Raymarching

Summary: Implements two basic raymarchers.

Reviewed By: gkioxari

Differential Revision: D24064250

fbshipit-source-id: 18071bd039995336b7410caa403ea29fafb5c66f

Author: davnov134

pytorch3d/renderer/__init__.py

@@ -20,6 +20,7 @@
     look_at_rotation,
     look_at_view_transform,
 )
+from .implicit import AbsorptionOnlyRaymarcher, EmissionAbsorptionRaymarcher
 from .lighting import DirectionalLights, PointLights, diffuse, specular
 from .materials import Materials
 from .mesh import (

pytorch3d/renderer/implicit/__init__.py

@@ -0,0 +1,6 @@
+# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
+
+from .raymarching import AbsorptionOnlyRaymarcher, EmissionAbsorptionRaymarcher
+
+
+__all__ = [k for k in globals().keys() if not k.startswith("_")]

pytorch3d/renderer/implicit/raymarching.py

@@ -0,0 +1,223 @@
+# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
+import warnings
+from typing import Optional, Tuple, Union
+
+import torch
+
+
+class EmissionAbsorptionRaymarcher(torch.nn.Module):
+    """
+    Raymarch using the Emission-Absorption (EA) algorithm.
+
+    The algorithm independently renders each ray by analyzing density and
+    feature values sampled at (typically uniformly) spaced 3D locations along
+    each ray. The density values `rays_densities` are of shape
+    `(..., n_points_per_ray)`, their values should range between [0, 1], and
+    represent the opaqueness of each point (the higher the less transparent).
+    The feature values `rays_features` of shape
+    `(..., n_points_per_ray, feature_dim)` represent the content of the
+    point that is supposed to be rendered in case the given point is opaque
+    (i.e. its density -> 1.0).
+
+    EA first utilizes `rays_densities` to compute the absorption function
+    along each ray as follows:
+        ```
+        absorption = cumprod(1 - rays_densities, dim=-1)
+        ```
+    The value of absorption at position `absorption[..., k]` specifies
+    how much light has reached `k`-th point along a ray since starting
+    its trajectory at `k=0`-th point.
+
+    Each ray is then rendered into a tensor `features` of shape `(..., feature_dim)`
+    by taking a weighed combination of per-ray features `rays_features` as follows:
+        ```
+        weights = absorption * rays_densities
+        features = (rays_features * weights).sum(dim=-2)
+        ```
+    Where `weights` denote a function that has a strong peak around the location
+    of the first surface point that a given ray passes through.
+
+    Note that for a perfectly bounded volume (with a strictly binary density),
+    the `weights = cumprod(1 - rays_densities, dim=-1) * rays_densities`
+    function would yield 0 everywhere. In order to prevent this,
+    the result of the cumulative product is shifted `self.surface_thickness`
+    elements along the ray direction.
+    """
+
+    def __init__(self, surface_thickness: int = 1):
+        """
+        Args:
+            surface_thickness: Denotes the overlap between the absorption
+                function and the density function.
+        """
+        super().__init__()
+        self.surface_thickness = surface_thickness
+
+    def forward(
+        self,
+        rays_densities: torch.Tensor,
+        rays_features: torch.Tensor,
+        eps: float = 1e-10,
+        **kwargs,
+    ) -> torch.Tensor:
+        """
+        Args:
+            rays_densities: Per-ray density values represented with a tensor
+                of shape `(..., n_points_per_ray, 1)` whose values range in [0, 1].
+            rays_features: Per-ray feature values represented with a tensor
+                of shape `(..., n_points_per_ray, feature_dim)`.
+            eps: A lower bound added to `rays_densities` before computing
+                the absorbtion function (cumprod of `1-rays_densities` along
+                each ray). This prevents the cumprod to yield exact 0
+                which would inhibit any gradient-based learning.
+
+        Returns:
+            features_opacities: A tensor of shape `(..., feature_dim+1)`
+                that concatenates two tensors alonng the last dimension:
+                    1) features: A tensor of per-ray renders
+                        of shape `(..., feature_dim)`.
+                    2) opacities: A tensor of per-ray opacity values
+                        of shape `(..., 1)`. Its values range between [0, 1] and
+                        denote the total amount of light that has been absorbed
+                        for each ray. E.g. a value of 0 corresponds to the ray
+                        completely passing through a volume. Please refer to the
+                        `AbsorptionOnlyRaymarcher` documentation for the
+                        explanation of the algorithm that computes `opacities`.
+        """
+        _check_raymarcher_inputs(
+            rays_densities,
+            rays_features,
+            None,
+            z_can_be_none=True,
+            features_can_be_none=False,
+            density_1d=True,
+        )
+        _check_density_bounds(rays_densities)
+        rays_densities = rays_densities[..., 0]
+        absorption = _shifted_cumprod(
+            (1.0 + eps) - rays_densities, shift=self.surface_thickness
+        )
+        weights = rays_densities * absorption
+        features = (weights[..., None] * rays_features).sum(dim=-2)
