was added to biped after much bitching by animators that where tired of using programer-esk numbers to tweak curves. Biped still defaults to quaternion, and uses quaternion under the hood even when in Euler mode, but at least you can interact with curves in a much easier way.