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* Keyframe interpolation for smooth skeletal animations
*
* Provides interpolation between animation keyframes for smooth 60fps playback.
* Supports linear, ease-in, ease-out, and ease-in-out easing functions.
*
* @module systems/animation/KeyframeInterpolation
* @category Animation System
* @korean 키프레임보간
*/
import * as THREE from "three";
import type {
AnimationKeyframe,
SkeletalAnimation,
SkeletalRig,
} from "../../types/skeletal";
/**
* Easing function type
*
* @param t - Time value between 0 and 1
* @returns Eased value between 0 and 1
*
* @korean 이징함수타입
*/
type EasingFunction = (t: number) => number;
/**
* Linear easing (no easing)
*
* @param t - Input time (0-1)
* @returns Same as input
*
* @korean 선형이징
*/
export const easeLinear: EasingFunction = (t: number): number => t;
/**
* Ease-in (slow start, fast end)
*
* @param t - Input time (0-1)
* @returns Eased value
*
* @korean 이즈인
*/
export const easeIn: EasingFunction = (t: number): number => t * t;
/**
* Ease-out (fast start, slow end)
*
* @param t - Input time (0-1)
* @returns Eased value
*
* @korean 이즈아웃
*/
export const easeOut: EasingFunction = (t: number): number => t * (2 - t);
/**
* Ease-in-out (slow start, slow end)
*
* @param t - Input time (0-1)
* @returns Eased value
*
* @korean 이즈인아웃
*/
export const easeInOut: EasingFunction = (t: number): number => {
if (t < 0.5) {
return 2 * t * t;
}
return -1 + (4 - 2 * t) * t;
};
/**
* Get easing function by name
*
* @param name - Easing function name
* @returns Easing function
*
* @korean 이징함수구하기
*/
export const getEasingFunction = (
name: "linear" | "ease-in" | "ease-out" | "ease-in-out" = "linear"
): EasingFunction => {
switch (name) {
case "ease-in":
return easeIn;
case "ease-out":
return easeOut;
case "ease-in-out":
return easeInOut;
default:
return easeLinear;
}
};
/**
* Find keyframes surrounding current time
*
* Returns the two keyframes to interpolate between for current animation time.
*
* @param animation - Skeletal animation
* @param currentTime - Current time in animation (seconds)
* @returns Tuple of [previousKeyframe, nextKeyframe, interpolationFactor]
*
* @korean 주변키프레임찾기
*/
export const findSurroundingKeyframes = (
animation: SkeletalAnimation,
currentTime: number
): [AnimationKeyframe, AnimationKeyframe, number] => {
const { keyframes } = animation;
// Clamp time to animation duration
const clampedTime = Math.max(
0,
Math.min(currentTime, animation.duration)
);
// Find keyframes
let prevKeyframe = keyframes[0];
let nextKeyframe = keyframes[keyframes.length - 1];
let interpolationFactor = 0;
for (let i = 0; i < keyframes.length - 1; i++) {
const current = keyframes[i];
const next = keyframes[i + 1];
if (clampedTime >= current.time && clampedTime <= next.time) {
prevKeyframe = current;
nextKeyframe = next;
// Calculate interpolation factor (0 to 1)
const timeDelta = next.time - current.time;
if (timeDelta > 0) {
interpolationFactor = (clampedTime - current.time) / timeDelta;
}
break;
}
}
return [prevKeyframe, nextKeyframe, interpolationFactor];
};
/**
* Interpolate between two Euler rotations
*
* Performs spherical linear interpolation (slerp) for smooth rotation.
*
* @param from - Start rotation
* @param to - End rotation
* @param t - Interpolation factor (0-1)
* @param easingFn - Easing function to apply
* @returns Interpolated rotation
*
* @korean 회전보간
*/
export const interpolateRotation = (
from: THREE.Euler,
to: THREE.Euler,
t: number,
easingFn: EasingFunction = easeLinear
): THREE.Euler => {
const easedT = easingFn(t);
// Convert Euler to Quaternion for proper slerp
const fromQuat = new THREE.Quaternion().setFromEuler(from);
const toQuat = new THREE.Quaternion().setFromEuler(to);
// Slerp between quaternions
const result = new THREE.Quaternion().slerpQuaternions(
fromQuat,
toQuat,
easedT
);
// Convert back to Euler
const resultEuler = new THREE.Euler().setFromQuaternion(result);
return resultEuler;
};
/**
* Interpolate between two Vector3 positions
*
* Performs linear interpolation for smooth position changes.
*
* @param from - Start position
* @param to - End position
* @param t - Interpolation factor (0-1)
* @param easingFn - Easing function to apply
* @returns Interpolated position
*
* @korean 위치보간
*/
export const interpolatePosition = (
from: THREE.Vector3,
to: THREE.Vector3,
t: number,
easingFn: EasingFunction = easeLinear
): THREE.Vector3 => {
const easedT = easingFn(t);
return new THREE.Vector3().lerpVectors(from, to, easedT);
};
/**
* Get interpolated keyframe at current time
*
* Calculates bone transformations by interpolating between keyframes.
