voxel_grid_implicit_function

Reviewed By: shapovalov

Differential Revision: D40622304

fbshipit-source-id: 277515a55c46d9b8300058b439526539a7fe00a0

Author: bottler

projects/implicitron_trainer/tests/experiment.yaml

@@ -394,6 +394,168 @@ model_factory_ImplicitronModelFactory_args:
         in_features: 256
         out_features: 3
         ray_dir_in_camera_coords: false
+    implicit_function_VoxelGridImplicitFunction_args:
+      harmonic_embedder_xyz_density_args:
+        n_harmonic_functions: 6
+        omega_0: 1.0
+        logspace: true
+        append_input: true
+      harmonic_embedder_xyz_color_args:
+        n_harmonic_functions: 6
+        omega_0: 1.0
+        logspace: true
+        append_input: true
+      harmonic_embedder_dir_color_args:
+        n_harmonic_functions: 6
+        omega_0: 1.0
+        logspace: true
+        append_input: true
+      decoder_density_class_type: MLPDecoder
+      decoder_color_class_type: MLPDecoder
+      use_multiple_streams: true
+      xyz_ray_dir_in_camera_coords: false
+      scaffold_calculating_epochs: []
+      scaffold_resolution:
+      - 128
+      - 128
+      - 128
+      scaffold_empty_space_threshold: 0.001
+      scaffold_occupancy_chunk_size: 'inf'
+      scaffold_max_pool_kernel_size: 3
+      scaffold_filter_points: true
+      volume_cropping_epochs: []
+      voxel_grid_density_args:
+        voxel_grid_class_type: FullResolutionVoxelGrid
+        extents:
+        - 2.0
+        - 2.0
+        - 2.0
+        translation:
+        - 0.0
+        - 0.0
+        - 0.0
+        init_std: 0.1
+        init_mean: 0.0
+        hold_voxel_grid_as_parameters: true
+        param_groups: {}
+        voxel_grid_CPFactorizedVoxelGrid_args:
+          align_corners: true
+          padding: zeros
+          mode: bilinear
+          n_features: 1
+          resolution_changes:
+            0:
+            - 128
+            - 128
+            - 128
+          n_components: 24
+          basis_matrix: true
+        voxel_grid_FullResolutionVoxelGrid_args:
+          align_corners: true
+          padding: zeros
+          mode: bilinear
+          n_features: 1
+          resolution_changes:
+            0:
+            - 128
+            - 128
+            - 128
+        voxel_grid_VMFactorizedVoxelGrid_args:
+          align_corners: true
+          padding: zeros
+          mode: bilinear
+          n_features: 1
+          resolution_changes:
+            0:
+            - 128
+            - 128
+            - 128
+          n_components: null
+          distribution_of_components: null
+          basis_matrix: true
+      voxel_grid_color_args:
+        voxel_grid_class_type: FullResolutionVoxelGrid
+        extents:
+        - 2.0
+        - 2.0
+        - 2.0
+        translation:
+        - 0.0
+        - 0.0
+        - 0.0
+        init_std: 0.1
+        init_mean: 0.0
+        hold_voxel_grid_as_parameters: true
+        param_groups: {}
+        voxel_grid_CPFactorizedVoxelGrid_args:
+          align_corners: true
+          padding: zeros
+          mode: bilinear
+          n_features: 1
+          resolution_changes:
+            0:
+            - 128
+            - 128
+            - 128
+          n_components: 24
+          basis_matrix: true
+        voxel_grid_FullResolutionVoxelGrid_args:
+          align_corners: true
+          padding: zeros
+          mode: bilinear
+          n_features: 1
+          resolution_changes:
+            0:
+            - 128
+            - 128
+            - 128
+        voxel_grid_VMFactorizedVoxelGrid_args:
+          align_corners: true
+          padding: zeros
+          mode: bilinear
+          n_features: 1
+          resolution_changes:
+            0:
+            - 128
+            - 128
+            - 128
+          n_components: null
+          distribution_of_components: null
+          basis_matrix: true
+      decoder_density_ElementwiseDecoder_args:
+        scale: 1.0
+        shift: 0.0
+        operation: IDENTITY
+      decoder_density_MLPDecoder_args:
+        param_groups: {}
+        network_args:
+          n_layers: 8
+          output_dim: 256
+          skip_dim: 39
+          hidden_dim: 256
+          input_skips:
+          - 5
+          skip_affine_trans: false
+          last_layer_bias_init: null
+          last_activation: RELU
+          use_xavier_init: true
+      decoder_color_ElementwiseDecoder_args:
+        scale: 1.0
+        shift: 0.0
+        operation: IDENTITY
+      decoder_color_MLPDecoder_args:
+        param_groups: {}
+        network_args:
+          n_layers: 8
+          output_dim: 256
+          skip_dim: 39
+          hidden_dim: 256
+          input_skips:
+          - 5
+          skip_affine_trans: false
+          last_layer_bias_init: null
+          last_activation: RELU
+          use_xavier_init: true
     view_metrics_ViewMetrics_args: {}
     regularization_metrics_RegularizationMetrics_args: {}
