def to_a
      width.times.map do |x|
        height.times.map do |y|
          sample_distance x, y
        end
      end
    end
def to_a
      width.times.map do |x|
        height.times.map do |y|
          sample_distance x, y
        end
      end
    end
Convert into a nested array of sample values.
 @return [Array<Array<Integer>>]