class_docstr: A base class for Neural Radiance Fields (NeRF) models.
  
  Args:
      num_input_dims (int): Number of input dimensions.
      bound (torch.Tensor): The bounding box tensor.
      density_activation (DensityActivationEnum): Activation function for density.
      blob_radius (float): Radius for the blob.
      blob_density (float): Density for the blob.
      normal_type (Optional[NormalTypeEnum]): Method to compute normals.
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docstr: Compute the density blob for the given positions.
  
  This method computes a density blob for each position in the tensor. It is
  used to add a density value based on the distance of each position from the origin.
  
  Args:
      positions (torch.Tensor): Input positions tensor with shape [B*N, D].
  
  Returns:
      torch.Tensor: Density blob tensor with shape [B*N, 1].
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code: def density_blob(self, positions: torch.Tensor) -> torch.Tensor:
      """
      Compute the density blob for the given positions.
  
      This method computes a density blob for each position in the tensor. It is
      used to add a density value based on the distance of each position from the origin.
  
      Args:
          positions (torch.Tensor): Input positions tensor with shape [B*N, D].
  
      Returns:
          torch.Tensor: Density blob tensor with shape [B*N, 1].
      """
  
      # Compute the squared distance for each position
      d = (positions**2).sum(-1)
  
      # Compute the density blob based on the activation function
      if self.density_activation == DensityActivationEnum.EXP:
          g = self.blob_density * torch.exp(
              -d / (2 * self.blob_radius**2)
          )
      elif self.density_activation == DensityActivationEnum.SOFTPLUS:
          g = self.blob_density * (1 - torch.sqrt(d) / self.blob_radius)
      else:
          raise NotImplementedError("Invalid density activation.")
  
      return g
  
imports: from enum import Enum
  from typing import Optional, Tuple
  import mcubes
  import numpy
  import torch
  import torch.nn
  import torch.nn.functional
  import trimesh
  from nemo.collections.multimodal.modules.nerf.utils.activation import trunc_exp
label_desc: Use image node_id 9 for calling the NeRFBase obj's underlying member method code with example usage: obj.density_blob(positions) and returns: g