 optimizer_factory_ImplicitronOptimizerFactory_args:

pytorch3d/implicitron/models/generic_model.py

@@ -52,6 +52,9 @@
     SRNHyperNetImplicitFunction,
     SRNImplicitFunction,
 )
+from .implicit_function.voxel_grid_implicit_function import (  # noqa
+    VoxelGridImplicitFunction,
+)
 
 from .renderer.base import (
     BaseRenderer,

pytorch3d/implicitron/models/implicit_function/voxel_grid_implicit_function.py

@@ -0,0 +1,616 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+import math
+import warnings
+from dataclasses import fields
+from typing import Callable, Dict, Optional, Tuple, Union
+
+import torch
+
+from omegaconf import DictConfig
+
+from pytorch3d.implicitron.models.implicit_function.base import ImplicitFunctionBase
+from pytorch3d.implicitron.models.implicit_function.decoding_functions import (
+    DecoderFunctionBase,
+)
+from pytorch3d.implicitron.models.implicit_function.voxel_grid import VoxelGridModule
+from pytorch3d.implicitron.models.renderer.base import ImplicitronRayBundle
+from pytorch3d.implicitron.tools.config import (
+    enable_get_default_args,
+    get_default_args_field,
+    registry,
+    run_auto_creation,
+)
+from pytorch3d.renderer import ray_bundle_to_ray_points
+from pytorch3d.renderer.cameras import CamerasBase
+from pytorch3d.renderer.implicit import HarmonicEmbedding
+
+enable_get_default_args(HarmonicEmbedding)
+
+
+@registry.register
+# pyre-ignore[13]
+class VoxelGridImplicitFunction(ImplicitFunctionBase, torch.nn.Module):
+    """
+    This implicit function consists of two streams, one for the density calculation and one
+    for the color calculation. Each of these streams has three main parts:
+        1) Voxel grids:
+            They take the (x, y, z) position and return the embedding of that point.
+            These components are replaceable, you can make your own or choose one of
+            several options.
+        2) Harmonic embeddings:
+            Convert each feature into series of 'harmonic features', feature is passed through
+            sine and cosine functions. Input is of shape [minibatch, ..., D] output
+            [minibatch, ..., (n_harmonic_functions * 2 + int(append_input)) * D]. Appends
+            input by default. If you want it to behave like identity, put n_harmonic_functions=0
+            and append_input=True.
+        3) Decoding functions:
+            The decoder is an instance of the DecoderFunctionBase and converts the embedding
+            of a spatial location to density/color. Examples are Identity which returns its
+            input and the MLP which uses fully connected nerual network to transform the input.
+            These components are replaceable, you can make your own or choose from
+            several options.
+
+    Calculating density is done in three steps:
+        1) Evaluating the voxel grid on points
+        2) Embedding the outputs with harmonic embedding
+        3) Passing through the Density decoder
+
+    To calculate the color we need the embedding and the viewing direction, it has five steps:
+        1) Transforming the viewing direction with camera
+        2) Evaluating the voxel grid on points
+        3) Embedding the outputs with harmonic embedding
+        4) Embedding the normalized direction with harmonic embedding
+        5) Passing everything through the Color decoder
+
+    If using the Implicitron configuration system the input_dim to the decoding functions will
+    be set to the output_dim of the Harmonic embeddings.
+
+    A speed up comes from using the scaffold, a low resolution voxel grid.
+    The scaffold is referenced as "binary occupancy grid mask" in TensoRF paper and "AlphaMask"
+    in official TensoRF implementation.
+    The scaffold is used in:
+        1) filtering points in empty space
+            - controlled by `scaffold_filter_points` boolean. If set to True, points for which
+                scaffold predicts that are in empty space will return 0 density and
+                (0, 0, 0) color.
+        2) calculating the bounding box of an object and cropping the voxel grids
+            - controlled by `volume_cropping_epochs`.
+            - at those epochs the implicit function will find the bounding box of an object
+                inside it and crop density and color grids. Cropping of the voxel grids means
+                preserving only voxel values that are inside the bounding box and changing the
+                resolution to match the original, while preserving the new cropped location in
+                world coordinates.
+
+    The scaffold has to exist before attempting filtering and cropping, and is created on
+    `scaffold_calculating_epochs`. Each voxel in the scaffold is labeled as having density 1 if
+    the point in the center of it evaluates to greater than `scaffold_empty_space_threshold`.
+    3D max pooling is performed on the densities of the points in 3D.
+    Scaffold features are off by default.
