WPF Rub out top image to reveal image underneath

As the title suggests, using WPF I want to have a scenario where I have a bitmap image with another bitmap overlayed on top of it and, using the mouse, "paint" over the t开发者_运维百科op bitmap so that it reveals the bitmap underneath. Is this just as simple as painting with a transparent brush? I have tried this to no avail but maybe I am missing something.

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You can use a VisualBrush to create an OpacityMask. Here is a code example of a dark green rectangle being replaced by a light green one. Using this approach the two rectangles could be any elements including images.

<Grid MouseMove="Grid_MouseMove">
    <Rectangle Fill="DarkGreen"/>
    <Rectangle Fill="LightGreen">
        <Rectangle.OpacityMask>
            <VisualBrush Stretch="None" AlignmentX="Left" AlignmentY="Top">
                <VisualBrush.Visual>
                    <Path Name="path" Stroke="Black" StrokeThickness="10"/>
                </VisualBrush.Visual>
            </VisualBrush>
        </Rectangle.OpacityMask>
    </Rectangle>
</Grid>

and here is the code-behind:

private void Grid_MouseMove(object sender, MouseEventArgs e)
{
    if (e.LeftButton == MouseButtonState.Pressed)
    {
        var point = e.GetPosition(sender as Grid);
        if (lastPoint != nullPoint)
            AddSegment(lastPoint, point);
        lastPoint = point;
    }
    else
    {
        lastPoint = nullPoint;
    }
}

private void AddSegment(Point point1, Point point2)
{
    if (segments.Count == 0 || segments[segments.Count - 1].Point != point1)
        segments.Add(new LineSegment(point1, false));
    segments.Add(new LineSegment(point2, true));
    var figures = new PathFigureCollection();
    figures.Add(new PathFigure(new Point(), segments, false));
    var geometry = new PathGeometry();
    geometry.Figures = figures;
    path.Data = geometry;
}

List<LineSegment> segments = new List<LineSegment>();
private static readonly Point nullPoint = new Point(-1, -1);
Point lastPoint = nullPoint;