title: Expected behavior when using CIAdditionCompositing to add pure black?
body: <p>I am attempting to understand how <a href="https://developer.apple.com/library/content/documentation/GraphicsImaging/Reference/CoreImageFilterReference/index.html#//apple_ref/doc/filter/ci/CIAdditionCompositing" rel="noreferrer"><code>CIAdditionCompositing</code></a> works.</p>
  
  <p>As part of my testing, I have created a square mid-gray image:</p>
  
  <p><img src="https://i.stack.imgur.com/mtFHX.png" alt=""></p>
  
  <p><img src="https://i.stack.imgur.com/6TVPLm.png" alt=""></p>
  
  <p>and a square black image:</p>
  
  <p><img src="https://i.stack.imgur.com/BSc0I.png" alt=""></p>
  
  <p><img src="https://i.stack.imgur.com/aXQ2dm.png" alt=""></p>
  
  <p>When I combined these two square images using a <code>CIAdditionCompositing</code> patch, I expected to see a gray square whose color matched the original mid-gray square exactly (because all color components of the black image have value 0). However, the final result is actually <strong>brighter</strong> than the original gray image:</p>
  
  <p><img src="https://i.stack.imgur.com/xsyX3.png" alt=""></p>
  
  <p><img src="https://i.stack.imgur.com/sU9Wum.png" alt=""></p>
  
  <p>I don't understand how this result is produced. What am I misunderstanding about how <code>CIAdditionCompositing</code> works?</p>