+
+    Members:
+        voxel_grid_density (VoxelGridBase): voxel grid to use for density estimation
+        voxel_grid_color   (VoxelGridBase): voxel grid to use for color   estimation
+
+        harmonic_embedder_xyz_density (HarmonicEmbedder): Function to transform the outputs of
+            the voxel_grid_density
+        harmonic_embedder_xyz_color (HarmonicEmbedder): Function to transform the outputs of
+            the voxel_grid_color for density
+        harmonic_embedder_dir_color (HarmonicEmbedder): Function to transform the outputs of
+            the voxel_grid_color for color
+
+        decoder_density (DecoderFunctionBase): decoder function to use for density estimation
+        color_density   (DecoderFunctionBase): decoder function to use for color   estimation
+
+        use_multiple_streams (bool): if you want the density and color calculations to run on
+            different cuda streams set this to True. Default True.
+        xyz_ray_dir_in_camera_coords (bool): This is true if the directions are given in
+            camera coordinates. Default False.
+
+        voxel_grid_scaffold (VoxelGridModule): which holds the scaffold. Extents and
+            translation of it are set to those of voxel_grid_density.
+        scaffold_calculating_epochs (Tuple[int, ...]): at which epochs to recalculate the
+            scaffold. (The scaffold will be created automatically at the beginning of
+            the calculation.)
+        scaffold_resolution (Tuple[int, int, int]): (width, height, depth) of the underlying
+            voxel grid which stores scaffold
+        scaffold_empty_space_threshold (float): if `self.get_density` evaluates to less than
+            this it will be considered as empty space and the scaffold at that point would
+            evaluate as empty space.
+        scaffold_occupancy_chunk_size (str or int): Number of xy scaffold planes to calculate
+            at the same time. To calculate the scaffold we need to query `get_density()` at
+            every voxel, this calculation can be split into scaffold depth number of xy plane
+            calculations if you want the lowest memory usage, one calculation to calculate the
+            whole scaffold, but with higher memory footprint or any other number of planes.
+            Setting to 'inf' calculates all planes at the same time. Defaults to 'inf'.
+        scaffold_max_pool_kernel_size (int): Size of the pooling region to use when
+            calculating the scaffold. Defaults to 3.
+        scaffold_filter_points (bool): If set to True the points will be filtered using
+            `self.voxel_grid_scaffold`. Filtered points will be predicted as having 0 density
+            and (0, 0, 0) color. The points which were not evaluated as empty space will be
+            passed through the steps outlined above.
+        volume_cropping_epochs: on which epochs to crop the voxel grids to fit the object's
+            bounding box. Scaffold has to be calculated before cropping.
+    """
+
+    # ---- voxel grid for density
+    voxel_grid_density: VoxelGridModule
+
+    # ---- voxel grid for color
+    voxel_grid_color: VoxelGridModule
+
+    # ---- harmonic embeddings density
+    harmonic_embedder_xyz_density_args: DictConfig = get_default_args_field(
+        HarmonicEmbedding
+    )
+    harmonic_embedder_xyz_color_args: DictConfig = get_default_args_field(
+        HarmonicEmbedding
+    )
+    harmonic_embedder_dir_color_args: DictConfig = get_default_args_field(
+        HarmonicEmbedding
+    )
+
+    # ---- decoder function for density
+    decoder_density_class_type: str = "MLPDecoder"
+    decoder_density: DecoderFunctionBase
+
+    # ---- decoder function for color
+    decoder_color_class_type: str = "MLPDecoder"
+    decoder_color: DecoderFunctionBase
+
+    # ---- cuda streams
+    use_multiple_streams: bool = True
+
+    # ---- camera
+    xyz_ray_dir_in_camera_coords: bool = False
+
+    # --- scaffold
+    # voxel_grid_scaffold: VoxelGridModule
+    scaffold_calculating_epochs: Tuple[int, ...] = ()
+    scaffold_resolution: Tuple[int, int, int] = (128, 128, 128)
+    scaffold_empty_space_threshold: float = 0.001
+    scaffold_occupancy_chunk_size: Union[str, int] = "inf"
+    scaffold_max_pool_kernel_size: int = 3
+    scaffold_filter_points: bool = True
+
+    # --- cropping
+    volume_cropping_epochs: Tuple[int, ...] = ()
+
+    def __post_init__(self) -> None:
+        super().__init__()
+        run_auto_creation(self)
+        # pyre-ignore[16]
+        self.voxel_grid_scaffold = self._create_voxel_grid_scaffold()
+        # pyre-ignore[16]
+        self.harmonic_embedder_xyz_density = HarmonicEmbedding(
+            **self.harmonic_embedder_xyz_density_args
+        )
+        # pyre-ignore[16]
+        self.harmonic_embedder_xyz_color = HarmonicEmbedding(
+            **self.harmonic_embedder_xyz_color_args
+        )
+        # pyre-ignore[16]
+        self.harmonic_embedder_dir_color = HarmonicEmbedding(
+            **self.harmonic_embedder_dir_color_args
+        )
+        # pyre-ignore[16]
+        self._scaffold_ready = False
+        if type(self.scaffold_occupancy_chunk_size) != int:
+            if self.scaffold_occupancy_chunk_size != "inf":
+                raise ValueError(
+                    "`scaffold_occupancy_chunk_size` has to be int or 'inf'."
