title: Draw rectangles over bitmapimage in WPF
body: <p>I'm trying to figure out the best way to draw rectangles over a Image in wpf. When the user clicks somewhere in a thumbnail Image the region the user clicks should be marked with a semi-transparent rectangle. This is easy. I just add a rectangle with some opacity on a canvas that is placed over the BitmapImage. The problem is when the user clicks the same region multiple times, then a new rectangle will be added over the previous rectangle and because of the semi-transparency the combined rectangles will the be darker. Please look at the image:</p>
  
  <p><img src="https://i.stack.imgur.com/GPADc.png" alt="enter image description here"></p>
  
  <p>So, my question is: How should I draw the rectangles to prevent this behavior? Not matter how many times the user clicks a region or overlapped region the area will always have the same semi-transparent color. </p>
  
  <p>Thanks</p>