var saveAsLocalImage = function () { 
    var canvas = document.getElementById("theCanvasId"); 
    //直接改图片的mimeType，强制改成steam流类型的。比如"image/octet-stream"
    var image = canvas.toDataURL("image/png").replace("image/png", "image/octet-stream"); 
    window.location.href = image;
}