code: def update_field(self, poses=None):
          
          m = np.clip(self.particle_field, 0, 1)
          part_color = np.zeros(self._image.shape)
          for a in range(4): part_color[:,:,:,a] = self.part_col[a]
          self.field = np.zeros(self._image.shape)
          for a in range(4):
              self.field[:,:,:,a] = m*part_color[:,:,:,a] + (1-m) * self._image[:,:,:,a]
docs: updates self.field