Rigging and animating walkers

Legged walkers such as striders, mechs, and walking fortresses are the hardest rigs to make read as believable, 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 such as the AT-TE and AT-AT are built.

This is author-facing design background. It is neither a rig a case pins nor content for a brief. A case declares no parts, joints, or pose angles: its [model] table fixes 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, and it reads as a heavy machine rather than flailing.

The brief carries only that requirement. The mechanics on this page are how a walk is achieved, and they stay out of a brief. That covers segment counts, joint angles, counter-rotation to hold a foot flat, knee direction, and gait phasing. A brief specifies what rather than 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 rather than giving a spec to reproduce. The parts, joints, animations, and F-curves they build on are defined by the voxel binaries.

Leg structure

A believable walker leg 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, moving the leg up and down as well as fore and 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. Modelling a left or right bank of legs as a single part on one shared pivot drives the rear feet down through the ground while the front feet lift, because a fore-and-aft spread of feet is rotating about one point. Independent per-leg chains are what stop the feet clipping.

The AT-TE

The AT-TE’s three leg pairs use two distinct designs. Angles below take 0° as flat and forward, and −90° as straight down.

Rear legs use three segments and two joints, running 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 use two segments and two joints, running 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°.
• The lower joint exists to keep the foot flat, tilting it only about ±15°.

Front legs are the rear legs mirrored: the same three-segment, two-joint design, with the middle segment sitting forward of the joint and travelling roughly +30° to +60°.

The through-line is big motion at the top joint, small motion lower down, and a foot that stays nearly flat. A leg that splits a large rotation evenly down the chain, or lets the foot tilt with the shin, reads as a spider tiptoeing rather than a heavy machine walking.

The planted stance phase

A believable walk cycle has two phases per leg, and the stance phase is the one most often omitted.

In stance, the foot is planted flat and translates straight backward relative to the body. The walk is authored in place, so the body’s origin does not travel across the scene and the planted foot slides straight back under the body through stance, like a treadmill belt. A consuming game moves the whole unit forward at that same speed, which is what 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.

In 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 whose foot is in a continuous arc the whole time and never sits still on the ground makes a walker look like it is flailing its legs instead of pushing itself forward. 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, and a hexapod walks two alternating tripods, three planted legs at all times, a half-period apart.

In-place authoring

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 and then swinging forward. A consuming game plays the clip while it drives the unit’s real world movement, so a clip that also translated the body would compound with that and rocket the unit forward. Authoring the walk in place is what lets a game reuse it.

When you author the cycle:

• Keep the root or body part centered. 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, then a lifting arc forward during swing.
• The review viewer plays the clip in place, so a correct walk shows the body holding station while the feet cycle underneath.

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

Foot angle and knee direction

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

The lower joint must bend the way a real walker’s does, a reverse or digitigrade knee. The common failure is the lower segment rotating the wrong way relative to the upper, a knee bending inside-out, which instantly reads as broken. Fix the sign of the knee’s motion as well as its range.

World-space angle versus relative rotation

The example angles throughout this page describe the world orientation of each segment, how it points in the scene. A joint does not set its segment’s world angle: a joint’s rotation is applied relative to its parent segment and stacks on top of everything above it, so a segment’s world orientation is the sum of its parent’s world orientation and its own local joint rotation. Mistaking one for the other is the most common reason a foot refuses to stay flat.

Keeping the foot flat in the world is therefore 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 at a roughly constant world angle, the ankle must counter-rotate by the negative of that accumulated hip and knee rotation, tracking it frame by frame. That is a moving local angle. A foot pinned to a fixed local angle visibly tips as the leg folds and extends.

Two consequences follow. 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, and a narrow-range ankle cannot stay flat through the stride. The foot’s track should also be thought about in the world frame, where the foot stays flat while the shin swings back, letting the relative ankle keyframes fall out of that goal.

Curved interpolation

Legs carry weight, and weight means the motion is not a constant-speed slide between poses. Author the joint tracks as F-curves rather than 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, with gentle acceleration and deceleration at each key (ease-in-out) giving a slow, ponderous roll. The AT-TE’s front and rear legs are about 80% smooth and then accelerate hard into the foot-plant, an ease-in on the final descent that gives the satisfying thump of a heavy foot landing, while its middle legs stay smooth.

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

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 from that path.

1. Define the foot path in the body’s frame: a flat, ground-level segment moving straight back for stance, then a lifting arc forward for swing, with the foot held flat throughout.
2. At several sample times, solve the leg’s joint angles that place the foot on that path, hip, knee, and 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 with a clearly folded knee. A near-straight leg has no room to extend and fold, so the foot cannot stay planted as the body passes over it.