Get visible square of image/rectangle - wpf

Question

There is a canvas(Gray color) with an image on it (Blue color). I make some manipulations with the image and want to find the visible part(Red color) of the image after manipulations.
For that i use this method:
private double GetSquare()
{
Rect rect = GetBounds(Image, Canvas);
var canvasRect = RootGeometry.Rect;
canvasRect.Intersect(rect);
return canvasRect.Height * canvasRect.Width;
}
private static Rect GetBounds(FrameworkElement of, FrameworkElement from)
{
// Might throw an exception if of and from are not in the same visual tree
GeneralTransform transform = of.TransformToVisual(from);
return transform.TransformBounds(new Rect(0, 0, of.ActualWidth, of.ActualHeight));
}
XAML:
<Canvas Grid.Row="1" x:Name="ImageCanvas" Background="Gray">
<Canvas.Clip>
<RectangleGeometry x:Name="RootGeometry"/>
</Canvas.Clip>
<Image x:Name="Image" Stretch="Fill"
ManipulationStarted="OnManipulationStarted"
ManipulationDelta="OnManipulationDelta"
ManipulationCompleted="OnManipulationCompleted" Source="{...}"
ImageOpened="OnImageOpened">
<Image.RenderTransform>
<TransformGroup>
<MatrixTransform x:Name="previousTransform" />
<TransformGroup x:Name="currentTransform">
<ScaleTransform x:Name="scaleTransform" />
<RotateTransform x:Name="rotateTransform" />
<TranslateTransform x:Name="translateTransform" />
</TransformGroup>
</TransformGroup>
</Image.RenderTransform>
</Image>
</Canvas>
The method works correct for 2-d picture. But for 3-th image the method returns incorrect result. Maybe someone can explain why the method returns incorrect result and also explain the way how to get the visible part of image (3-th)?