def sample_distance(x, y)
      x = [[x, width - 1].min, 0].max
      y = [[y, height - 1].min, 0].max
      # Could be checking any of red/blue/green.
      @field.red((x / @scale).round, (y / @scale).round) - ZERO_DISTANCE
    end
def sample_distance(x, y)
      x = [[x, width - 1].min, 0].max
      y = [[y, height - 1].min, 0].max
      # Could be checking any of red/blue/green.
      @field.red((x / @scale).round, (y / @scale).round) - ZERO_DISTANCE
    end
If positive, distance, in pixels, to the nearest opaque pixel.
 If negative, distance in pixels to the nearest transparent pixel.