+        opacities = 1.0 - torch.prod(1.0 - rays_densities, dim=-1, keepdim=True)
+
+        return torch.cat((features, opacities), dim=-1)
+
+
+class AbsorptionOnlyRaymarcher(torch.nn.Module):
+    """
+    Raymarch using the Absorption-Only (AO) algorithm.
+
+    The algorithm independently renders each ray by analyzing density and
+    feature values sampled at (typically uniformly) spaced 3D locations along
+    each ray. The density values `rays_densities` are of shape
+    `(..., n_points_per_ray, 1)`, their values should range between [0, 1], and
+    represent the opaqueness of each point (the higher the less transparent).
+    The algorithm only measures the total amount of light absorbed along each ray
+    and, besides outputting per-ray `opacity` values of shape `(...,)`,
+    does not produce any feature renderings.
+
+    The algorithm simply computes `total_transmission = prod(1 - rays_densities)`
+    of shape `(..., 1)` which, for each ray, measures the total amount of light
+    that passed through the volume.
+    It then returns `opacities = 1 - total_transmission`.
+    """
+
+    def __init__(self):
+        super().__init__()
+
+    def forward(
+        self, rays_densities: torch.Tensor, **kwargs
+    ) -> Union[None, torch.Tensor]:
+        """
+        Args:
+            rays_densities: Per-ray density values represented with a tensor
+                of shape `(..., n_points_per_ray)` whose values range in [0, 1].
+
+        Returns:
+            opacities: A tensor of per-ray opacity values of shape `(..., 1)`.
+                Its values range between [0, 1] and denote the total amount
+                of light that has been absorbed for each ray. E.g. a value
+                of 0 corresponds to the ray completely passing through a volume.
+        """
+
+        _check_raymarcher_inputs(
+            rays_densities,
+            None,
+            None,
+            features_can_be_none=True,
+            z_can_be_none=True,
+            density_1d=True,
+        )
+        rays_densities = rays_densities[..., 0]
+        _check_density_bounds(rays_densities)
+        total_transmission = torch.prod(1 - rays_densities, dim=-1, keepdim=True)
+        opacities = 1.0 - total_transmission
+        return opacities
+
+
+def _shifted_cumprod(x, shift=1):
+    """
+    Computes `torch.cumprod(x, dim=-1)` and prepends `shift` number of
+    ones and removes `shift` trailing elements to/from the last dimension
+    of the result.
+    """
+    x_cumprod = torch.cumprod(x, dim=-1)
+    x_cumprod_shift = torch.cat(
+        [torch.ones_like(x_cumprod[..., :shift]), x_cumprod[..., :-shift]], dim=-1
+    )
+    return x_cumprod_shift
+
+
+def _check_density_bounds(
+    rays_densities: torch.Tensor, bounds: Tuple[float, float] = (0.0, 1.0)
+):
+    """
+    Checks whether the elements of `rays_densities` range within `bounds`.
+    If not issues a warning.
+    """
+    if ((rays_densities > bounds[1]) | (rays_densities < bounds[0])).any():
+        warnings.warn(
+            "One or more elements of rays_densities are outside of valid"
+            + f"range {str(bounds)}"
+        )
+
+
+def _check_raymarcher_inputs(
+    rays_densities: torch.Tensor,
+    rays_features: Optional[torch.Tensor],
+    rays_z: Optional[torch.Tensor],
+    features_can_be_none: bool = False,
+    z_can_be_none: bool = False,
+    density_1d: bool = True,
+):
+    """
+    Checks the validity of the inputs to raymarching algorithms.
+    """
+    if not torch.is_tensor(rays_densities):
+        raise ValueError("rays_densities has to be an instance of torch.Tensor.")
+
+    if not z_can_be_none and not torch.is_tensor(rays_z):
+        raise ValueError("rays_z has to be an instance of torch.Tensor.")
+
+    if not features_can_be_none and not torch.is_tensor(rays_features):
+        raise ValueError("rays_features has to be an instance of torch.Tensor.")
+
+    if rays_densities.ndim < 1:
+        raise ValueError("rays_densities have to have at least one dimension.")
+
+    if density_1d and rays_densities.shape[-1] != 1:
+        raise ValueError(
+            "The size of the last dimension of rays_densities has to be one."
+        )
+
+    rays_shape = rays_densities.shape[:-1]
+
+    if not z_can_be_none and rays_z.shape != rays_shape:
+        raise ValueError("rays_z have to be of the same shape as rays_densities.")
+
+    if not features_can_be_none and rays_features.shape[:-1] != rays_shape:
+        raise ValueError(
+            "The first to previous to last dimensions of rays_features"
+            " have to be the same as all dimensions of rays_densities."
+        )

tests/bm_raymarching.py

@@ -0,0 +1,19 @@
+# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
+
+import itertools
+
+from fvcore.common.benchmark import benchmark
+from pytorch3d.renderer import AbsorptionOnlyRaymarcher, EmissionAbsorptionRaymarcher
+from test_raymarching import TestRaymarching
+
+
+def bm_raymarching() -> None:
+    case_grid = {
+        "raymarcher_type": [EmissionAbsorptionRaymarcher, AbsorptionOnlyRaymarcher],
+        "n_rays": [10, 1000, 10000],
+        "n_pts_per_ray": [10, 1000, 10000],
+    }
+    test_cases = itertools.product(*case_grid.values())
+    kwargs_list = [dict(zip(case_grid.keys(), case)) for case in test_cases]
+
+    benchmark(TestRaymarching.raymarcher, "RAYMARCHER", kwargs_list, warmup_iters=1)