def pixel_difference_filter(pixel, x, y)
      chans = []
      [:r, :b, :g].each do |chan| 
        chans << channel_difference(chan, pixel, x, y)
      end
      @images.last[x,y] = ChunkyPNG::Color.rgb(chans[0], chans[1], chans[2])
    end
def pixel_difference_filter(pixel, x, y)
      chans = []
      [:r, :b, :g].each do |chan| 
        chans << channel_difference(chan, pixel, x, y)
      end
      @images.last[x,y] = ChunkyPNG::Color.rgb(chans[0], chans[1], chans[2])
    end
Changes the pixel color to be the opposite RGB value