code: def flat_shading(
      meshes, fragments, lights, cameras, materials, texels
  ) -> torch.Tensor:
      """
      Apply per face shading. Use the average face position and the face normals
      to compute the ambient, diffuse and specular lighting. Apply the ambient
      and diffuse color to the pixel color and add the specular component to
      determine the final pixel color.
  
      Args:
          meshes: Batch of meshes
          fragments: Fragments named tuple with the outputs of rasterization
          lights: Lights class containing a batch of lights parameters
          cameras: Cameras class containing a batch of cameras parameters
          materials: Materials class containing a batch of material properties
          texels: texture per pixel of shape (N, H, W, K, 3)
  
      Returns:
          colors: (N, H, W, K, 3)
      """
      verts = meshes.verts_packed()  # (V, 3)
      faces = meshes.faces_packed()  # (F, 3)
      face_normals = meshes.faces_normals_packed()  # (V, 3)
      faces_verts = verts[faces]
      face_coords = faces_verts.mean(
          dim=-2
      )  # (F, 3, XYZ) mean xyz across verts
  
      # Replace empty pixels in pix_to_face with 0 in order to interpolate.
      mask = fragments.pix_to_face == -1
      pix_to_face = fragments.pix_to_face.clone()
      pix_to_face[mask] = 0
  
      N, H, W, K = pix_to_face.shape
      idx = pix_to_face.view(N * H * W * K, 1).expand(N * H * W * K, 3)
  
      # gather pixel coords
      pixel_coords = face_coords.gather(0, idx).view(N, H, W, K, 3)
      pixel_coords[mask] = 0.0
      # gather pixel normals
      pixel_normals = face_normals.gather(0, idx).view(N, H, W, K, 3)
      pixel_normals[mask] = 0.0
  
      # Calculate the illumination at each face
      ambient, diffuse, specular = _apply_lighting(
          pixel_coords, pixel_normals, lights, cameras, materials
      )
      colors = (ambient + diffuse) * texels + specular
      return colors
  
file_path: pytorch3d/pytorch3d/renderer/mesh/shading.py
imports: from typing import Tuple
  import torch
  from pytorch3d.ops import interpolate_face_attributes
  from .textures import TexturesVertex
label_desc: Use image node_id 5 for calling a global function with example usage: flat_shading(meshes, fragments, lights, cameras, materials, texels) and returns: colors