Measure whether feet touch the ground on the rendered result, not with a formula — fit contact, stride and slope posture to the mesh every frame

A character walks through your game or 3D web scene. It looked fine on flat ground when you first tuned it, but then you lengthened a leg bone, swapped the walk cycle, or put it on a hillside, and now it hovers a few centimetres above the ground, its feet skate along the surface (foot sliding), or one foot sinks into the slope. The symptom is the same in three.js or any other engine. It usually shows up when contact and stride were fitted with a closed-form formula like "height = leg length × cos(angle)" or "stride = 2 × leg length × sin(swing angle)".

Why you should not fit ground contact and stride with a formula

Whether a foot touches the ground is measured on the rendered result, not computed from a formula. Contact height, stride length and posture on a slope come out of bone lengths, joint bends, ankle rotation, skinning weights and terrain curvature all at once. Change any one of them and the closed-form answer goes quietly wrong. Measure the mesh and the terrain that will actually be drawn, every frame, and the answer follows whatever changed.

Formulas usually break in three places:

• Joints stretch and bend. Bend the shin and turn the ankle and the "hip to sole" distance changes every frame. The formula assumes a straight leg.
• Bones are not skin. The foot bone usually sits at the ankle. The sole (the skin) is below it. Measure the bone and your test passes while the character is buried in the floor.
• The ground is not a plane. A body placed at the height of one point sinks its front or back feet on a slope, and a body fitted to the straight line between two points sinks in the middle on a convex hilltop.

How do you measure ground contact?

Do this every frame:

1. Update the animation to this frame's pose.
2. Compute the skinned vertices near the feet in world space and find the lowest y. In three.js, SkinnedMesh.applyBoneTransform moves a vertex to its bone-transformed position. Pre-select a few dozen sole vertices instead of the whole mesh and it stays cheap.
3. Move the root bone (pelvis) up or down by the gap between that lowest point and the terrain height under it.

Leave the bones that move the feet (thigh, shin) alone. Only the root moves. That splits the work cleanly: the clip or IK (inverse kinematics) decides the shape of the walk, and measurement decides the distance to the ground.

How many points do you need on a slope?

Sample several points under the body: along the direction of travel, over the body's length. Five is enough for most terrain.

• Use the front and back points to get the slope and pitch the body along the direction of travel.
• For all five points, check whether the ground there is higher than the underside of the body, and pick the height that clears every one of them.
• Measure ground and body with the same ruler. If you sampled the ground by horizontal distance, compare the body by horizontal distance too. Mixing arc length on one side with horizontal distance on the other leaves a few millimetres of float even on a straight slope.
• Get the body's length (how far it touches front to back) from its bounding box in local space. In world axes, a character walking sideways to the axis gives you its width instead of its length.

The pitch rotation has a trap of its own. With three.js's default Euler order XYZ, an X rotation meant to "lean forward" rolls the character sideways when it faces east. Set rotation.order = 'YXZ' so the heading (Y) is applied first and the lean happens around the local axis.

How do you measure stride length?

A wrong stride makes the feet slide: the body moves forward at a constant speed while the planted foot moves backward at a different one. And you will never see sliding in an in-place preview — nothing moves forward, so there is nothing to compare against.

1. Play the clip in place for one cycle.
2. During the stance phase (while the foot is planted), measure how far that foot's lowest skin vertex travels backward. Stride is that distance × 2.
3. Movement speed = stride ÷ cycle duration. Feed that into locomotion.
4. If you also have a formula value, compare. Within 1–3%, both are trustworthy. If they disagree, the measured value wins.

If you build the walk procedurally, there is one more catch. Swing the stance-leg angle as a sine wave and the planted foot moves backward at a different speed each frame, so it must slide while the body moves at constant speed. Don't pick the angles first: place the foot positions at constant speed first, then solve the thigh and shin angles from them (IK). Bend the knee only in the swing phase, while the foot is in the air.

What to re-measure when you swap a character or clip

When you replace a character with one rigged and animated by an outside service, lock these down with tests that load the real model file (glb or similar) in your engine:

• Stride — re-measure as above. Don't carry over the old character's value
• Contact — measure the lowest skinned vertex, not the foot bone. The imported clip may already have the soles below ground, or bobbing
• Bones — don't delete every bone except the ones your code looks up by name. The animation channels on the deleted bones (shoulder rotation, spine bend) disappear with them. Keep all bones and only rename
• Axes — if your contact fix assumes "the root bone's local Y is up", an imported rig may break that. Move it along the world axis

Lines to put in your agent's instructions:

Do not compute a character's ground height, stride or slope posture from a formula.
Every frame, measure the lowest skinned sole vertex and the terrain height under it, and move only the root bone.
On slopes, sample terrain height at five points under the body along the direction of travel and place the body so no point penetrates.
Measure stride by playing the clip: the distance the planted foot travels backward × 2. If it disagrees with the formula, use the measured value.
When the rig or clip changes, rerun tests that measure all three on the real model file.

What happened to us

We built a caravan of people and pack horses walking over real trail terrain in an ink-painting-style 3D web scene (three.js), and kept coming back to the same spot three times.

• Stretching the joints broke formula-based contact. Leg length changed every frame and the characters floated. After switching to measuring the lowest sole vertex every frame and lowering the pelvis, no joint change made them float again. With a sine-wave stance angle, each step slid about 7 cm (measured as the world-space movement of the planted foot).
• On slopes, a horse's front legs sank entirely. At the trail's steepest grade, about 20°, the drop between the horse's front and back was about a person's height. Pitching it from two points still left it buried by up to 79 cm on curved hills (6 cm for a walking person), measured along the real trail; with five points it became 6 cm for the horse and 0 cm for people (a test that measures body underside against terrain height along the whole trail).
• An imported walk clip could not be used as-is. Its soles sat below ground and bobbed about 11.5 cm, and keeping only the 13 bones our code uses cut the animation channels from 85 to 42, so the shoulders stopped turning. We locked stride, contact and channel count with tests that run the real model file.