docs: def visibility(self, strip_pos, particle_pos):
          """
          Compute particle visibility based on distance between current
          strip position being rendered and particle position. A value
          of 0.0 is returned if they are >= one aperture away, values
          between 0.0 and 1.0 are returned if they are less than one
          aperature apart.
          """
          dist = abs(particle_pos - strip_pos)
          if dist > self.half_size:
              dist = self._size - dist
          if dist < self.aperture:
              return (self.aperture - dist) / self.aperture
          else:
              return 0