Javascript 如何在画布中绘制imageUrl？

我将html5

canvas

与

signar

一起使用。 我通过这个得到了画布的Url

var refreshCanvas = canvasParent.toDataURL();
whiteBoard.server.refreshCanvas(refreshCanvas);

我还通过以下方式获取客户端数据，作为url字符串：

whiteBoard.client.refreshCanvas = function (refreshCanvas){
    //alert(refreshCanvas);
    var img = new Image;
    img.onload = function () {
        contextParent.drawImage(img, 0, 0);
    };
    img.src = refreshCanvas;
}

我已经检查过imagedata Url是否以字符串形式出现，但我的问题是如何将imageUrl数据绘制到画布上

我的画布名称是“canvasParent”，上下文名称是“contextParent”

我得到的图像Url为：

data:image/png;base64,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