CATMotion also integrates CAT's retargeting system, which has two advantages over standard IK: Retargeting eliminates the problem of snapping at the knee joint caused by overextension of the IK chain, and because the leg pushes the pelvis up, the character automatically has “weight.”
Because retargeting enables completely straight legs, you can use it to achieve walks that would be difficult or impossible using IK.
in CATMotion, retargeting is controlled by a combination of two controllers: KneeAngle and LegWeight.
KneeAngle
The walk cycle is divided up into four knee angles. Control over the knee angles is accessed in the KneeAngle controller, under Legs in the CATMotion editor.
For clarity we call the four knee angles Lift, MidLift, Plant and MidPlant.
1. MidPlant
2. Lift
3. MidLift
4. Plant
- MidPlant
-
The angle of the knee between Plant and Lift.
- Lift
-
The angle of the knee as the foot is about to leave the ground.
- MidLift
-
The angle of the knee in the middle of its path through the air.
- Plant
-
The angle of the knee as the foot is placed back on the ground.
LegWeight
The other controller important to retargeting is called LegWeight. It can be found with KneeAngle under Legs in the CATMotion editor.
The LegWeight graph controls the how much the foot platform is being pulled down onto the ground plane. When LegWeight has a value of 0 increasing the knee angle will pull the foot higher into the air. When LegWeight has a value of 1.0 increasing the knee angle will pull the pelvis down.
LegWeight can also be thought of as Leg Priority. The left side of the graph is the Lift Priority and the right side is the Plant Priority. This becomes important when you try to push the angles further than is possible to physically achieve. Then by adjusting the leg weights so that one is set to and the other to zero you can control where the error will be most. Usually the planting leg should get priority.
Error within the system is absorbed in the ankles. If you notice the ankles tend to go straight up or otherwise behave strangely, try adjusting either LegWeight or KneeAngle.
Usually the first thing to try is to reduce the Maximum Stride Length or increase or decrease various knee angles.
If the movement looks a bit clunky it could be the LegWeight graph needs smoothing out. If the ankles are going too straight as the foot lifts it could be the LegWeight on the lift part of the graph is too long.
Running and Retargeting
While retargeting is useful for walking, it isn’t an ideal solution for running. At this point it is pretty much impossible to get a useful blend between where the leg weights let go and “flight” takes over.
For now we recommend that you use IK for running and jumping.