+                )
+
+    def forward(
+        self,
+        ray_bundle: ImplicitronRayBundle,
+        fun_viewpool=None,
+        camera: Optional[CamerasBase] = None,
+        global_code=None,
+        **kwargs,
+    ) -> Tuple[torch.Tensor, torch.Tensor, Dict]:
+        """
+        The forward function accepts the parametrizations of 3D points sampled along
+        projection rays. The forward pass is responsible for attaching a 3D vector
+        and a 1D scalar representing the point's RGB color and opacity respectively.
+
+        Args:
+            ray_bundle: An ImplicitronRayBundle object containing the following variables:
+                origins: A tensor of shape `(minibatch, ..., 3)` denoting the
+                    origins of the sampling rays in world coords.
+                directions: A tensor of shape `(minibatch, ..., 3)`
+                    containing the direction vectors of sampling rays in world coords.
+                lengths: A tensor of shape `(minibatch, ..., num_points_per_ray)`
+                    containing the lengths at which the rays are sampled.
+            fun_viewpool: an optional callback with the signature
+                    fun_fiewpool(points) -> pooled_features
+                where points is a [N_TGT x N x 3] tensor of world coords,
+                and pooled_features is a [N_TGT x ... x N_SRC x latent_dim] tensor
+                of the features pooled from the context images.
+            camera: A camera model which will be used to transform the viewing
+                directions
+
+        Returns:
+            rays_densities: A tensor of shape `(minibatch, ..., num_points_per_ray, 1)`
+                denoting the opacitiy of each ray point.
+            rays_colors: A tensor of shape `(minibatch, ..., num_points_per_ray, 3)`
+                denoting the color of each ray point.
+        """
+        # ########## convert the ray parametrizations to world coordinates ########## #
+        # points.shape = [minibatch x n_rays_width x n_rays_height x pts_per_ray x 3]
+        # pyre-ignore[6]
+        points = ray_bundle_to_ray_points(ray_bundle)
+        directions = ray_bundle.directions.reshape(-1, 3)
+        input_shape = points.shape
+        points = points.view(-1, 3)
+
+        # ########## filter the points using the scaffold ########## #
+        if self._scaffold_ready and self.scaffold_filter_points:
+            # pyre-ignore[29]
+            non_empty_points = self.voxel_grid_scaffold(points)[..., 0] > 0
+            points = points[non_empty_points]
+            directions = directions[non_empty_points]
+            if len(points) == 0:
+                warnings.warn(
+                    "The scaffold has filtered all the points."
+                    "The voxel grids and decoding functions will not be run."
+                )
+                return (
+                    points.new_zeros((*input_shape[:-1], 1)),
+                    points.new_zeros((*input_shape[:-1], 3)),
+                    {},
+                )
+
+        # ########## calculate color and density ########## #
+        rays_densities, rays_colors = self.calculate_density_and_color(
+            points, directions, camera
+        )
+
+        if not (self._scaffold_ready and self.scaffold_filter_points):
+            return (
+                rays_densities.view((*input_shape[:-1], rays_densities.shape[-1])),
+                rays_colors.view((*input_shape[:-1], rays_colors.shape[-1])),
+                {},
+            )
+
+        # ########## merge scaffold calculated points ########## #
+        # Create a zeroed tensor corresponding to a point with density=0 and fill it
+        # with calculated density for points which are not in empty space. Do the
+        # same for color
+        rays_densities_combined = rays_densities.new_zeros(
+            (math.prod(input_shape[:-1]), rays_densities.shape[-1])
+        )
+        rays_colors_combined = rays_colors.new_zeros(
+            (math.prod(input_shape[:-1]), rays_colors.shape[-1])
+        )
+        # pyre-ignore[61]
+        rays_densities_combined[non_empty_points] = rays_densities
+        # pyre-ignore[61]
+        rays_colors_combined[non_empty_points] = rays_colors
+
+        return (
+            rays_densities_combined.view((*input_shape[:-1], rays_densities.shape[-1])),
+            rays_colors_combined.view((*input_shape[:-1], rays_colors.shape[-1])),
+            {},
+        )
+
+    def calculate_density_and_color(
+        self,
+        points: torch.Tensor,
+        directions: torch.Tensor,
+        camera: Optional[CamerasBase] = None,
+    ) -> Tuple[torch.Tensor, torch.Tensor]:
+        """
+        Calculates density and color at `points`.
+        If enabled use cuda streams.
+
+        Args:
+            points: points at which to calculate density and color.
+                Tensor of shape [..., 3].
+            directions: from which directions are the points viewed
+                Tensor of shape [..., 3].
