实例html5-canvas中实现背景图片的移动

Posted 2020-08-04 网络虫

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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

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<!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>

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