*
* @param animation - Skeletal animation
* @param currentTime - Current time in animation (seconds)
* @returns Interpolated keyframe
*
* @korean 보간된키프레임구하기
*/
export const getInterpolatedKeyframe = (
animation: SkeletalAnimation,
currentTime: number
): AnimationKeyframe => {
const [prevKeyframe, nextKeyframe, t] =
findSurroundingKeyframes(animation, currentTime);
// Get easing function from next keyframe
const easingFn = getEasingFunction(nextKeyframe.easing);
// Interpolate bone rotations
const boneRotations = new Map<string, THREE.Euler>();
const allBoneNames = new Set([
...prevKeyframe.boneRotations.keys(),
...nextKeyframe.boneRotations.keys(),
]);
for (const boneName of allBoneNames) {
const prevRotation =
prevKeyframe.boneRotations.get(boneName) ?? new THREE.Euler();
const nextRotation =
nextKeyframe.boneRotations.get(boneName) ?? new THREE.Euler();
const interpolated = interpolateRotation(
prevRotation,
nextRotation,
t,
easingFn
);
boneRotations.set(boneName, interpolated);
}
// Interpolate bone positions
const bonePositions = new Map<string, THREE.Vector3>();
const allPositionBones = new Set([
...prevKeyframe.bonePositions.keys(),
...nextKeyframe.bonePositions.keys(),
]);
for (const boneName of allPositionBones) {
const prevPosition =
prevKeyframe.bonePositions.get(boneName) ?? new THREE.Vector3();
const nextPosition =
nextKeyframe.bonePositions.get(boneName) ?? new THREE.Vector3();
const interpolated = interpolatePosition(
prevPosition,
nextPosition,
t,
easingFn
);
bonePositions.set(boneName, interpolated);
}
return {
time: currentTime,
boneRotations,
bonePositions,
easing: nextKeyframe.easing,
};
};
/**
* Apply keyframe to skeletal rig
*
* Updates all bone transformations based on keyframe data.
*
* @param rig - Skeletal rig to update
* @param keyframe - Keyframe to apply
*
* @korean 키프레임적용
*/
export const applyKeyframeToRig = (
rig: SkeletalRig,
keyframe: AnimationKeyframe
): void => {
// Apply bone rotations
keyframe.boneRotations.forEach((rotation, boneName) => {
const bone = rig.bones.get(boneName);
if (bone) {
bone.rotation.copy(rotation);
}
});
// Apply bone positions (offset from rest pose)
keyframe.bonePositions.forEach((position, boneName) => {
const bone = rig.bones.get(boneName);
if (bone) {
// Add position offset to rest position
bone.position.copy(bone.restPosition).add(position);
}
});
};
/**
* Blend between two keyframes
*
* Creates smooth transition between two animations for animation blending.
* Useful for transitioning between stance change and attack, etc.
*
* @param keyframe1 - First keyframe
* @param keyframe2 - Second keyframe
* @param blendFactor - Blend amount (0 = keyframe1, 1 = keyframe2)
* @returns Blended keyframe
*
* @korean 키프레임블렌드
*/
export const blendKeyframes = (
keyframe1: AnimationKeyframe,
keyframe2: AnimationKeyframe,
blendFactor: number
): AnimationKeyframe => {
const clampedBlend = Math.max(0, Math.min(1, blendFactor));
// Blend rotations
const boneRotations = new Map<string, THREE.Euler>();
const allBones = new Set([
...keyframe1.boneRotations.keys(),
...keyframe2.boneRotations.keys(),
]);
allBones.forEach((boneName) => {
const rot1 = keyframe1.boneRotations.get(boneName) ?? new THREE.Euler();
const rot2 = keyframe2.boneRotations.get(boneName) ?? new THREE.Euler();
const blended = interpolateRotation(rot1, rot2, clampedBlend);
boneRotations.set(boneName, blended);
});
// Blend positions
const bonePositions = new Map<string, THREE.Vector3>();
const allPositionBones = new Set([
...keyframe1.bonePositions.keys(),
...keyframe2.bonePositions.keys(),
]);
allPositionBones.forEach((boneName) => {
const pos1 = keyframe1.bonePositions.get(boneName) ?? new THREE.Vector3();
const pos2 = keyframe2.bonePositions.get(boneName) ?? new THREE.Vector3();
const blended = interpolatePosition(pos1, pos2, clampedBlend);
bonePositions.set(boneName, blended);
});
return {
time: 0,
boneRotations,
bonePositions,
easing: "linear",
};
};
/**
* Update animation state
*
* Advances animation time and returns current interpolated keyframe.
* Handles looping animations automatically.
*
* @param animation - Skeletal animation
* @param currentTime - Current time in animation
* @param deltaTime - Time since last update (seconds)
* @param playbackSpeed - Speed multiplier (1.0 = normal)
* @returns Updated time and current keyframe
*
* @korean 애니메이션상태업데이트
*/
export const updateAnimation = (
animation: SkeletalAnimation,
currentTime: number,
deltaTime: number,
playbackSpeed = 1.0
): { time: number; keyframe: AnimationKeyframe; completed: boolean } => {
// Advance time
let newTime = currentTime + deltaTime * playbackSpeed;
let completed = false;
// Handle looping or completion
if (newTime >= animation.duration) {
if (animation.loop) {
newTime = newTime % animation.duration;
} else {
newTime = animation.duration;
completed = true;
}
}
// Get interpolated keyframe
const keyframe = getInterpolatedKeyframe(animation, newTime);
return {
time: newTime,
keyframe,
completed,
};
};
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