+            camera: A camera model which will be used to transform the viewing
+                directions
+        Returns:
+               Tuple of color (tensor of shape [..., 3]) and density
+                (tensor of shape [..., 1])
+        """
+        if self.use_multiple_streams and points.is_cuda:
+            current_stream = torch.cuda.current_stream(points.device)
+            other_stream = torch.cuda.Stream(points.device)
+            other_stream.wait_stream(current_stream)
+
+            with torch.cuda.stream(other_stream):
+                # rays_densities.shape =
+                # [minibatch x n_rays_width x n_rays_height x pts_per_ray x density_dim]
+                rays_densities = self.get_density(points)
+
+            # rays_colors.shape =
+            # [minibatch x n_rays_width x n_rays_height x pts_per_ray x color_dim]
+            rays_colors = self.get_color(points, camera, directions)
+
+            current_stream.wait_stream(other_stream)
+        else:
+            # Same calculation as above, just serial.
+            rays_densities = self.get_density(points)
+            rays_colors = self.get_color(points, camera, directions)
+        return rays_densities, rays_colors
+
+    def get_density(self, points: torch.Tensor) -> torch.Tensor:
+        """
+        Calculates density at points:
+            1) Evaluates the voxel grid on points
+            2) Embeds the outputs with harmonic embedding
+            3) Passes everything through the Density decoder
+
+        Args:
+            points: tensor of shape [..., 3]
+                where the last dimension is the points in the (x, y, z)
+        Returns:
+            calculated densities of shape [..., density_dim], `density_dim` is the
+                feature dimensionality which `decoder_density` returns
+        """
+        embeds_density = self.voxel_grid_density(points)
+        # pyre-ignore[29]
+        harmonic_embedding_density = self.harmonic_embedder_xyz_density(embeds_density)
+        # shape = [..., density_dim]
+        return self.decoder_density(harmonic_embedding_density)
+
+    def get_color(
+        self,
+        points: torch.Tensor,
+        camera: Optional[CamerasBase],
+        directions: torch.Tensor,
+    ) -> torch.Tensor:
+        """
+        Calculates color at points using the viewing direction:
+            1) Transforms the viewing direction with camera
+            2) Evaluates the voxel grid on points
+            3) Embeds the outputs with harmonic embedding
+            4) Embeds the normalized direction with harmonic embedding
+            5) Passes everything through the Color decoder
+        Args:
+            points: tensor of shape (..., 3)
+                where the last dimension is the points in the (x, y, z)
+            camera: A camera model which will be used to transform the viewing
+                directions
+            directions: A tensor of shape `(..., 3)`
+                containing the direction vectors of sampling rays in world coords.
+        """
+        # ########## transform direction ########## #
+        if self.xyz_ray_dir_in_camera_coords:
+            if camera is None:
+                raise ValueError("Camera must be given if xyz_ray_dir_in_camera_coords")
+            directions = directions @ camera.R
+
+        # ########## get voxel grid output ########## #
+        # embeds_color.shape = [..., pts_per_ray, n_features]
+        embeds_color = self.voxel_grid_color(points)
+
+        # ########## embed with the harmonic function ########## #
+        # Obtain the harmonic embedding of the voxel grid output.
+        # pyre-ignore[29]
+        harmonic_embedding_color = self.harmonic_embedder_xyz_color(embeds_color)
+
+        # Normalize the ray_directions to unit l2 norm.
+        rays_directions_normed = torch.nn.functional.normalize(directions, dim=-1)
+        # Obtain the harmonic embedding of the normalized ray directions.
+        # pyre-ignore[29]
+        harmonic_embedding_dir = self.harmonic_embedder_dir_color(
+            rays_directions_normed
+        )
+
+        n_rays = directions.shape[0]
+        points_per_ray: int = points.shape[0] // n_rays
+
+        harmonic_embedding_dir = torch.repeat_interleave(
+            harmonic_embedding_dir, points_per_ray, dim=0
+        )
+
+        # total color embedding is concatenation of the harmonic embedding of voxel grid
+        # output and harmonic embedding of the normalized direction
+        total_color_embedding = torch.cat(
+            (harmonic_embedding_color, harmonic_embedding_dir), dim=-1
+        )
+
+        # ########## evaluate color with the decoding function ########## #
+        # rays_colors.shape = [..., pts_per_ray, 3] in [0-1]
+        return self.decoder_color(total_color_embedding)
+
+    @staticmethod
+    def allows_multiple_passes() -> bool:
+        """
+        Returns True as this implicit function allows
+        multiple passes. Overridden from ImplicitFunctionBase.
+        """
+        return True
+
+    def subscribe_to_epochs(self) -> Tuple[Tuple[int, ...], Callable[[int], bool]]:
+        """
+        Method which expresses interest in subscribing to optimization epoch updates.
+        This implicit function subscribes to epochs to calculate the scaffold and to
+        crop voxel grids, so this method combines wanted epochs and wraps their callbacks.
+
+        Returns:
+            list of epochs on which to call a callable and callable to be called on
+                particular epoch. The callable returns True if parameter change has
+                happened else False and it must be supplied with one argument, epoch.
