Rendering a WPF canvas as a specificly sized bitmap

Question

I have a WPF Canvas that I want to make a bitmap of.

Specifically, I want to render it actual size on a 300dpi bitmap. The "actual size" of the objects on the canvas is 10 device independent pixels = 1" in real life. Theoretically, WPF device independent pixels are 96dpi.

I've spent days trying to get this to work and am coming up flummoxed.

My understanding is that the general procedure is roughly:

var masterBitmap = new RenderTargetBitmap((int)(canvas.ActualWidth * ?SomeFactor?),
                                          (int)(canvas.ActualHeight * ?SomeFactor?),
                                          BitmapDpi, BitmapDpi, PixelFormats.Default);
masterBitmap.Render(canvas);

and that I need to set the canvas's LayoutTransform to a ScaleTransform of ?SomeOtherFactor? and then do a measure and arrange of the canvas to ?SomeDesiredSize?

What I am stuck on is what to use for the values of ?SomeFactor?, ?SomeOtherFactor? and ?SomeDesiredSize? to make this work. MSDN documentation gives no indication of what factors to use.

score 0 · Answer 1 · answered Dec 03 '12 at 20:40

I use this code to display images with 1:1 pixel accuracy.

double dpiXFactor, dpiYFactor;  
Matrix m = PresentationSource.FromVisual(Application.Current.MainWindow).CompositionTarget.TransformToDevice;
if (m.M11 > 0 && m.M22 > 0)
{
    dpiXFactor = m.M11;
    dpiYFactor = m.M22;
}
else
{
    // Sometimes this can return a matrix with 0s.
    // Fall back to assuming normal DPI in this case.
    dpiXFactor = 1;
    dpiYFactor = 1;
}

double width = widthPixels / dpiXFactor;
double height = heightPixels / dpiYFactor;

Don't forget to enable UseLayoutRounding on the control as well.

Rendering a WPF canvas as a specificly sized bitmap

1 Answers1