Foot Slides During a Walk Cycle After Retargeting
A rigger retargets a 30-frame walk cycle onto a new character whose leg proportions are about 15 percent shorter than the source. The cycle plays at the correct speed, but the planted foot slides backward during contact frames and the knee pops at frame 12. The rig uses IK legs with a reverse-foot setup. Root motion is baked into the hips. The animator has two days before delivery and cannot re-author the cycle from scratch.
1. Explain why the foot slides and propose a fix that preserves the source timing.
Read the worked answer
The slide comes from proportion mismatch under IK. The source cycle was authored for longer legs; when retargeted to shorter legs, the IK chain cannot reach the same hip-to-foot distances, so the solver either over-extends or the hips drift relative to the planted foot. Because root motion is baked into the hips, the hip translation no longer matches the new stride length, and the contact foot appears to slide.
Fix that preserves timing:
- Unbake hip translation to a separate root control so stride length can be adjusted independently of timing.
- Scale the hip translation amplitude by roughly the leg-length ratio (about 0.85) so the stride matches the shorter legs.
- Re-key the IK foot controls at contact frames so the planted foot stays fixed in world space; use IK handle snapping or a locator constraint.
- Check the reverse-foot attributes; a 15 percent proportion change often needs the foot roll pivots repositioned.
Timing (30 frames) stays untouched; only translation amplitude and foot placement change.
2. What is causing the knee pop at frame 12, and how do you resolve it?
Read the worked answer
A knee pop at a single frame usually means the IK chain hits full extension or crosses its preferred angle plane, forcing the solver to flip the bend direction. With shorter legs, the hip-to-foot distance at frame 12 likely exceeds the chain length, so the knee snaps straight.
Resolve it by:
- Checking the IK handle's preferred angle and re-setting it while the knee is slightly bent.
- Adding a small bend to the knee in the rest pose so the solver always has a direction to follow.
- If the leg is over-extended, either lower the hip at that frame or bring the foot control slightly closer so the chain is never fully straight.
- Inspect the pole vector constraint; a pole vector that drifts can also flip the knee.
After fixing, scrub frames 10-14 and confirm the knee bend is continuous rather than snapping between two solutions.