title: Convert RGB image to yuv420 image by matlab
body: <p>I want to convert RGB image to yuv420 image and this is my Matlab code:</p>
  
  <pre><code>ColorImageRGB = imread('view1.png');
  ColorImageYUV = rgb2ycbcr(ColorImageRGB);
  
  [rows cols d] = size(ColorImageRGB);
  dims = [cols rows];
  dimsUV = uint8(dims / 2);
  Yd = zeros(dimsUV);
  UVd = zeros(dimsUV);
  
  Y = Yd';
  U = UVd';
  V = UVd';
  Y = uint8(Y);
  U = uint8(U);
  V = uint8(V);
  
  %sample:420
  for i = 1 : rows
      for j = 1 : cols
          %sample Y in every row
          r = ColorImageRGB(i,j,1);
          g = ColorImageRGB(i,j,2);
          b = ColorImageRGB(i,j,3);
          Y(i,j) = 0.299 * r + 0.587 * g + 0.114 * b + 16;
          %old line : sample 1/2 U
          if mod(i,2) == 1
              index_i = uint8(i / 2);
              index_j = uint8(j / 2);
              U(index_i,index_j) = -0.147 * r - 0.289 * g + 0.436 * b + 128;
          end
          %even line : sample 1/2 V
          if mod(i,2) == 0
              index_i = uint8(i / 2);
              index_j = uint8(j / 2);
              V(index_i,index_j) = 0.615 * r - 0.515 * g - 0.1 * b + 128;
          end
      end
  end
  
  filename = 'view1.yuv';
  fid=fopen(filename,'w');
  count = fwrite(fid,Y','ubit8');
  count = fwrite(fid,U','ubit8');
  count = fwrite(fid,V','ubit8');
  count = fwrite(fid,Y','ubit8');
  count = fwrite(fid,U','ubit8');
  count = fwrite(fid,V','ubit8');
  fclose(fid);
  </code></pre>
  
  <p>view1.png：</p>
  
  <p><img src="https://i.stack.imgur.com/gNIes.png" alt="enter image description here"></p>
  
  <p>When I get the yuv image ,I read it like this（I have tried to read other yuv image,and it this code can work well）:</p>
  
  <pre><code>[Y U V] = yuv_import('view1.yuv',[cols rows],2);
  rgb=yuv2rgb(Y{1},U{1},V{1});
  imwrite(rgb,'test.bmp','bmp');
  </code></pre>
  
  <p>But test.bmp is no the same as view1.png：
  <img src="https://i.stack.imgur.com/RcIwS.png" alt="enter image description here"></p>
  
  <p>So ,the question is,what is wrong with the code that convert rgb image to yuv image?Thx。</p>