
Find pixels where the color has a large rate-of-change.

For each pixel (x, y), approximate the length of the
color gradient vector at (x, y).

                +-------+
                |       |
   y-1          |  cA   |
                |       |
        +-------+-------+-------+
        |       |       |       |
  y     |  cL   |  c0   |  cR   |
        |       |       |       |
        +-------+-------+-------+
                |       |
   y+1          |  cB   |
                |       |
                +-------+
         x-1        x      x+1


   deltaH(x,y) = (cR - cL) / 2
   deltaV(x,y) = (cA - cB) / 2
     grad(x,y) = sqrt( deltaH*deltaH + deltaV*deltaV )
