html5 canvas 图像处理

1.图像放大缩小

 <script>
        var cvs = document.getElementById("canvas");
        cvs.width = 320
        cvs.height = 213;
        var context = cvs.getContext("2d");
        var range = document.getElementById("range");
        var img = new Image();
        window.onload = function () {
            img.src = "imgs/back.jpg";
            img.onload = function () {
                drawImg(1);
            }
        }
        function drawImg(scale) {
            context.clearRect(0, 0, cvs.width, cvs.height);
            var w = scale * img.width;
            var h = scale * img.height;
            var dx = cvs.width / 2 - w / 2;
            var dy = cvs.height / 2 - h / 2;
            context.drawImage(img, dx, dy, w, h);
        }
        range.onmousemove = function () {
            var v = this.value;
            drawImg(v);
        }
    </script>

 <canvas id="canvas"></canvas>
    
    <input id="range" type="range" min="0.5" max="3.0" value="1.0" style="600px;" step="0.1" />

上面是通过滑动来放大缩小图像的，在canvas上使用 dragImage()，可以把一张图像绘制到画布上。

drawImage不仅能加载图片，还能加载canvas:

2.对图像加水印

可以在一个画布上加载另一个画布的方法来对图片加水印

<canvas id="canvas"></canvas>
    <input id="range" type="range" min="0.5" max="3.0" value="1.0" style="600px;" step="0.1" />
    <canvas id="canvas1" style="display:none"></canvas>

 <script>
        var cvs = document.getElementById("canvas");
        cvs.width = 600
        cvs.height = 600;
        var context = cvs.getContext("2d");
        var range = document.getElementById("range");

        var cvs1 = document.getElementById("canvas1");
        cvs1.width = 600;
        cvs1.height = 100;

        context1 = cvs1.getContext("2d");
        var img = new Image();
        window.onload = function () {
            img.src = "images/1-1.jpg";
            img.onload = function () {
                drawImg(1);
            }

            context1.font = "bold 50px Arial";
            context1.fillStyle = "rgba(255,255,255,0.5)";
            context1.textBaseline = "middle";
            context1.fillText("==www.sina.mtn==", 20, 50);

        }
        function drawImg(scale) {
            context.clearRect(0, 0, cvs.width, cvs.height);
            var w = scale * img.width;
            var h = scale * img.height;
            var dx = cvs.width / 2 - w / 2;
            var dy = cvs.height / 2 - h / 2;
            context.drawImage(img, dx, dy, w, h);
            context.drawImage(canvas1, cvs.width - cvs1.width, cvs.height - cvs1.height);
        }
        range.onmousemove = function () {
            var v = this.value;
            drawImg(v);
        }
    </script>

3.图片复制：从画布A得到画布图像，放到画布B上

 　　var dirImg= ctxA.getImageData(0, 0, cvsA.width, cvsA.height);
 

　　ctxB.putImageData(dirImg, 0, 0);

putImageData还有四个参数，可以参考下图：