def map_pixel
      Image.new(@width, @height) do |x,y|
        yield(x,y,get_pixel(x,y))
      end
    end
def map_pixel
      Image.new(@width, @height) do |x,y|
        yield(x,y,get_pixel(x,y))
      end
    end
Creates a new image of same dimensions in which each pixel is replaced with the value returned
 by the block passed as parameter, the block receives the coordinates and color of each pixel

 Example

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