title: Whiten skin color in an image
body: <p>Is there any theory or image filter for whitening the skin color in an image on iPhone ?</p>
  
  <p>I have the RGB data on hand. But whatever I change, it cannot archieve my needs. I alter the RGB as follows:</p>
  
  <p>New value of R/G/B = (int)roundf(Old R/G/B - 128) * contrast_value + 128 + brightness_value);</p>
  
  <p>Where contrast_value = 1 to 1.3 and brightness_value = 0 to 50</p>
  
  <p>But I found that it comes to pale yellow ...</p>