HTML5_canvas_图片加载_双缓冲

HTML5_canvas_图片加载_双缓冲_跳帧闪烁问题

canvas 图片加载

pen.drawImage(ele, showX, showY, imgWidth, imgHeight);

ele 将 img 元素加载到画布上

步骤

1. 创建一个 <img> 对象

var imgNode = new Image();

imgNode.src = "./img/bird.png";

2. 等待图片加载完成，再进行下一步操作

imgNode.onload = function(){

3. 图片显示到画布上

pen.drawImage(imgNode, 0, 0, 100, 100);

};

跳帧闪烁问题

闪烁的原因：
- 清空了画布，然后加载图片，再等图片加载完，最后画下一帧。
- 这个空白延迟，主要是因为等待图片加载完成时间太久

解法1：

在加载图片之前，不清空上一帧图像
加载完成后，再清空画布，最后画下一帧。

<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title>帧闪烁解决</title>
        
        <style type="text/css">
            * {
                margin: 0;
                padding: 0;
            }
            
            body{
                text-align: center;
            }
            
            #myCanvas{
                border: 1px solid ;
            }
        </style>
    </head>
    
    <body>
        <canvas id="myCanvas" width="800" height="400"></canvas>
        
        
        
        <!-- javascript 代码 -->
        <script type="text/javascript">
            window.onload = function () {
                var myCanvas = document.getElementById('myCanvas');
                
                var painting = myCanvas.getContext('2d');
                
                var num = 0;
                var speed = 0;
                setInterval(function(){
                    speed += 20;
                    if(speed > myCanvas.width){
                        speed = -150
                        //                speed = 0
                    };
                    
                    num++;
                    if(num > 8){
                        num = 1;
                    };
                    
                    painting.beginPath();
                    
                    //1.img对象
                    var imgNode = new Image();
                    
                    //2.img对象 设置 src
                    imgNode.src = 'img/q_r' + num + '.jpg';
                    
                    //3.等图片加载完成后再去设置图片显示
                    imgNode.onload = function () {
                        //4.图片显示在画布上
                        painting.clearRect(0, 0, myCanvas.width, myCanvas.height)
                        painting.drawImage(imgNode, speed, 150, 150, 90);
                    };
                }, 340);
            };
        </script>
    </body>
</html>

飞鸟案例（双缓冲，解决跳帧闪烁问题）

<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8" />
        <title>canvas 双缓冲案例</title>

        <style type="text/css">
            body {
                width: 100%;
                color: #000;
                background: #96b377;
                font: 14px Helvetica, Arial, sans-serif;
            }
            
            .wrap {
                display: flex;
                flex-direction: column;
                justify-content: center;
                align-items: center;
            }
        </style>
    </head>
    
    <body>
        
        
        <div id="box" class="wrap">
        </div>
        
        
        
        
        <!-- javascript 代码 -->
        <script type="text/javascript">
            //                 创建 画布的width  画布的height  背景颜色 父元素
            function createCanvasTo(canvasWidth, canvasHeight, bgColor, parentObj){
                var myCanvas = document.createElement("canvas");
                myCanvas.width = canvasWidth;
                myCanvas.height = canvasHeight;
                myCanvas.innerText = " 您的浏览器不支持 canvas，建议更新或者更换浏览器。";
                if(bgColor){
                    myCanvas.style.backgroundColor = bgColor;
                };
                if(parentObj){
                    parentObj.appendChild(myCanvas);
                };
                
                return myCanvas;
            };
            
            var box = document.getElementById("box");
            
            var topCanvas = createCanvasTo(600, 83, "#fff", box);
            movingPic(topCanvas, "./img/q_r", "jpg", -150, 0, 150, 83);
            
            var bottomCanvas = createCanvasTo(600, 300, "#fff", box);
            movingPic(bottomCanvas, "./img/walking", "png", -130, 27, 130, 246);
            
            
            //                 画布对象 图片路径 图片类型 起始x 起始y 图片width 图片height
            function movingPic(theCanvas, imgPath, imgType, posX, posY, imgWidth, imgHeight){
                var cacheCanvas = document.createElement("canvas");
                cacheCanvas.width = theCanvas.width;
                cacheCanvas.height = theCanvas.height;
                var cachePen = cacheCanvas.getContext("2d");
                
                var num = 1;
                var pos = 0;
                window.setInterval(function(){
                    pen = theCanvas.getContext("2d");    // 坑1: 一定要放在循环里面
                    pen.clearRect(0, 0, theCanvas.width, theCanvas.height);
                    
                    // 图形位移变换
                    num++;
                    if(num > 8){
                        num = 1;
                    };
                    
                    pos += 15;
                    if(posX+pos > theCanvas.width){
                        pos = -130;
                    };
                    
                    // 双缓冲绘制    先画到临时 canvas
                    cachePen.save();
                    cachePen.beginPath();
                    var imgObj = new Image();
                    imgObj.src = imgPath+num+"."+imgType;
                    imgObj.onload = function(){
                        cachePen.drawImage(imgObj, posX+pos, posY, imgWidth,imgHeight);
                    };
                    cachePen.restore();
                    
                    // 再转到正式 canvas
                    pen.save();
                    pen.drawImage(cacheCanvas, 0, 0, cacheCanvas.width, cacheCanvas.height);
                    pen.restore();
                    
                    // 坑2: 一定要在 取走缓冲内容后 再清除缓冲
                    cachePen.clearRect(0, 0, cacheCanvas.width, cacheCanvas.height);
                }, 100);
            };
        </script>
    </body>
</html>

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