要使用HTML5 Canvas创建二次运动动画，我们可以为每个帧增加vx（水平速度），vy（垂直速度）或对象的vx和vy，然后更新对象的位置， 根据加速度方程：
距离=速度*时间+ 1/2 *加速度*时间^ 2
html代码：
<!DOCTYPE HTML>
<html>
<head>
<style>
body {
margin: 0px;
padding: 0px;
}
</style>
</head>
<body>
<canvas id="myCanvas" width="578" height="200"></canvas>
<script>
window.requestAnimFrame = (function(callback) {
return window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || window.oRequestAnimationFrame || window.msRequestAnimationFrame ||
function(callback) {
window.setTimeout(callback, 1000 / 60);
};
})();
function drawRectangle(myRectangle, context) {
context.beginPath();
context.rect(myRectangle.x, myRectangle.y, myRectangle.width, myRectangle.height);
context.fillStyle = '#8ED6FF';
context.fill();
context.lineWidth = myRectangle.borderWidth;
context.strokeStyle = 'black';
context.stroke();
}
function animate(myRectangle, canvas, context, startTime) {
// update
var time = (new Date()).getTime() - startTime;
// pixels / second^2
var gravity = 200;
myRectangle.y = 0.5 * gravity * Math.pow(time / 1000, 2);
if(myRectangle.y > canvas.height - myRectangle.height - myRectangle.borderWidth / 2) {
myRectangle.y = canvas.height - myRectangle.height - myRectangle.borderWidth / 2;
}
lastTime = time;
// clear
context.clearRect(0, 0, canvas.width, canvas.height);
// draw
drawRectangle(myRectangle, context);
// request new frame
requestAnimFrame(function() {
animate(myRectangle, canvas, context, startTime);
});
}
var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');
var myRectangle = {
x: 239,
y: 0,
width: 100,
height: 50,
borderWidth: 5
};
drawRectangle(myRectangle, context);
// wait one second before dropping rectangle
setTimeout(function() {
var startTime = (new Date()).getTime();
animate(myRectangle, canvas, context, startTime);
}, 2000);
</script>
</body>
</html>
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