【实例】html5-canvas中实现背景图片的移动
本例来自于《HTML5 Canvas核心技术 图形、动画与游戏开发》
在线演示 (图有点多,请多刷新几次)
本例还有一点代码不理解,我用注释和问号标注出来了,有大神看到求解答,谢谢
本例子难点主要在通过当前的FPS计算图像下一帧的显示坐标,这也是我不理解的地方
还有就是requestAnimationFrame这个,这个是用来以浏览器最合适的方式循环执行一些代码
- <!DOCTYPE html>
- <html>
- <head>
- <meta charset="utf-8">
- <title>图片运动实例</title>
- <style type="text/css">
- body {
- background: #DDDDDD;
- }
- #canvas {
- border: thin solid black;
- }
- input {
- margin-left: 15px;
- }
- </style>
- </head>
- <body>
- <canvas id="canvas" width="1000" height="440">
- 您的浏览器不支持canvas,请更新到最新的浏览器
- </canvas>
- <input type="button" name="animateButton" id="animateButton" value="运动" />
- <hr />
- <div>
- <table border="1" cellspacing="" cellpadding="">
- <tr>
- <td>FPS</td>
- <td><span id="FPS"></span></td>
- </tr>
- <tr>
- <td>SKY_VELOCITY/fps</td>
- <td><span id="SKY_VELOCITY"></span></td>
- </tr>
- <tr>
- <td>GRASS_VELOCITY/fps</td>
- <td><span id="GRASS_VELOCITY"></span></td>
- </tr>
- <tr>
- <td>TREE_VELOCITY/fps</td>
- <td><span id="TREE_VELOCITY"></span></td>
- </tr>
- <tr>
- <td>FAST_TREE_VELOCITY/fps</td>
- <td><span id="FAST_TREE_VELOCITY"></span></td>
- </tr>
- </table>
- </div>
- </body>
- <script type="text/javascript">
- var canvas = document.getElementById("canvas"),
- context = canvas.getContext("2d"),
- animateButton = document.getElementById("animateButton"),
- //创建多个图像对象
- sky = new Image(),
- tree = new Image(),
- nearTree = new Image(),
- grass = new Image(),
- grass2 = new Image(),
- redRect = new Image,
- paused = true,
- lastTime = 0,
- fps = 0, //当前的帧速率
- //都是不同图像
- skyOffset = 0,
- grassOffset = 0,
- treeOffset = 0,
- nearTreeOffset = 0,
- TREE_VELOCITY = 20,
- FAST_TREE_VELOCITY = 40,
- SKY_VELOCITY = 8,
- GRASS_VELOCITY = 75;
- //以下是检测对象
- var SKY_VELOCITYFps = document.getElementById("SKY_VELOCITY");
- var GRASS_VELOCITYFps = document.getElementById("GRASS_VELOCITY");
- var TREE_VELOCITYFps = document.getElementById("TREE_VELOCITY");
- var FAST_TREE_VELOCITYFps = document.getElementById("FAST_TREE_VELOCITY");
- var FPS = document.getElementById("FPS");
- //擦除方法
- function erase() {
- context.clearRect(0, 0, canvas.width, canvas.height);
- }
- function draw() {
- context.save();
- //fps是共用的
- //应该是确定当前图像位置的坐标X
- //???以下四行还是有点不太理解,只知道这个可以用来循环的显示image对象,至于SKY_VELOCITY/fps是什么还不清楚
- skyOffset = skyOffset < canvas.width ? skyOffset + SKY_VELOCITY/fps : 0;
- grassOffset = grassOffset < canvas.width ? grassOffset + GRASS_VELOCITY/fps : 0;
- treeOffset = treeOffset < canvas.width ? treeOffset + TREE_VELOCITY/fps : 0;
- nearTreeOffset = nearTree < canvas.width ? nearTree + FAST_TREE_VELOCITY/fps : 0;
- //检测数据
- SKY_VELOCITYFps.innerHTML = SKY_VELOCITY/fps;
- GRASS_VELOCITYFps.innerHTML = GRASS_VELOCITY/fps;
