title: Interpolation algorithms when downscaling
body: <p>Im trying to understand downscaling. I can see how interpolation algorithms such as bicubic and nearest neighbour can be used when when upscaling, to "fill in the blanks" between the old, known points (pixels, in case of images).</p>
  
  <p>But downscaling? I cant see how any interpolation technique can be used there. There are no blanks to fill!</p>
  
  <p>Ive been stuck with this for far to long, give me a nudge in the right direction. How do you interpolate when you, in fact, remove known data?</p>
  
  <p><strong>Edit</strong>: Lets assume we have a one dimensional image, with one colour channel per point. A downscale algorithm scaling 6 to 3 points by average pixel value looks like this:
  1,2,3,4,5,6 = (1+2)/2,(3+4)/2,(5+6)/2
  Am I on the right track here? Is this interpolation in downscaling rather than just discarding data?</p>