title: Why pixels have not the same weight?
body: <p>I don't understand :
  if we considerate the value 00001111 (15) is a byte and a RGB pixel (220,180,155) it's 3 bytes whatever the values of the pixel.
  so why when i reduce the values of my pixels (with bitshift operation or whatever) the size of
  my image is not = pixel numbers x 3. when i say &quot;pixel numbers&quot; i mean &quot;pixel numbers bigger than fully black&quot;.</p>
  <p>how the mechanism works ? is it counted in bits and then divided by eight as an average ?
  if i have a 3MB picture and i do a bitshift (factor 2 on each 3 RGB channel) i found a 300 KB picture.</p>
  <p>Don't tell me 90% of my pixels turned fully black.<br />
  Thanks.</p>