+        """
+
+        def callback(epoch) -> bool:
+            change = False
+            if epoch in self.scaffold_calculating_epochs:
+                change = self._get_scaffold(epoch)
+            if epoch in self.volume_cropping_epochs:
+                change = self._crop(epoch) or change
+            return change
+
+        # remove duplicates
+        call_epochs = list(
+            set(self.scaffold_calculating_epochs) | set(self.volume_cropping_epochs)
+        )
+        return call_epochs, callback
+
+    def _crop(self, epoch: int) -> bool:
+        """
+        Finds the bounding box of an object represented in the scaffold and crops
+        density and color voxel grids to match that bounding box. If density of the
+        scaffold is 0 everywhere (there is no object in it) no change will
+        happen.
+
+        Args:
+            epoch: ignored
+        Returns:
+            True (indicating that parameter change has happened) if there is
+            an object inside, else False.
+        """
+        # find bounding box
+        # pyre-ignore[16]
+        points = self.voxel_grid_scaffold.get_grid_points(epoch=epoch)
+        assert self._scaffold_ready, "Scaffold has to be calculated before cropping."
+        # pyre-ignore[29]
+        occupancy = self.voxel_grid_scaffold(points)[..., 0] > 0
+        non_zero_idxs = torch.nonzero(occupancy)
+        if len(non_zero_idxs) == 0:
+            return False
+        min_indices = tuple(torch.min(non_zero_idxs, dim=0)[0])
+        max_indices = tuple(torch.max(non_zero_idxs, dim=0)[0])
+        min_point, max_point = points[min_indices], points[max_indices]
+
+        # crop the voxel grids
+        self.voxel_grid_density.crop_self(min_point, max_point)
+        self.voxel_grid_color.crop_self(min_point, max_point)
+        return True
+
+    @torch.no_grad()
+    def _get_scaffold(self, epoch: int) -> bool:
+        """
+        Creates a low resolution grid which is used to filter points that are in empty
+        space.
+
+        Args:
+            epoch: epoch on which it is called, ignored inside method
+        Returns:
+             Always False: Modifies `self.voxel_grid_scaffold` member.
+        """
+
+        planes = []
+        # pyre-ignore[16]
+        points = self.voxel_grid_scaffold.get_grid_points(epoch=epoch)
+
+        chunk_size = (
+            self.scaffold_occupancy_chunk_size
+            if type(self.scaffold_occupancy_chunk_size) == int
+            else points.shape[-1]
+        )
+        for k in range(0, points.shape[-1], chunk_size):
+            points_in_planes = points[..., k : k + chunk_size]
+            planes.append(self.get_density(points_in_planes)[..., 0])
+
+        density_cube = torch.cat(planes, dim=-1)
+        density_cube = torch.nn.functional.max_pool3d(
+            density_cube[None, None],
+            kernel_size=self.scaffold_max_pool_kernel_size,
+            padding=self.scaffold_max_pool_kernel_size // 2,
+            stride=1,
+        )
+        occupancy_cube = density_cube > self.scaffold_empty_space_threshold
+        # pyre-ignore[16]
+        self.voxel_grid_scaffold.params["voxel_grid"] = occupancy_cube.float()
+        # pyre-ignore[16]
+        self._scaffold_ready = True
+
+        return False
+
+    @classmethod
+    def decoder_density_tweak_args(cls, type, args: DictConfig) -> None:
+        args.pop("input_dim", None)
+
+    def create_decoder_density_impl(self, type, args: DictConfig) -> None:
+        """
+        Decoding functions come after harmonic embedding and voxel grid. In order to not
+        calculate the input dimension of the decoder in the config file this function
+        calculates the required input dimension and sets the input dimension of the
+        decoding function to this value.
+        """
+        grid_args = self.voxel_grid_density_args
+        # pyre-ignore[6]
+        grid_output_dim = VoxelGridModule.get_output_dim(grid_args)
+
+        embedder_args = self.harmonic_embedder_xyz_density_args
+        input_dim = HarmonicEmbedding.get_output_dim_static(
+            grid_output_dim,
+            embedder_args["n_harmonic_functions"],
+            embedder_args["append_input"],
+        )
+
+        cls = registry.get(DecoderFunctionBase, type)
+        need_input_dim = any(field.name == "input_dim" for field in fields(cls))
+        if need_input_dim:
+            self.decoder_density = cls(input_dim=input_dim, **args)
+        else:
+            self.decoder_density = cls(**args)
+
+    @classmethod
+    def decoder_color_tweak_args(cls, type, args: DictConfig) -> None:
+        args.pop("input_dim", None)
+
+    def create_decoder_color_impl(self, type, args: DictConfig) -> None:
+        """
+        Decoding functions come after harmonic embedding and voxel grid. In order to not
+        calculate the input dimension of the decoder in the config file this function
+        calculates the required input dimension and sets the input dimension of the
+        decoding function to this value.
