HTML5 Canvas draw方法制作動畫效果示例
編輯：HTML5教程
HTML5 Canvas動畫效果演示
主要思想：
首先要准備一張有連續幀的圖片，然後利用HTML5 Canvas的draw方法在不同的時間間隔繪制不同的幀，這樣看起來就像動畫在播放。
關鍵技術點：
JavaScript 函數setTimeout()有兩個參數，第一個是參數可以傳遞一個JavaScript方法，
另外一個參數代表間隔時間，單位為毫秒數。代碼示例：
setTimeout( update, 1000/30);
Canvas的API-drawImage()方法，需要指定全部9個參數：
ctx.drawImage(myImage, offw, offh, width,height, x2, y2, width, height);
其中offw, offh是指源圖像的起始坐標點，width, height表示源圖像的寬與高，x2,y2表
示源圖像在目標Canvas上的起始坐標點。
一個22幀的大雁飛行圖片實現的效果：
源圖像：
程序代碼：
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="X-UA-Compatible" content="chrome=IE8">
<meta http-equiv="Content-type" content="text/html;charset=UTF-8">
<title>Canvas Mouse Event Demo</title>
<link href="default.css" rel="stylesheet" />
<script>
var ctx = null; // global variable 2d context
var started = false;
var mText_canvas = null;
var x = 0, y =0;
var frame = 0; // 22 5*5 + 2
var imageReady = false;
var myImage = null;
var px = 300;
var py = 300;
var x2 = 300;
var y2 = 0;
window.onload = function() {
var canvas = document.getElementById("animation_canvas");
console.log(canvas.parentNode.clientWidth);
canvas.width = canvas.parentNode.clientWidth;
canvas.height = canvas.parentNode.clientHeight;
if (!canvas.getContext) {
console.log("Canvas not supported. Please install a HTML5 compatible browser.");
return;
}
// get 2D context of canvas and draw rectangel
ctx = canvas.getContext("2d");
ctx.fillStyle="black";
ctx.fillRect(0, 0, canvas.width, canvas.height);
myImage = document.createElement('img');
myImage.src = "../robin.png";
myImage.onload = loaded();
}
function loaded() {
imageReady = true;
setTimeout( update, 1000/30);
}
function redraw() {
ctx.clearRect(0, 0, 460, 460)
ctx.fillStyle="black";
ctx.fillRect(0, 0, 460, 460);
// find the index of frames in image
var height = myImage.naturalHeight/5;
var width = myImage.naturalWidth/5;
var row = Math.floor(frame / 5);
var col = frame - row * 5;
var offw = col * width;
var offh = row * height;
// first robin
px = px - 5;
py = py - 5;
if(px < -50) {
px = 300;
}
if(py < -50) {
py = 300;
}
//var rate = (frame+1) /22;
//var rw = Math.floor(rate * width);
//var rh = Math.floor(rate * height);
ctx.drawImage(myImage, offw, offh, width, height, px, py, width, height);
// second robin
x2 = x2 - 5;
y2 = y2 + 5;
if(x2 < -50) {
x2 = 300;
y2 = 0;
}
ctx.drawImage(myImage, offw, offh, width, height, x2, y2, width, height);
}
function update() {
redraw();
frame++;
if (frame >= 22) frame = 0;
setTimeout( update, 1000/30);
}
</script>
</head>
<body>
<h1>HTML Canvas Animations Demo - By Gloomy Fish</h1>
<pre>Play Animations</pre>
<div id="my_painter">
<canvas id="animation_canvas"></canvas>
</div>
</body>
</html>
發現上傳透明PNG格式有點問題，所以我上傳不透明的圖片。可以用其它圖片替換，替換以後請修改最大幀數從22到你的實際幀數即可運行。
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