问题

先前在项目中用d3画了树图等svg图形，然后需求是需要以png格式上传这个图片到服务器保存以供其他地方调用，可以先生成svg码流，然后在canvas上绘图，最后在利用canvas.toDataUrl()生成png码流，然后上传到服务器，但是上传后发现一个问题就是png码流生成的图片丢失了canvas的背景色，背景色变为了透明的。

问题原因

查询问题后在canvas草案中发现原因是：

For image types that do not support an alpha channel, the image must be composited onto a solid black background using the source-over operator, and the resulting image must be the one used to create the data: URL.

当你利用toDataUrl输出的码流生成图片时候，根据图片格式不同会是透明或者黑色。

解决方案

直接上代码：

//Returns contents of a canvas as a png based data url, with the specified
//background color
function canvasToImage(backgroundColor)
{
    //cache height and width        
    var w = canvas.width;
    var h = canvas.height;

    var data;       

    if(backgroundColor)
    {
        //get the current ImageData for the canvas.
        data = context.getImageData(0, 0, w, h);

        //store the current globalCompositeOperation
        var compositeOperation = context.globalCompositeOperation;

        //set to draw behind current content
        context.globalCompositeOperation = "destination-over";

        //set background color
        context.fillStyle = backgroundColor;

        //draw background / rect on entire canvas
        context.fillRect(0,0,w,h);
    }

    //get the image data from the canvas
    var imageData = this.canvas.toDataURL("image/png");

    if(backgroundColor)
    {
        //clear the canvas
        context.clearRect (0,0,w,h);

        //restore it with original / cached ImageData
        context.putImageData(data, 0,0);        

        //reset the globalCompositeOperation to what it was
        context.globalCompositeOperation = compositeOperation;
    }

    //return the Base64 encoded data url string
    return imageData;
}

主要几个步骤

1. 从canvas中得到ImageData
2. 将globalCompositeOperation属性设置为destination-over. 然后将会在当前图形之下绘制新的图形
3. 画一个rectangle填充你想要的背景色
4. 此时再生成码流
5. 后面就是清除canvas恢复到最初状态即可

参考资料

1. 解决方案
2. MDN