重要公式：

（1）简单缓动

dx = targetX - object.x;

dy = targetY - object.y;

vx = dx * easing;

vy = dy * easing;

object.x += vx;

object.y += vy;

可精简：

vx = (targetX - object.x) * easing;

vy = (targetY - object.y) * easing;

object.x += vx;

object.y += vy;

再精简：

object.x += (targetX - object.x) * easing;

object.y += (targetY - object.y) * easing;

（2）简单弹动

ax = (targetX - object.x) * spring;

ay = (targetY - object.y) * spring;

vx += ax;

vy += ay;

vx *= friction;

vy *= friction;

object.x += vx;

object.y += vy;

可精简：

vx += (targetX - object.x) * spring;

vy += (targetY - object.y) * spring;

vx *= friction;

vy *= friction;

object.x += vx;

object.y += vy;

再精简：

vx += (targetX - object.x) * spring;

vy += (targetY - object.y) * spring;

object.x += (vx *= friction);

object.y += (vy *= friction);

（3）有偏移的弹动

dx = object.x - fixedX;

dy = object.y - fixedY;

targetX = fixedX + Math.cos(angle) * springLength;

targetY = fixedY + Math.sin(angle) * springLength;

边界检测重要公式：

（1）矩形边界碰撞检测

tool.intersects = function(bodyA,bodyB){

  return !(bodyA.x + bodyA.width < bodyB.x ||

          bodyB.x + bodyB.width < bodyA.x ||

          bodyA.y + bodyA.height < bodyB.y ||

          bodyB.y + bodyB.height < bodyA.y);

};

（2）基于距离的碰撞检测

dx = objectB.x - objectA.x;

dy = objectB.y - objectA.y;

dist = Math.sqrt(dx * dx + dy * dy);

if(dist < objectA.radius + objectB.radius){}

（3）多物体碰撞检测

for(var i = 0; i < objects.length; i++){

  var objectA = objects[i];

  for(var j = i + 1; j < objects.length; j++){

    var objectB = objects[j];

    if(tool.intersects(objectA,objectB){}

  }

};

弹动的公式和检测矩形边界的公式