WPF 300DPI High Resoluation Printing

            PrintDialog dlg = new PrintDialog();
            GeneralTransform transformToRoot = this.TransformToAncestor(win_root);

            Rect rect = new Rect(transformToRoot.Transform(new Point(0, 0)), transformToRoot.Transform(new Point(this.ActualWidth, this.ActualHeight)));

            double scale = 300 / 96;

            RenderTargetBitmap bitmap = new RenderTargetBitmap((int)(scale * (rect.Width + 1)), (int)(scale * (rect.Height + 1)),  scale * 96, scale * 96, PixelFormats.Default);
            bitmap.Render(g); // g=grid

            Image img = new Image();
            img.Source = bitmap;
            img.Stretch = Stretch.None;

            img.Measure(new Size(dlg.PrintableAreaWidth,
                                 dlg.PrintableAreaHeight));
            Size sizeImage = img.DesiredSize;
            img.Arrange(new Rect(new Point(0, 0), sizeImage));

            dlg.PrintVisual(img, "Test300DPI");

For 3D:

Rect = Viewport3DVisual.ViewPort and it comes from visual brush:

      <Border>
            <Border.Background>
                <VisualBrush Stretch="None">
                    <VisualBrush.Visual>
        <Viewport3DVisual x:Name="visual3d" Viewport="0 0 384 384">