动画原理——边界
书籍名称：HTML5-Animation-with-JavaScript
书籍源码：https://github.com/lamberta/html5-animation
1.判断球体是否出界程序
一.画板的边界就是整个画板，也就是画板的形成的矩形。
var left = 0, top = 0, right = canvas.width, bottom = canvas.height;
二.判断一个球体是否跑出边界，要考虑球的半径，位置，和边界。用代码表示，if中的条件，分别对应右左上下的出界条件。
if (ball.x – ball.radius > canvas.width || ball.x + ball.radius < 0 || ball.y – ball.radius > canvas.height || ball.y + ball.radius < 0) { balls.splice(balls.indexOf(ball), 1); }
三.判断球体出界程序
01-removal.html
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Removal</title>
<link rel="stylesheet" href="../include/style.css">
</head>
<body>
<header>
Example from <a href="http://amzn.com/1430236655?tag=html5anim-20"><em>Foundation HTML5 Animation with JavaScript</em></a>
</header>
<canvas id="canvas" width="400" height="400"></canvas>
<textarea id="log"></textarea>
<script src="../include/utils.js"></script>
<script src="./classes/ball.js"></script>
<script>
window.onload = function () {
var canvas = document.getElementById('canvas'),
context = canvas.getContext('2d'),
log = document.getElementById('log'),
balls = [],
numBalls = 10;
for (var ball, i = 0; i < numBalls; i++) {
ball = new Ball(20);
ball.id = "ball" + i;
ball.x = Math.random() * canvas.width;
ball.y = Math.random() * canvas.height;
ball.vx = Math.random() * 2 - 1;
ball.vy = Math.random() * 2 - 1;
balls.push(ball);
}
function draw (ball, pos) {
ball.x += ball.vx;
ball.y += ball.vy;
if (ball.x - ball.radius > canvas.width ||
ball.x + ball.radius < 0 ||
ball.y - ball.radius > canvas.height ||
ball.y + ball.radius < 0) {
balls.splice(pos, 1); //remove ball from array
if (balls.length > 0) {
log.value = "Removed " + ball.id;
} else {
log.value = "All gone!";
}
}
ball.draw(context);
}
(function drawFrame () {
window.requestAnimationFrame(drawFrame, canvas);
context.clearRect(0, 0, canvas.width, canvas.height);
var i = balls.length;
while (i--) {
draw(balls[i], i);
}
}());
};
</script>
</body>
</html>
2.重新生成已经出界的对象
02-fountain.html
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Fountain</title>
<link rel="stylesheet" href="../include/style.css">
</head>
<body>
<header>
Example from <a href="http://amzn.com/1430236655?tag=html5anim-20"><em>Foundation HTML5 Animation with JavaScript</em></a>
</header>
<canvas id="canvas" width="400" height="400"></canvas>
<script src="../include/utils.js"></script>
<script src="./classes/ball.js"></script>
<script>
window.onload = function () {
var canvas = document.getElementById('canvas'),
context = canvas.getContext('2d'),
balls = [],
numBalls = 80,
gravity = 0.5;
for (var ball, i = 0; i < numBalls; i++) {
ball = new Ball(2, Math.random() * 0xffffff);
ball.x = canvas.width / 2;
ball.y = canvas.height;
ball.vx = Math.random() * 2 - 1;
ball.vy = Math.random() * -10 - 10;
balls.push(ball);
}
function draw (ball) {
ball.vy += gravity;
ball.x += ball.vx;
ball.y += ball.vy;
if (ball.x - ball.radius > canvas.width ||
ball.x + ball.radius < 0 ||
ball.y - ball.radius > canvas.height ||
ball.y + ball.radius < 0) {
ball.x = canvas.width / 2;
ball.y = canvas.height;
ball.vx = Math.random() * 2 - 1;
ball.vy = Math.random() * -10 - 10;
}
ball.draw(context);
}
(function drawFrame () {
window.requestAnimationFrame(drawFrame, canvas);
context.clearRect(0, 0, canvas.width, canvas.height);
balls.forEach(draw);
}());
};
</script>
</body>
</html>
3.无限屏幕
这个概念很简单，是对重新生成方法的拓展。就是当目标从右侧出去，他讲从左侧进入。
03-ship-sim-2.html
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Ship Sim 2</title>
<link rel="stylesheet" href="../include/style.css">
<style>
#canvas {
background-color: #000000;
}
</style>
</head>
<body>
<header>
Example from <a href="http://amzn.com/1430236655?tag=html5anim-20"><em>Foundation HTML5 Animation with JavaScript</em></a>
</header>
<canvas id="canvas" width="400" height="400"></canvas>
<aside>Press left and right arrow keys to rotate ship, up to add thrust.</aside>
<script src="../include/utils.js"></script>
<script src="./classes/ship.js"></script>
<script>
window.onload = function () {
var canvas = document.getElementById('canvas'),
