Eraser equivalent for a drawing app, c#, silverlight, wp7

Question

I'm working on a simple drawing app to further advance my skills and I can't seem to get the logic down for an eraser tool. The app simply uses the Line class to create lines as the user moves their finger. For the eraser tool I tried using the VisualTreeHelper as follows:

List<UIElement> elements = (List<UIElement>)VisualTreeHelper.FindElementsInHostCoordinates(e.GetPosition(tree), ContentPanelCanvas);

            foreach (UIElement element in elements)
            {
                if (element is Line)
                {
                    this.ContentPanelCanvas.Children.Remove(element);
                }
            }

It at some points but can be very slow and laggy. Sometimes I would have to touch the area more than 5 times to get rid of the line there.

Is there an alternative to this?

Answer 1

The e.GetPosition(tree) will be returning a point. Try instead using a Rect with the position as its center.

const double fingerMargin = 10.0;

Point p = e.GetPosition(tree);
Rect r = new Rect(p.X - fingerMargin, p.Y - fingerMargin, fingerMargin * 2, fingerMargin * 2); 

var elements = VisualTreeHelper.FindElementsInHostCoordinates(r, ContentPanelCanvas);

Line lineToRemove = elements.OfType<Line>().FirstOrDefault();
if (lineToRemove != null)
{
    ContentPanelCanvas.Children.Remove(lineToRemove);
}

Note don't cast the result of FindElementsInHostCoordinates to List<T> , that is an implementation detail, the documentation only guarantees it to be an IEnumerable<UIElement> , besides which it is an unnecessary cast.

Answer 2

You are actually looking for the set of elements that match the hit test of a single pixel. If your lines are narrow, then it's like a needle in a haystack; it's very hard to hit the line precisely to remove it.

Instead you need to use a fuzzy match using a rectangle instead of a point. You can use the same API, just the rectangle version of it:

• VisualTreeHelper.FindElementsInHostCoordinates Method (Rect, UIElement)

Answer 3

VisualTreeHelper.FindElementsInHostCoordinates(r, MainCanvas); Is not returning any Elements.