getting a DrawingContext for a wpf WriteableBitmap

Question

Is there a way to get a DrawingContext (or something similar) for a WriteableBitmap? I.e. something to allow you to call simple DrawLine/DrawRectangle/etc kinds of methods, rather than manipulate the raw pixels directly.

Answer 1

I'm wondering the same thing, as currently I do something like:

DrawingVisual drawingVisual = new DrawingVisual();
using (DrawingContext drawingContext = drawingVisual.RenderOpen())
{
   //
   // ... draw on the drawingContext
   //
   RenderTargetBitmap bmp = new RenderTargetBitmap(width, height, dpi, dpi, PixelFormats.Default);
   bmp.Render(drawingVisual);
   image.Source = bmp;
}

I'm trying to use the WriteableBitmap to allow multithreaded access to the pixel buffer, which is currently not allowed with neither a DrawingContext nor a RenderTargetBitmap. Maybe some sort of WritePixels routine based off of what you've retrieved from the RenderTargetBitmap would work?

Answer 2

It appears the official word is no.

Still, happy to hear if anyone has any non-wpf alternatives for simple bitmap operations like DrawLine and friends.

Answer 3

If you don't mind using System.Drawing you could do something like:

var wb = new WriteableBitmap( width, height, dpi, dpi, 
                              PixelFormats.Pbgra32, null );
wb.Lock();
var bmp = new System.Drawing.Bitmap( wb.PixelWidth, wb.PixelHeight,
                                     wb.BackBufferStride, 
                                     PixelFormat.Format32bppPArgb, 
                                     wb.BackBuffer );
var g = new System.Drawing.Graphics.FromImage( bmp ); // Good old Graphics

g.DrawLine( ... ); // etc...

// ...and finally:
g.Dispose(); 
bmp.Dispose();
wb.AddDirtyRect( ... );
wb.Unlock();

Answer 4

I found sixlettervariables' solution the most workable one. However, there's a "drawingContext.Close()" missing. According to MSDN, "A DrawingContext must be closed before its content can be rendered". The result is the following utility function:

public static BitmapSource CreateBitmap(
    int width, int height, double dpi, Action<DrawingContext> render)
{
    DrawingVisual drawingVisual = new DrawingVisual();
    using (DrawingContext drawingContext = drawingVisual.RenderOpen())
    {
        render(drawingContext);
    }
    RenderTargetBitmap bitmap = new RenderTargetBitmap(
        width, height, dpi, dpi, PixelFormats.Default);
    bitmap.Render(drawingVisual);

    return bitmap;
}

This can then easily be used like this:

BitmapSource image = ImageTools.CreateBitmap(
    320, 240, 96,
    drawingContext =>
    {
        drawingContext.DrawRectangle(
            Brushes.Green, null, new Rect(50, 50, 200, 100));
        drawingContext.DrawLine(
            new Pen(Brushes.White, 2), new Point(0, 0), new Point(320, 240));
    });