context = canvas.getContext('2d'),
ship = new Ship(),
vr = 0,
vx = 0,
vy = 0,
thrust = 0;
ship.x = canvas.width / 2;
ship.y = canvas.height / 2;
window.addEventListener('keydown', function (event) {
switch (event.keyCode) {
case 37: //left
vr = -3;
break;
case 39: //right
vr = 3;
break;
case 38: //up
thrust = 0.05;
ship.showFlame = true;
break;
}
}, false);
window.addEventListener('keyup', function () {
vr = 0;
thrust = 0;
ship.showFlame = false;
}, false);
(function drawFrame () {
window.requestAnimationFrame(drawFrame, canvas);
context.clearRect(0, 0, canvas.width, canvas.height);
ship.rotation += vr * Math.PI / 180;
var angle = ship.rotation, //in radians
ax = Math.cos(angle) * thrust,
ay = Math.sin(angle) * thrust,
left = 0,
right = canvas.width,
top = 0,
bottom = canvas.height;
vx += ax;
vy += ay;
ship.x += vx;
ship.y += vy;
//screen wrapping
if (ship.x - ship.width / 2 > right) {
ship.x = left - ship.width / 2;
} else if (ship.x + ship.width / 2 < left) {
ship.x = right + ship.width / 2;
}
if (ship.y - ship.height / 2 > bottom) {
ship.y = top - ship.height / 2;
} else if (ship.y < top - ship.height / 2) {
ship.y = bottom + ship.height / 2;
}
ship.draw(context);
}());
};
</script>
</body>
</html>
4.反弹
一.反弹
当一个球体的边界碰到左边的边界，重新设置球体的x轴的速度为反向，Y不变，这样就实现了反弹。
04-bouncing-1.html
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Bouncing 1</title>
<link rel="stylesheet" href="../include/style.css">
</head>
<body>
<header>
Example from <a href="http://amzn.com/1430236655?tag=html5anim-20"><em>Foundation HTML5 Animation with JavaScript</em></a>
</header>
<canvas id="canvas" width="400" height="400"></canvas>
<script src="../include/utils.js"></script>
<script src="./classes/ball.js"></script>
<script>
window.onload = function () {
var canvas = document.getElementById('canvas'),
context = canvas.getContext('2d'),
ball = new Ball(),
vx = Math.random() * 10 - 5,
vy = Math.random() * 10 - 5;
ball.x = canvas.width / 2;
ball.y = canvas.height / 2;
(function drawFrame () {
window.requestAnimationFrame(drawFrame, canvas);
context.clearRect(0, 0, canvas.width, canvas.height);
var left = 0,
right = canvas.width,
top = 0,
bottom = canvas.height;
ball.x += vx;
ball.y += vy;
if (ball.x + ball.radius > right) {
ball.x = right - ball.radius;
vx *= -1;
} else if (ball.x - ball.radius < left) {
ball.x = left + ball.radius;
vx *= -1;
}
if (ball.y + ball.radius > bottom) {
ball.y = bottom - ball.radius;
vy *= -1;
} else if (ball.y - ball.radius < top) {
ball.y = top + ball.radius;
vy *= -1;
}
ball.draw(context);
}());
};
</script>
</body>
</html>
二.反弹能量减少
先前的实例是建立在反弹能量不减少的情况，上例系数为-1,而生活中，反弹都会流失一部分能量，所以系数的绝对值是要小于1的，0.7是一个比较逼真的数字。
05-bouncing-2.html
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Bouncing 2</title>
<link rel="stylesheet" href="../include/style.css">
</head>
<body>
<header>
Example from <a href="http://amzn.com/1430236655?tag=html5anim-20"><em>Foundation HTML5 Animation with JavaScript</em></a>
</header>
<canvas id="canvas" width="400" height="400"></canvas>
<script src="../include/utils.js"></script>
<script src="./classes/ball.js"></script>
<script>
window.onload = function () {
var canvas = document.getElementById('canvas'),
context = canvas.getContext('2d'),
ball = new Ball(),
vx = Math.random() * 10 - 5,
vy = Math.random() * 10 - 5,
bounce = -0.7,
gravity = 0.1;
ball.x = canvas.width / 2;
ball.y = canvas.height / 2;
(function drawFrame () {
window.requestAnimationFrame(drawFrame, canvas);
context.clearRect(0, 0, canvas.width, canvas.height);
var left = 0,
right = canvas.width,
top = 0,
bottom = canvas.height;
vy += gravity;
ball.x += vx;
ball.y += vy;
if (ball.x + ball.radius > right) {
ball.x = right - ball.radius;
vx *= bounce;
} else if (ball.x - ball.radius < left) {
ball.x = left + ball.radius;
vx *= bounce;
}
if (ball.y + ball.radius > bottom) {
ball.y = bottom - ball.radius;
vy *= bounce;
} else if (ball.y - ball.radius < top) {
ball.y = top + ball.radius;
vy *= bounce;
}
ball.draw(context);
}());
};
</script>
</body>
</html>
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