+        """
+        grid_args = self.voxel_grid_color_args
+        # pyre-ignore[6]
+        grid_output_dim = VoxelGridModule.get_output_dim(grid_args)
+
+        embedder_args = self.harmonic_embedder_xyz_color_args
+        input_dim0 = HarmonicEmbedding.get_output_dim_static(
+            grid_output_dim,
+            embedder_args["n_harmonic_functions"],
+            embedder_args["append_input"],
+        )
+
+        dir_dim = 3
+        embedder_args = self.harmonic_embedder_dir_color_args
+        input_dim1 = HarmonicEmbedding.get_output_dim_static(
+            dir_dim,
+            embedder_args["n_harmonic_functions"],
+            embedder_args["append_input"],
+        )
+
+        input_dim = input_dim0 + input_dim1
+
+        cls = registry.get(DecoderFunctionBase, type)
+        need_input_dim = any(field.name == "input_dim" for field in fields(cls))
+        if need_input_dim:
+            self.decoder_color = cls(input_dim=input_dim, **args)
+        else:
+            self.decoder_color = cls(**args)
+
+    def _create_voxel_grid_scaffold(self) -> VoxelGridModule:
+        """
+        Creates object to become self.voxel_grid_scaffold:
+            -  makes `self.voxel_grid_scaffold` have same world to local mapping as
+                    `self.voxel_grid_density`
+        """
+        return VoxelGridModule(
+            # pyre-ignore[29]
+            extents=self.voxel_grid_density_args["extents"],
+            # pyre-ignore[29]
+            translation=self.voxel_grid_density_args["translation"],
+            voxel_grid_class_type="FullResolutionVoxelGrid",
+            hold_voxel_grid_as_parameters=False,
+            voxel_grid_FullResolutionVoxelGrid_args={
+                "resolution_changes": {0: self.scaffold_resolution},
+                "padding": "zeros",
+                "align_corners": True,
+                "mode": "trilinear",
+            },
+        )

tests/implicitron/test_voxel_grid_implicit_function.py

@@ -0,0 +1,227 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+
+import unittest
+
+import torch
+
+from omegaconf import DictConfig, OmegaConf
+from pytorch3d.implicitron.models.implicit_function.voxel_grid_implicit_function import (
+    VoxelGridImplicitFunction,
+)
+from pytorch3d.implicitron.models.renderer.base import ImplicitronRayBundle
+
+from pytorch3d.implicitron.tools.config import expand_args_fields, get_default_args
+from pytorch3d.renderer import ray_bundle_to_ray_points
+from tests.common_testing import TestCaseMixin
+
+
+class TestVoxelGridImplicitFunction(TestCaseMixin, unittest.TestCase):
+    def setUp(self) -> None:
+        torch.manual_seed(42)
+        expand_args_fields(VoxelGridImplicitFunction)
+
+    def _get_simple_implicit_function(self, scaffold_res=16):
+        default_cfg = get_default_args(VoxelGridImplicitFunction)
+        custom_cfg = DictConfig(
+            {
+                "voxel_grid_density_args": {
+                    "voxel_grid_FullResolutionVoxelGrid_args": {"n_features": 7}
+                },
+                "decoder_density_class_type": "ElementwiseDecoder",
+                "decoder_color_class_type": "MLPDecoder",
+                "decoder_color_MLPDecoder_args": {
+                    "network_args": {
+                        "n_layers": 2,
+                        "output_dim": 3,
+                        "hidden_dim": 128,
+                    }
+                },
+                "scaffold_resolution": (scaffold_res, scaffold_res, scaffold_res),
+            }
+        )
+        cfg = OmegaConf.merge(default_cfg, custom_cfg)
+        return VoxelGridImplicitFunction(**cfg)
+
+    def test_forward(self) -> None:
+        """
+        Test one forward of VoxelGridImplicitFunction.
+        """
+        func = self._get_simple_implicit_function()
+
+        n_grids, n_points = 10, 9
+        raybundle = ImplicitronRayBundle(
+            origins=torch.randn(n_grids, 2, 3, 3),
+            directions=torch.randn(n_grids, 2, 3, 3),
+            lengths=torch.randn(n_grids, 2, 3, n_points),
+            xys=0,
+        )
+        func(raybundle)
+
+    def test_scaffold_formation(self):
+        """
+        Test calculating the scaffold.
+
+        We define a custom density function and make the implicit function use it
+        After calculating the scaffold we compare the density of our custom
+        density function with densities from the scaffold.
