在图像/画布上绘制图像而不显示

Question

我想创建一个拼贴画（图像上的不同图像）。这适用于在我的XAML中定义和显示的画布。但是如何在不显示画布的情况下实现这一目标呢？

System.Windows.Controls.Image img = new System.Windows.Controls.Image();
img.Source = new BitmapImage(new System.Uri(System.Environment.CurrentDirectory + "/" + b.Wert, System.UriKind.RelativeOrAbsolute));

TransformGroup tg = new TransformGroup();
double scaleX = (double)w / (double)img.Source.Width;
double scaleY = (double)h / (double)img.Source.Height;
tg.Children.Add(new ScaleTransform(scaleX, scaleY));
if (b.Rotation != null)
    tg.Children.Add(new RotateTransform(b.Rotation, w/2, h/2));
tg.Children.Add(new TranslateTransform(x, y));
img.RenderTransform = tg;

canvas.Children.Add(img);

double dpi = 96d;
RenderTargetBitmap rtb = new RenderTargetBitmap(collage.Breite, collage.Hoehe, dpi, dpi, System.Windows.Media.PixelFormats.Default);
canvas.UpdateLayout();
rtb.Render(canvas);

// save as png
BitmapEncoder pngEncoder = new PngBitmapEncoder();
pngEncoder.Frames.Add(BitmapFrame.Create(rtb));
using (var fs = System.IO.File.OpenWrite("test.png"))
{
pngEncoder.Save(fs);
}