code: def test_background_z(self):
      self.assertTrue(
          torch.all(
              self.pixel_coords_screen[..., 2][self.background_mask]
              == 1.0
          )
      )
  
imports: import unittest
  import torch
  from pytorch3d.common.compat import meshgrid_ij
  from pytorch3d.renderer.cameras import FoVPerspectiveCameras
  from pytorch3d.renderer.splatter_blend import _compute_occlusion_layers, _compute_splatted_colors_and_weights, _compute_splatting_colors_and_weights, _get_splat_kernel_normalization, _normalize_and_compose_all_layers, _offset_splats, _precompute, _prepare_pixels_and_colors
  from .common_testing import TestCaseMixin
label_desc: Use image node_id 2 for calling the TestPreparPixelsAndColors obj's underlying member method code with example usage: obj.test_background_z() without return types