Gradientfall: procedural animation framework + Kern's movement pass

[danieltalbert]

Kern's locomotion is rebuilt on a new creature-agnostic animation stack under game/src/anim/, so every future villager, monster and boss inherits it rather than getting its own hand-tuned sine waves.

The load-bearing change

The gait cycle is phased by distance travelled, not by time:

phase += distance_this_frame / stride_length

A planted foot's body-relative position then retreats at exactly the speed the body advances, so it holds still in world space as an algebraic identity — at any speed, mid-acceleration, across gait changes. The placeholder did phase += delta * rate while the body moved at whatever speed it liked, which is why the feet skated.

Framework (9 modules)

Module	Responsibility
gait_engine.gd	Distance-phased cycle; rigid-foot heel/toe rocker
gait_profile.gd	Gaits as biomechanics, blended continuously by speed
two_bone_ik.gd	Closed-form law-of-cosines leg solve
foot_planter.gd	Ground probes, leashed world plant-locks, pelvis drop
locomotion_profile.gd	Measures limbs off the rig; rescales by leg length
pose_stack.gd	Layered quaternion pose (euler layers do not compose)
emote_library.gd / emote_player.gd	16 emotes; blending; partial-body
anim_math.gd	Half-life damping, exact critically-damped springs
creature_animator.gd	Coordinator: lean, air, landing, idle life

Gaits blend continuously, so there is no walk/run threshold to pop at — duty factor slides through 0.5 and double-support becomes a flight phase the way a real gait transition does.

Kern

Analog walk/jog/run/sprint (stick deflection is the speed), crouch with capsule resize and ceiling check, slope-aware pace, step-up, landing absorption, turning that costs momentum, idle fidget scheduler, head look-at with travel anticipation, and 16 emotes/dances on a radial wheel (B / right-stick click).

Dances are named out of the world's own vocabulary: Gradient Descent, The Backprop, Overfit, Dropout, Convergence, Weight Shuffle, Epoch Step, Softmax.

Verification

New instrumented bench scenes/dev/locomotion_lab.tscn drives the real controller through 20 scripted segments on a four-zone obstacle course (flat, ramps, stairs, broken ground) via the Input singleton.

Metric	Before	After
Foot slip (overall)	147 mm/m	47 mm/m
Foot slip (walk)	148 mm/m	19-23 mm/m
Ground error	60 mm	7.4 mm
Backward-knee frames	8182	0
Worst pose jerk	58 rad/s	47 rad/s

Idle, hard-stop and turn-in-place measure ~0.2 mm. Game boots clean headless; validate_content.py passes 111 entries. Gait and dance frames eyes-verified against ground stripes.

Fixed in passing (both pre-existing)

• The placeholder gait bent Kern's knees backwards at every speed — positive X on the shin is hyperextension on this rig. The IK now derives knee direction from a pole vector so it cannot recur, and the bench fails the run if it does.
• The boot shaft stopped 65 mm below the trouser cuff, leaving a visible ring of nothing between boot and trouser whenever the knee bent.

Known limitations (documented in DEVLOG)

• Residual slip concentrates in the heel/toe rocker at jog and above; flat-sole slip is near zero, so the plant lock itself is sound.
• Kern's rig gives him a 0.81 m leg on a 1.78 m body (a real adult is ~0.87 m) because his ankle sits ~4 cm high. That forces a stride cap and a slightly brisk cadence. Lowering the ankle would buy it back but moves the boot meshes and the imported-rig retarget, so it is its own change.
• NPCs and monsters are still un-skeletoned meshes; rigging them to adopt CreatureAnimator is the next milestone.

🤖 Generated with Claude Code