title: Using get() and put() to access pixel values in OpenCV for Java
body: <p>I am a beginner in using OpenCV for JAVA. I want to access individual pixel values of an image matrix. Since, JAVA jar for OpenCV doesn't offer nice functions like C++, I ran into some trouble. After lot of searching, I found out two different methods to do that though they are not explained properly (not even in documentation). We can do that either using get() and put() functions or by converting the mat data into a primitive java type such as arrays. I tried both but getting different output results! Please help explaining what am I doing wrong. Am I using them wrong or some other silly problem. I am still a newbie so please forgive if its a stupid question. :)</p>
  
  <p><strong>CASE 1: Using get() function</strong></p>
  
  <pre><code>Mat A = Highgui.imread(image_addr); \\"image_addr" is the address of the image
  Mat C = A.clone();
  Size sizeA = A.size();
  for (int i = 0; i &lt; sizeA.height; i++)
      for (int j = 0; j &lt; sizeA.width; j++) {
          double[] data = A.get(i, j);
          data[0] = data[0] / 2;
          data[1] = data[1] / 2;
          data[2] = data[2] / 2;
          C.put(i, j, data);
      }
  </code></pre>
  
  <p><strong>CASE 2: Using Array</strong></p>
  
  <pre><code>Mat A = Highgui.imread(image_addr); \\"image_addr" is the address of the image
  Mat C = A.clone();
  int size = (int) (A.total() * A.channels());
  byte[] temp = new byte[size];
  A.get(0, 0, temp);
  for (int i = 0; i &lt; size; i++)
     temp[i] = (byte) (temp[i] / 2);
  C.put(0, 0, temp);
  </code></pre>
  
  <p>Now according to my understanding they both should do the same thing. They both access the individual pixel values (all 3 channels) and making it half. I am getting no error after running. But, the output image I am getting is different in these two cases. Can someone please explain what is the issue? May be I don't understand exactly how get() function works? Is it because of the byte() casting? Please help.</p>
  
  <p>Thanks!</p>