def draw(im, draw_value = 255.0)

      im.each do |ic|

        if (yield ic) then

          im.setValue(ic, draw_value)

        end

      end

    end
def draw(im, draw_value = 255.0)

      im.each do |ic|

        if (yield ic) then

          im.setValue(ic, draw_value)

        end

      end

    end
A basic drawing method that iterates through an entire image.  At each coordinate,
 an attached block is evaluated for a boolean response that determines whether that
 coordinate is overwritten with a specified value.  The attached block will be given
 a single parameter, which is the current ImageCoordinate.

 @param [WritableImage] im the image being drawn on
 @param [Numeric] draw_value the value to which a pixel will be set if the block
  evaluates to a true value at that coordinate

 @return [void]