def map_pixel!
      each_pixel do |x,y,color|
        set_pixel(x,y, yield(x,y,get_pixel(x,y)))
      end

      self
    end
def map_pixel!
      each_pixel do |x,y,color|
        set_pixel(x,y, yield(x,y,get_pixel(x,y)))
      end

      self
    end
Replace each pixel of the image with the value returned
 by the block passed as parameter, the block receives the coordinates and color of each pixel

 Example

   image.map_pixel!{|x,y,color| color.r = 0; color } # remove color channel of all pixels