我们可以在Windows Phone 8中清除透明度吗？

Question

Actually i am wondering to erase an image to transparency. like i have an image on page background and another image above that. Now i want that if i erase above image by finger then lower image should be appear, simply means to say image will become transparent.i'm doing something like this but its not meet my requirements. Your suggestions are Welcome :)

private void Canvas_MouseMove(object sender, System.Windows.Input.MouseEventArgs e)
{

    currentPoint = e.GetPosition(this.canvas);
    //Initialize line according to currentpoint position.
    Line line = new Line() 
                { 
                    X1 = currentPoint.X, 
                    Y1 = currentPoint.Y, 
                    X2 = oldPoint.X, 
                    Y2 =oldPoint.Y 
                };          


    line.StrokeDashCap = PenLineCap.Round;
    line.StrokeEndLineCap = PenLineCap.Round;
    line.StrokeLineJoin = PenLineJoin.Round;
    line.StrokeThickness = 20;
    line.Stroke = new SolidColorBrush(Colors.Black) ;
    ////////////////////////////////
    //Set color & thickness of line. 

    //Line add in canvas children to draw image & assign oldpoint.
    this.canvas.Children.Add(line);
    oldPoint = currentPoint;
}

Answer 1

You can do it with 3 different ways:

1. Using opaque overlay and UIElement.Clip property. But you need to deal with Geometry . And I'm afraid it will be very CPU cost.

2. Using WriteableBitmap and changing an alpha channel of image. You can do it using WriteableBitmapEx .

3. Using opaque overlay and UIElement.OpacityMask property. I think it's the best way to accomplish that, as you're not limited to use bitmap (so you can place any XAML control below overlay) as in the second way.