Fix Foot Sliding with IK Retargeter

When using the IK Retargeter to retarget animation between different characters, the target character may exhibit noticeable feet sliding. This can occur if leg lengths and stances between the source and target characters are very different, causing leg retargeting to be imperfect. To resolve this, you can use Speed Planting workflows which pin IK Goals in place when they should not be moving.

This document provides instructions on how to use Speed Planting to resolve foot sliding in your retargeted animations.

Prerequisites

• You have retargeted animations from one character to another using the IK Retargeter. Refer to the Retargeting Bipeds with IK Rig page for how to do this.
• You are familiar with creating Full Body IK solvers in IK Rig.

Source Animation Setup

The first step for setting up Speed Planting is to create Animation Curves in the source animation that will define the speed of each foot bone throughout the animation. You can do this using Animation Data Modifiers to generate the curve so that it produces accurate motion data quickly. The intent of this step is to use Animation Curve data to determine at what point during the animation the feet should be planted.

Animation Data Modifier

In your source Animation Sequence, from the main menu, go to Window > Animation Data Modifiers.

Next, add a Motion Extractor Modifier by clicking Add Modifier > MotionExtractorModifier. Do this for each foot bone within the source animation. In this example, two Motion Extractor Modifiers are added.

Within both Motion Extractor Modifiers, set the following properties:

• Bone Name: The name of the foot bone to watch for its speed. This example uses the ball_r and ball_l bones.
• Motion Type: Translation Speed.
• Axis: XYZ.

Generate Speed Curve

Once your Animation Data Modifiers are set up, click Apply All Modifiers to generate speed curves for the specified bones.

Double-click the curve track to open the Curve Editor where you can see more detailed information about it. Select or hover your cursor over the keyframes to see the values. This information can be used to determine the threshold of speed at which the foot should be planted. In most cases, this means that the lower flat parts of the curve are where the foot is stationary, whereas the peaks are where it is in motion.

Because you modified this Animation Sequence and the Skeleton (due to creating a new curve), make sure you save both Assets now.

IK Rig Setup

Continuing the setup process, you must now create IK Goals and Solvers for the leg chains on the target character. This example uses a Full Body IK (FBIK) solver, however depending on your character, you could choose other kinds of IK Rig Solvers. This step is required in order to pin the legs using IK when they become stationary during the animation.

Create FBIK

Open the target character's IK Rig Asset used for retargeting, navigate to the Hierarchy panel, right-click the end bone in one of the leg retarget chains, and select New IK Goal.

In the following prompts, ensure Full Body IK is set as the solver and click Add Solver, then click Assign Goal.

Next, ensure that the newly created Full Body IK solver is selected in the Solver Stack panel, right-click the other foot bone, select New IK Goal, then click Assign Goal.

Finally, select the Full Body IK solver, then right-click the root bone in the hierarchy and select Set Root Bone on Selected Solver. Typically, the root bone for an FBIK solver is the pelvis or hips.

FBIK Settings

Depending on the bone structure of your character's legs, you may also need to create additional bone settings for bones that are controlled by the FBIK solver. This resolves stiffness or incorrect joint rotation that can result from the FBIK solve. To create a Bone setting, select the Full Body IK Solver, then right-click a bone you want to create settings for and select Add Settings to Selected Bone.

In this example, to resolve rotational stiffness for some leg bones, bone settings were created for these bones with the following properties:

• thigh_l
  • Use Preferred Angles: Enabled
  • Preferred Angles: 0, 0, -45
• calf_l
  • Use Preferred Angles: Enabled
  • Preferred Angles: 0, 0, 45
• thigh_r
  • Use Preferred Angles: Enabled
  • Preferred Angles: 0, 0, -45
• calf_r
  • Use Preferred Angles: Enabled
  • Preferred Angles: 0, 0, 45

Depending on your character's needs, there may be other bone settings required to ensure your FBIK solver functions correctly, such as Stiffness, Limits, and Mass Multiplier. Refer to the Full Body IK section of the Solvers page for more information.

Speed Planting in IK Retargeter

Next, open the IK Retargeter Asset for your characters. In the viewport select either the IK Goal or the chain, and in the Details panel enable Speed Planting.

Set the following properties in the Speed Planting section:

• Set Speed Curve Name to the name of the corresponding leg curve that was generated earlier on the source character. In this example it is set to ball_l_translation_speed_XYZ.
• Depending on the generated curve values, you may need to adjust Speed Threshold to a number that is slightly higher than where the curve stays flat. If you notice that your character's feet are still unplanting inappropriately, then increase this number. In this example, it is set to 30.

Do this for all necessary leg chains on your target character.

Final Result

Now when you play animations in the retargeter, you should see better feet planting for your target character. You can now export the retargeted animation to a new Animation Sequence.