- TREE_VELOCITYFps.innerHTML = TREE_VELOCITY/fps;
- FAST_TREE_VELOCITYFps.innerHTML = FAST_TREE_VELOCITY/fps;
- FPS.innerHTML = fps;
- //像下面几行这样写的后果:canvas运动到后面的时候就可能没有图像对象了
- // skyOffset += SKY_VELOCITY/fps;
- // grassOffset += GRASS_VELOCITY/fps;
- // treeOffset += TREE_VELOCITY/fps;
- // nearTreeOffset += FAST_TREE_VELOCITY/fps;
- //开始绘制天空
- context.save();
- //改变坐标原点
- context.translate(-skyOffset, 0);
- //绘制一个天空
- context.drawImage(sky, 0, 0);
- //在即将天空结尾的时候在绘制一个天空用来衔接
- context.drawImage(sky, sky.width-2, 0);
- //将改变的坐标原点改回来
- context.restore();
- //开始绘制远处的树
- //此处绘制的位置有的地方出现在canvas.width之外
- context.save();
- //改变坐标原点
- context.translate(-treeOffset, 0);
- //以改变原点后的坐标上绘制好多个树
- context.drawImage(tree, 100, 240);
- context.drawImage(tree, 1100, 240);
- context.drawImage(tree, 400, 240);
- context.drawImage(tree, 1400, 240);
- context.drawImage(tree, 700, 240);
- context.drawImage(tree, 1700, 240);
- //恢复坐标原点
- context.restore();
- //绘制近处的树木,他的运行速度更加的块
- context.save();
- context.translate(-nearTreeOffset, 0);
- context.drawImage(nearTree, 250, 220);
- context.drawImage(nearTree, 1250, 220);
- context.drawImage(nearTree, 800, 220);
- context.drawImage(nearTree, 1800, 220);
- context.restore();
- //绘制绿地
- context.save();
- context.translate(-grassOffset, 0);
- //在canvas底部绘制草地image对象
- context.drawImage(grass, 0, canvas.height-grass.height);
- context.drawImage(grass, grass.width, canvas.height-grass.height);
- context.drawImage(grass2, 0, canvas.height-grass2.height);
- context.drawImage(grass2, grass2.width, canvas.height-grass2.height);
- context.restore();
- }
- //计算当前的FPS
- function calculateFps(now) {
- //1000毫秒除以距离上一次计算FPS的时间意思是,或者距离上一次绘制图像
- //这段时间里面执行了多少次
- //除以1000是因为now和lastTime都是以毫秒为单位的
- //反正fps就是一秒内能够执行绘制多少次嘛~所以的
- var fps = 1000 / (now - lastTime);
- lastTime = now;
- return fps;
- }
- function animate(now) {
- //如果当前时间还没有定义
- if(now === undefined) {
- //就把当前时间赋值给now
- now = +new Date;
- }
- fps = calculateFps(now);
- //只要不是暂停状态就执行
- if(!paused) {
- //擦除canvas
- //原例子中是用兼容的手法实现requestAnimationFrame,所以需要erase
- // erase();
- //马上绘制新的
- draw();
- }
- //以最适合的速度执行animate中的内容,这个比较适合动画的处理,类似于setTimeout
- requestAnimationFrame(animate);
- }
- animateButton.onclick = function(e) {
- paused = paused ? false : true;
- if(paused) {
- animateButton.value = "运动";
- } else {
- animateButton.value = "暂停";
- }
- };
- // canvas.width = canvas.width;
- // canvas.height = canvas.height;
- sky.src = "sky.png";
- tree.src = "smalltree.png";
- nearTree.src = "tree-twotrunks.png";
- grass.src = "grass.png";
- grass2.src = "grass2.png";
- sky.onload = function(e) {
- draw();
- };
- // requestAnimationFrame(animate);
- //启动
- animate();
- </script>
- </html>