+        """
+        device = "cuda" if torch.cuda.is_available() else "cpu"
+        func = self._get_simple_implicit_function().to(device)
+        func.scaffold_max_pool_kernel_size = 1
+
+        def new_density(points):
+            """
+            Density function which returns 1 if p>(0.5, 0.5, 0.5) or
+            p < (-0.5, -0.5, -0.5) else 0
+            """
+            inshape = points.shape
+            points = points.view(-1, 3)
+            out = []
+            for p in points:
+                if torch.all(p > 0.5) or torch.all(p < -0.5):
+                    out.append(torch.tensor([[1.0]]))
+                else:
+                    out.append(torch.tensor([[0.0]]))
+            return torch.cat(out).view(*inshape[:-1], 1).to(device)
+
+        func.get_density = new_density
+        func._get_scaffold(0)
+
+        points = torch.tensor(
+            [
+                [0, 0, 0],
+                [1, 1, 1],
+                [1, 0, 0],
+                [0.1, 0, 0],
+                [10, 1, -1],
+                [-0.8, -0.7, -0.9],
+            ]
+        ).to(device)
+        expected = new_density(points).float().to(device)
+        assert torch.allclose(func.voxel_grid_scaffold(points), expected), (
+            func.voxel_grid_scaffold(points),
+            expected,
+        )
+
+    def test_scaffold_filtering(self, n_test_points=100):
+        """
+        Test that filtering points with scaffold works.
+
+        We define a scaffold and make the implicit function use it. We also
+        define new density and color functions which check that all passed
+        points are not in empty space (with scaffold function). In the end
+        we compare the result from the implicit function with one calculated
+        simple python, this checks that the points were merged correectly.
+        """
+        device = "cuda"
+        func = self._get_simple_implicit_function().to(device)
+
+        def scaffold(points):
+            """'
+            Function to deterministically and randomly enough assign a point
+            to empty or occupied space.
+            Return 1 if second digit of sum after 0 is odd else 0
+            """
+            return (
+                ((points.sum(dim=-1, keepdim=True) * 10**2 % 10).long() % 2) == 1
+            ).float()
+
+        def new_density(points):
+            # check if all passed points should be passed here
+            assert torch.all(scaffold(points)), (scaffold(points), points.shape)
+            return points.sum(dim=-1, keepdim=True)
+
+        def new_color(points, camera, directions):
+            # check if all passed points should be passed here
+            assert torch.all(scaffold(points))  # , (scaffold(points), points)
+            return points * 2
+
+        # check both computation paths that they contain only points
+        # which are not in empty space
+        func.get_density = new_density
+        func.get_color = new_color
+        func.voxel_grid_scaffold.forward = scaffold
+        func._scaffold_ready = True
+
+        bundle = ImplicitronRayBundle(
+            origins=torch.rand((n_test_points, 2, 1, 3), device=device),
+            directions=torch.rand((n_test_points, 2, 1, 3), device=device),
+            lengths=torch.rand((n_test_points, 2, 1, 4), device=device),
+            xys=None,
+        )
+        points = ray_bundle_to_ray_points(bundle)
+        result_density, result_color, _ = func(bundle)
+
+        # construct the wanted result 'by hand'
+        flat_points = points.view(-1, 3)
+        expected_result_density, expected_result_color = [], []
+        for point in flat_points:
+            if scaffold(point) == 1:
+                expected_result_density.append(point.sum(dim=-1, keepdim=True))
+                expected_result_color.append(point * 2)
+            else:
+                expected_result_density.append(point.new_zeros((1,)))
+                expected_result_color.append(point.new_zeros((3,)))
+        expected_result_density = torch.stack(expected_result_density, dim=0).view(
+            *points.shape[:-1], 1
+        )
+        expected_result_color = torch.stack(expected_result_color, dim=0).view(
+            *points.shape[:-1], 3
+        )
+
+        # check that thre result is expected
+        assert torch.allclose(result_density, expected_result_density), (
+            result_density,
+            expected_result_density,
+        )
+        assert torch.allclose(result_color, expected_result_color), (
+            result_color,
+            expected_result_color,
+        )
+
+    def test_cropping(self, scaffold_res=9):
+        """
+        Tests whether implicit function finds the bounding box of the object and sends
+        correct min and max points to voxel grids for rescaling.
+        """
+        device = "cuda" if torch.cuda.is_available() else "cpu"
+        func = self._get_simple_implicit_function(scaffold_res=scaffold_res).to(device)
+
+        assert scaffold_res >= 8
+        div = (scaffold_res - 1) / 2
+        true_min_point = torch.tensor(
+            [-3 / div, 0 / div, -3 / div],
+            device=device,
+        )
+        true_max_point = torch.tensor(
+            [1 / div, 2 / div, 3 / div],
+            device=device,
+        )
+
+        def new_scaffold(points):
+            # 1 if between true_min and true_max point else 0
+            # return points.new_ones((*points.shape[:-1], 1))
+            return (
+                torch.logical_and(true_min_point <= points, points <= true_max_point)
+                .all(dim=-1)
+                .float()[..., None]
+            )
+
+        called_crop = []
+
+        def assert_min_max_points(min_point, max_point):
+            called_crop.append(1)
+            self.assertClose(min_point, true_min_point)
+            self.assertClose(max_point, true_max_point)
+
+        func.voxel_grid_density.crop_self = assert_min_max_points
+        func.voxel_grid_color.crop_self = assert_min_max_points
+        func.voxel_grid_scaffold.forward = new_scaffold
+        func._scaffold_ready = True
+        func._crop(epoch=0)
+        assert len(called_crop) == 2