HTML5 Canvas双缓存实例
转自：http://www.108js.com/article/article3/30046.html?id=255
下面是用HTML5的<canvas>标签写的一个视差滚动动画的示例。采用了制作动画或者游戏编程中常用的双缓冲技术：获取到页面中的Canvas对象之后，创建了一个与页面Canvas同样大小的Canvas对象。绘图时先将图像绘制到缓冲Canvas中，等到每一桢的图像绘制完全后在把整个缓冲Canvas绘制到页面Canvas中。前景、中景、远景的视差通过控制移动速度来实现。整个动画的绘制部分只使用了Context对象的drawImage()方法。
<html>
<body>
<canvas id="canvas" width="600" height="400">
<p>Your browser does not support the canvas element</p>
</canvas>
</body>
<script>
const FPS = 30;
const SECONDS_BETWEEN_FRAMES = 1 / FPS;
var bg0 = new Image();
var bg1 = new Image();
var bg2 = new Image();
var x = 0;
const RATE = 50 * SECONDS_BETWEEN_FRAMES;
const WIDTH = 600;
const HEIGHT = 320;
var canvas;
var canvasBuffer;
var context;
var contextBuffer;
window.onload = init;
function init() {
bg0.src = "b0.png";
bg1.src = "b1.png";
bg2.src = "b2.png";
canvas = document.getElementById("canvas");
canvasBuffer = document.createElement("canvas");
canvasBuffer.width = canvas.width;
canvasBuffer.height = canvas.height;
context = canvas.getContext("2d");
contextBuffer = canvasBuffer.getContext("2d");
context.clearRect(0, 0, canvas.width, canvas.height)
contextBuffer.clearRect(0, 0, canvasBuffer.width, canvasBuffer.height);
setInterval(animation, SECONDS_BETWEEN_FRAMES);
}
function animation() {
x += RATE;
context.clearRect(0, 0, canvas.width, canvas.height)
contextBuffer.clearRect(0, 0, canvasBuffer.width, canvasBuffer.height);
drawBuffer(bg0, 0, 0, 0.5);
drawBuffer(bg1, 0, 100, 0.75);
drawBuffer(bg2, 0, 100, 1);
context.drawImage(canvasBuffer, 0, 0);
}
function drawBuffer(image, dx, dy, factor) {
var left = (x * factor) % image.width;
if (left + WIDTH >= image.width) {
var d0 = image.width - left;
var d1 = WIDTH - d0;
contextBuffer.drawImage(image, left, 0, d0, HEIGHT, dx, dy, d0, HEIGHT);
contextBuffer.drawImage(image, 0, 0, d1, HEIGHT, dx + d0, dy, d1, HEIGHT);
}
else {
contextBuffer.drawImage(image, left, 0, WIDTH, HEIGHT, dx, dy, WIDTH, HEIGHT);
}
}</script>
</html>
顺带一提的是在Canvas画布中绘制图形，一般情况下后绘制的会覆盖先绘制的，所以在绘制图像的时候需要先绘制远景b0，再绘制中景b1，最后绘制近景b2。
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