title: OpenCV Floodfill - Replace pixel color with transparent pixel
body: <p>I'm trying to do Floodfill on UIImage and ended up using OpenCV framework. I can replace the color with a solid color by defining the color as cv::Scalar(255,0,0). However I want the floodfill selection to be transparent. </p>
  
  <p>I don't know how I can define a transparent color in OpenCV and to the best of my knowledge it's not possible and the only option is to merge the image to a transparent background. Again it doesn't make much sense to Floodfill using a solid color and then merge it with a transparent layer as the result will be the original image with solid color in the fill areas.
  Please correct me if I'm wrong.</p>
  
  <p>Much appreciate your help in solving this.</p>
  
  <p>Cheers</p>