// 從 canvas 提取圖片 image
function convertCanvasToImage(canvas) {
    //新Image對象，可以理解為DOM
    var image = new Image();
    // canvas.toDataURL 返回一串Base64編碼的URL
    image.src = canvas.toDataURL("image/png");
    return image;
}
// 把image 轉(zhuǎn)換為 canvas對象
function convertImageToCanvas(image) {
    // 創(chuàng)建canvas DOM元素，并設(shè)置其寬高和圖片一樣
    var canvas = document.createElement("canvas");
    canvas.width = image.width;
    canvas.height = image.height;
    canvas.getContext("2d").drawImage(image, 0, 0);
    return canvas;
}