code: def setUp(self):
      self.device = torch.device("cpu")
      N, H, W, K = 2, 3, 4, 5
      self.pixel_coords_cameras = torch.randn(
          (N, H, W, K, 3), device=self.device, requires_grad=True
      )
      self.colors_before = torch.rand(
          (N, H, W, K, 3), device=self.device
      )
      self.cameras = FoVPerspectiveCameras(device=self.device)
      self.background_mask = (
          torch.rand((N, H, W, K), device=self.device) < 0.5
      )
      (
          self.pixel_coords_screen,
          self.colors_after,
      ) = _prepare_pixels_and_colors(
          self.pixel_coords_cameras,
          self.colors_before,
          self.cameras,
          self.background_mask,
      )
  
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 1 for calling the TestPreparPixelsAndColors obj's underlying member method code with example usage: obj.setUp() without return types