Rigging and animating walkers

Legged walkers — striders, mechs, walking fortresses — are the hardest voxel rigs to make read as believable rather than flailing, and they fail in consistent ways. This page is a reference for how a walker’s legs behave — how a believable leg is structured and how a convincing walk cycle moves — drawn from how established walkers (the AT-TE and AT-AT) are built.

This is author-facing design background, not a rig a case pins, and not content for a brief. A case no longer declares parts, joints, or pose angles: its [model] table fixes only the required animations (by name), and the model invents the parts, joints, pivots, and F-curves it needs to satisfy them. Use this page to understand what a convincing walk looks like so you can state that as a crisp behavioural requirement in the brief — the feet plant flat and the body advances over them; it reads as a heavy machine, not flailing.

The brief carries only that requirement. The mechanics on this page — segment counts, joint angles, counter-rotation to hold a foot flat, knee direction, gait phasing — are how a walk is achieved, and they never go into a brief: a brief specifies what, not how, is seeded into a sandbox with no access to these docs, and must never link here. Working the mechanics out from the behavioural requirement is the test.

The angles and segment breakdowns below are illustrative — they explain why real walkers read as heavy and grounded, not a spec to reproduce. The mechanics referenced here (parts, joints, model-authored animations, and F-curves) are defined in The voxel binaries and Manifests; this page is the design guidance that sits on top of them.

A leg is three segments and two joints

A believable walker leg is not one rigid part swung from a hip, and it is not a whole bank of legs sharing one pivot. It is an articulated chain: three segments (an upper thigh, a lower shin, and a short foot) joined by two moving joints (a hip and a knee), with the foot kept flat. Equivalently, the upper joint carries two degrees of freedom — it moves the leg up/down and fore/aft — so the foot can be lifted and placed rather than only swung.

Each leg is its own chain of parts on its own hip, positioned directly above its own foot. Do not model a left/right bank of legs as a single part on one shared pivot: rotating a fore-and-aft spread of feet about one point drives the rear feet down through the ground while the front feet lift. Independent per-leg chains are what stop the feet clipping.

The AT-TE, dissected

The AT-TE is the richest reference — its three leg pairs use two distinct designs. Angles below take 0° as flat/forward and −90° as straight down.

• Rear legs — three segments, two joints (foot → very short segment → joint → segment → joint → body):
  • the upper segment travels roughly −30° to −150° (a large sweep),
  • the middle segment travels roughly −120° to −150° (a small sweep, held well behind the upper joint),
  • the bottom segment is extremely short and barely moves — it stays almost vertical the whole cycle, and the foot itself tilts only about ±15°.
• Middle legs — two segments, two joints (foot → segment → joint → segment → joint → body):
  • the upper joint travels a semicircle: while the foot is planted it swings backward across the body, then lifts up and comes back down to place the foot forward again,
  • the top segment moves only a little, roughly −60° to −120° (≈ −70° to −110°),
  • the lower joint exists to keep the foot flat, tilting it only about ±15°.
• Front legs — the rear legs mirrored: the same three-segment/two-joint design, but the middle segment sits forward of the joint, travelling roughly +30° to +60° instead of behind.

The through-line: big motion at the top joint, small motion lower down, and a foot that stays nearly flat. A leg that instead splits a large rotation evenly down the chain — or lets the foot tilt with the shin — reads as a spider tiptoeing, not a heavy machine walking.

The gait needs a planted stance phase

This is the single most important rule, and the most common omission. A believable walk cycle has two phases per leg:

1. Stance — the foot is planted flat and translates straight backward relative to the body. Because the walk is authored in place (the body’s origin does not travel across the scene — see the next section), the planted foot slides straight back under the body through stance, like a treadmill belt; it is a consuming game moving the whole unit forward at that same speed that makes the foot read as anchored to the ground while the machine advances over it. The leg extends and folds (hip and knee working together) to carry the foot straight back along the ground line while the body holds station.
2. Swing — the foot lifts clear of the ground, travels forward, and plants again at the front of the stride, ready for the next stance.

A cycle with no planted stance phase — where the foot is in a continuous arc the whole time and never sits still on the ground — is what makes a walker look like it is flailing its legs instead of pushing itself forward. If you take nothing else from this page: there must be a segment of the cycle where the foot is flat and still on the ground while the body moves relative to it.

Phase the legs so the machine is always supported: a biped alternates the two legs in opposite phase; a quadruped moves diagonal pairs together; a hexapod walks two alternating tripods (three planted legs at all times) a half-period apart.

