WPF: Detect Image click only on non-transparent portion

Question

I have an Image control in WPF which contains an image with lots of transparent pixels. Right now, the MouseDown event on Image fires whenever I click within the full rectangular region of the Image control. I would like some way to detect if the mouse click occurred on a nontransparent portion of the image.

What would be the best way of doing this?

Answer 1

public class OpaqueClickableImage : Image
{
    protected override HitTestResult HitTestCore(PointHitTestParameters hitTestParameters)
    {
        var source = (BitmapSource)Source;
        var x = (int)(hitTestParameters.HitPoint.X / ActualWidth * source.PixelWidth);
        var y = (int)(hitTestParameters.HitPoint.Y / ActualHeight * source.PixelHeight);
        if (x == source.PixelWidth)
            x--;
        if (y == source.PixelHeight)
            y--;

        var pixels = new byte[4];
        source.CopyPixels(new Int32Rect(x, y, 1, 1), pixels, 4, 0);
        
        return (pixels[3] < 1) ? null : new PointHitTestResult(this, hitTestParameters.HitPoint);
    }
}