title: Problem when saving a png to jpg in opencv
body: <p>I'm running this piece of code and getting a wrong result:</p>
  
  <pre><code>        #saving image into a white bg
          img = cv2.imread(dir_img + id, cv2.IMREAD_UNCHANGED)
          img = cv2.cvtColor(img, cv2.COLOR_BGRA2BGR)
          print(img.shape)
          cv2.imwrite(dir_img + id, img, [int(cv2.IMWRITE_JPEG_QUALITY), 100])
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/xKbUX.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/xKbUX.jpg" alt="enter image description here"></a></p>
  
  <p>The original file is a png with a transparent background. I don't know why but it's saving with this grey pattern behind the bottle neck.</p>
  
  <p>Original File:
  <a href="https://i.stack.imgur.com/jY8bc.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jY8bc.png" alt="enter image description here"></a></p>