How to convert System.Windows.Media.DrawingImage into Stream?

Question

I'am trying convert DrawingImage into MemoryStream. My code looks like this:

public MemoryStream ImageStream(DrawingImage drawingImage)
    {
        MemoryStream stream = new MemoryStream();
        ImageSource imageSource = drawingImage;
        if (imageSource != null)
        {
            BitmapSource bitmap = imageSource as BitmapSource;
            if (bitmap != null)
            {
                BitmapEncoder encoder = new PngBitmapEncoder();
                encoder.Frames.Add(BitmapFrame.Create(bitmap));
                encoder.Save(stream);
            }     
        }

        return stream;
    }

But problem is after casting ImageSource into BitmapSource bitmap is always null. Any sugestion how to fix that?

Answer 1

The reason your bitmap variable is always null is because DrawingImage does not extend BitmapImage or vice-versa, so the cast is guaranteed to fail. A DrawingImage does not contain any pixel data of any kind. It references a Drawing that is used whenever the image needs to be rasterized.

How did you find yourself in a situation where you want to rasterize a DrawingImage and serialize it into a stream? I get the feeling you are going about something in an unusual way if you have need of a function like this.

Nevertheless, you could implement this function by drawing the DrawingImage to a DrawingVisual, rendering it to a RenderTargetBitmap, and then passing the render target to the encoder to serialize the raster data to a stream.

public MemoryStream ImageStream(DrawingImage drawingImage)
{
    DrawingVisual visual = new DrawingVisual();
    using (DrawingContext dc = visual.RenderOpen())
    {
        dc.DrawDrawing(drawingImage.Drawing);
        dc.Close();
    }
    RenderTargetBitmap target = new RenderTargetBitmap((int)visual.Drawing.Bounds.Right, (int)visual.Drawing.Bounds.Bottom, 96.0, 96.0, PixelFormats.Pbgra32);
    target.Render(visual);

    MemoryStream stream = new MemoryStream();
    BitmapEncoder encoder = new PngBitmapEncoder();
    encoder.Frames.Add(BitmapFrame.Create(target));
    encoder.Save(stream);

    return stream;
}

If you want something a little more generic, I would split this into two methods and change some of the types.

public BitmapSource Rasterize(Drawing drawing)
{
    DrawingVisual visual = new DrawingVisual();
    using (DrawingContext dc = visual.RenderOpen())
    {
        dc.DrawDrawing(drawing);
        dc.Close();
    }
    RenderTargetBitmap target = new RenderTargetBitmap((int)drawing.Bounds.Right, (int)drawing.Bounds.Bottom, 96.0, 96.0, PixelFormats.Pbgra32);
    target.Render(visual);

    return target;
}

public void SavePng(BitmapSource source, Stream target)
{
    BitmapEncoder encoder = new PngBitmapEncoder();
    encoder.Frames.Add(BitmapFrame.Create(source));
    encoder.Save(target);
}

Then you could use it with any kind of stream. For example, to save the drawing to a file:

using (FileStream file = File.Create("somepath.png"))
{
    SavePng(Rasterize(drawingImage.Drawing), file);
}