canvas image 相互转化

// 从 canvas 提取图片 image
function convertCanvasToImage(canvas) {
    //新Image对象，可以理解为DOM
    var image = new Image();
    // canvas.toDataURL 返回一串Base64编码的URL
    image.src = canvas.toDataURL("image/png");
    return image;
}
// 把image 转换为 canvas对象
function convertImageToCanvas(image) {
    // 创建canvas DOM元素，并设置其宽高和图片一样
    var canvas = document.createElement("canvas");
    canvas.width = image.width;
    canvas.height = image.height;
    canvas.getContext("2d").drawImage(image, 0, 0);
    return canvas;
}