title: Android, OpenCV: Detect color & Draw Contours
body: <p>How do i draw contours on only black object, and filling everything else in the background white?
  My code currently is able to draw contours on an image:</p>
  
  <pre><code>Bitmap b = BitmapFactory.decodeByteArray(getIntent().getByteArrayExtra("byteArray"),0,getIntent().getByteArrayExtra("byteArray").length);
  srcMat= new Mat();
  Utils.bitmapToMat(b,srcMat);
  
  Mat gray = new Mat();
  Imgproc.cvtColor(srcMat, gray, Imgproc.COLOR_RGBA2GRAY);
  
  Imgproc.Canny(gray, gray, 20, 20*3, 3, true);
  List&lt;MatOfPoint&gt; contours = new ArrayList&lt;&gt;();
  Mat hierarchy = new Mat();
  
  Imgproc.findContours(gray,contours,hierarchy,Imgproc.RETR_EXTERNAL, Imgproc.CHAIN_APPROX_SIMPLE);
  for (int contourIdx = 0; contourIdx &lt; contours.size(); contourIdx++) {
      Imgproc.drawContours(srcMat, contours, contourIdx, new Scalar(0, 0, 255), -1);
  }
  
  Utils.matToBitmap(gray, b);
  imgR.setImageBitmap(b);
  </code></pre>