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

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);
}