Author the walk in place — the body must not travel

A walk or march clip is a looping, in-place cycle: over one period the rig’s root does not translate across the scene — it starts and ends at the same place, with zero net displacement. Forward motion is conveyed entirely by the legs — the planted foot sliding straight back under the body during stance, then swinging forward — not by sliding the whole model across the volume. A consuming game plays the clip while it drives the unit’s real world movement; if the clip also translated the body, the two would compound and the unit would rocket forward. Authoring the walk in place is what lets a game reuse it.

When you author the cycle:

• Keep the root/body part centered — do not keyframe a steady forward translation onto the root. A small vertical bob (the body rising and settling with the stride) is right; a net forward drift across the loop is not.
• Express all forward motion as the foot path in the body’s frame — back during stance, a lifting arc forward during swing — exactly as in Authoring method below.
• The review viewer plays the clip in place, so a correct walk shows the body holding station while the feet cycle underneath (a treadmill), not the model marching off across the scene.

The same rule applies to any locomotion animation — a strider’s march, a flyer’s hover or cruise: the clip animates the motion in place, and the game supplies the travel.

Keep the foot flat, and bend the knee the right way

• Flat foot. The foot should tilt only about ±15° in the world across the whole cycle, held level by the foot/ankle joint counter-rotating against the leg. A foot that tilts far more than this (the tell-tale of a rigid two-joint arc with no foot control) reads as the machine walking on its toes and heels.

World-space angle vs. relative rotation — why feet don’t stay flat

This is the single most common reason a foot refuses to stay flat, and it is worth being explicit about. The example angles throughout this page (0° flat/forward, −90° straight down) describe the world orientation of each segment — how it points in the scene. But a joint does not set its segment’s world angle. A joint’s rotation is applied relative to its parent segment, and it stacks on top of everything above it: a segment’s world orientation is the sum of its parent’s world orientation and its own local joint rotation.

So keeping the foot flat in the world is not a matter of holding the ankle at a fixed local angle. As the hip and knee rotate through the stride, their rotations accumulate down the chain, and the foot inherits all of them. To hold the foot flat (a roughly constant world angle), the ankle must counter-rotate by the negative of that accumulated hip + knee rotation, tracking it frame by frame — a moving local angle, not a constant one. A foot pinned to a fixed local angle will visibly tip as the leg folds and extends.

Two consequences for the design:

• The ankle needs enough range to cancel the full swing of the joints above it. If the hip and knee together sweep a large arc, the ankle’s range must be able to absorb it; an ankle with a narrow range simply cannot stay flat through the stride.

• When you author (or ask the model to author) the foot’s track, think in the world frame — “the foot stays flat while the shin swings back” — and let the relative ankle keyframes fall out of that goal, rather than keyframing a fixed local ankle angle and hoping it looks level.

• Knee direction. The lower joint must bend the way a real walker’s does (a reverse / digitigrade knee). The common failure is the lower segment rotating the wrong way relative to the upper — the knee bending “inside-out” — which instantly reads as broken. Fix the sign of the knee’s motion, not just its range.

Animate with curves, not straight lines

Legs carry weight, and weight means the motion is not a constant-speed slide between poses. Author the joint tracks as F-curves (The voxel binaries), not linear interpolation — linear keys read as weightless, mechanical flailing however correct the poses are.

How much easing depends on the machine:

• The AT-AT walks largely smoothly — gentle acceleration and deceleration at each key (ease-in-out), a slow ponderous roll.
• The AT-TE’s front and rear legs are about 80% smooth, then accelerate hard into the foot-plant — an ease-in on the final descent that gives the satisfying “thump” of a heavy foot landing. Its middle legs stay smooth.

So a heavy walker typically eases most of its motion and reserves a sharp ease-in for the moment of contact. Match the curve to the weight you want the viewer to feel.

Authoring method: design the foot path, then solve the joints

Because the joints are driven by keyframed angles but the goal is a specific foot path (planted-flat during stance, a lift arc during swing), author a walk by working backward:

1. Define the foot path in the body’s frame: a flat, ground-level segment moving straight back (stance), then a lifting arc forward (swing), with the foot held flat throughout.
2. At several sample times, solve the leg’s joint angles (inverse kinematics) that place the foot on that path — hip, knee, and foot/ankle together.
3. Set those solved angles as the track keyframes, choose the easing per segment (smooth through the swing, a sharp ease-in into the plant), and phase the legs per the gait above.

Design the rest pose as a bent leg (a clearly folded knee), not a straight column — a near-straight leg has no room to extend and fold, so the foot cannot stay planted as the body passes over it.