Taking bitmap of Canvas is incorrectly based on top left corner of Window

I have a canvas in my Window and occasionally I want to take snapshots of the content.

I have the following method to do so:

private PngBitmapEncoder captureVisual(Visual v)
{
    RenderTargetBitmap bmp = new RenderTargetBitmap(546, 410, 120, 96, PixelFormats.Pbgra32);
    bmp.Render(v);
    PngBitmapEncoder encoder = new PngBitmapEncoder();
    encoder.Interlace = Png开发者_JS百科InterlaceOption.On;
    encoder.Frames.Add(BitmapFrame.Create(bmp));
    return encoder;
}

I call this method with the following method which is called by a press button action(the idea is to add the picture to another canvas which will display the "photo":

 private void take_photo(Object sender, RoutedEventArgs e)
        {
            System.Windows.Controls.Image photoImg = new System.Windows.Controls.Image();
            BitmapEncoder enc = captureVisual(videoCanvas);
            photoImg.Source = enc.Frames[0];
            photoCanvas.Children.Add(photoImg);
        }

The problem:

When I take the photo the origin of the photo/bitmap is not the origin of the canvas but the origin of the window. So the Canvas appears in the bitmap translated towards bottom/right in the same position as it is in relation to the top-left corner of the window. I have no idea why this is happening.

As an example see the pic below: There is white space to the top left and the actual content is translated to the bottom right. The origin should be where the actual image is located. I will try to put around a border to make it easier to visualize the problem(sorry I don't know how to put a border around the image if anyone knows please tell me or edit it yourself. Thanks).

---

Thanks to the comment of dowhilefor I got the answer from this link:

The problem is WPF implicitly adds the margins of the Canvas in relation to the parent container. In my case the parent container was a Grid and I didn't define any margins, the canvas was filling a specific cell. Nevertheless this somehow gets incorporated in the bitmap.

The solution, wrap the Canvas around another Canvas. That's stupid but it works. IMHO this is a bug in WPF:

<Canvas 
                Name="outerCanvas"
                HorizontalAlignment="Stretch"
                VerticalAlignment="Stretch"
                Grid.Row="1" Grid.Column="1">
                    <Canvas 
                        Name="videoCanvas"
                        Canvas.Left="0"
                        Canvas.Top="0"
                        HorizontalAlignment="Stretch"
                        VerticalAlignment="Stretch"
                        />
            </Canvas>