title: explain arguments meaning in res = cv2.bitwise_and(img,img,mask = mask)
body: <p>I am trying to extract blue colour of an input image. For that I create a blue HSV colour boundary and threshold HSV image by using the command</p>
  <pre><code>mask_img = cv2.inRange(hsv, lower_blue, upper_blue)
  </code></pre>
  <p>After that I used a <code>bitwise_and</code> on the input image and the threshold image by using</p>
  <pre><code>res = cv2.bitwise_and(img, img, mask = mask_img)
  </code></pre>
  <p>Where <code>img</code> is the input image. I got this code from opencv. But I didn't understand why are three arguments used in <code>bitwise_and</code> and what actually each arguments mean? Why the same image is used at src1 and src2 ?</p>
  <p>And also what is the use of <code>mask</code> keyword here? Please help me to find out the answer</p>