html5 – 如何将画布的一部分捕获到位图

例

var canvas,ctx,img,imgd,col;
window.onload = function () {
    canvas = document.getElementById('canvas');
    ctx = canvas.getContext('2d');
    col = {
        0: '#000',.25: '#0099f9',.55: '#03f',1: '#000'
    };
    img = document.getElementById('img');
    var w = h = canvas.width = canvas.height = 200;

    var grad = ctx.createRadialGradient(w / 2,w / 2,h);
    for (var i in col) {
        grad.addColorStop(i,col[i]); //just a funny mess,please don't bother :)
    }

    ctx.fillStyle = grad;
    ctx.fillRect(0,w,h);
    imgd = ctx.getImageData(50,50,20,20); //caching the image Data

}

function draw() {
    ctx.drawImage(img,20); //drawing Image on to canvas
}

function redraw() {
    ctx.putImageData(imgd,20); //redraw the cached Image Data
}

和一个简单的在线示例,只有你:)

Try it Here