Adapt RayPointRefiner and RayMarcher to support bins.

Summary:
## Context

Bins are used in mipnerf to allow to manipulate easily intervals. For example, by doing the following, `bins[..., :-1]` you will obtain all the left coordinates of your intervals, while doing `bins[..., 1:]` is equals to the right coordinates of your intervals.

We introduce here the support of bins like in MipNerf implementation.

## RayPointRefiner

Small changes have been made to modify RayPointRefiner.
- If bins is None

```
mids = torch.lerp(ray_bundle.lengths[..., 1:], ray_bundle.lengths[…, :-1], 0.5)
z_samples = sample_pdf(
		mids, # [..., npt]
		weights[..., 1:-1], # [..., npt - 1]
               ….
            )
```

- If bins is not None
In the MipNerf implementation the sampling is done on all the bins. It allows us to use the full weights tensor without slashing it.

```
z_samples = sample_pdf(
		ray_bundle.bins, # [..., npt + 1]
		weights, # [..., npt]
               ...
            )
```

## RayMarcher

Add a ray_deltas optional argument. If None, keep the same deltas computation from ray_lengths.

Reviewed By: shapovalov

Differential Revision: D46389092

fbshipit-source-id: d4f1963310065bd31c1c7fac1adfe11cbeaba606

Author: EmGarr

Diff for: pytorch3d/implicitron/models/renderer/multipass_ea.py

@@ -157,9 +157,13 @@ def _run_raymarcher(
             else 0.0
         )
 
+        ray_deltas = (
+            None if ray_bundle.bins is None else torch.diff(ray_bundle.bins, dim=-1)
+        )
         output = self.raymarcher(
             *implicit_functions[0](ray_bundle=ray_bundle),
             ray_lengths=ray_bundle.lengths,
+            ray_deltas=ray_deltas,
             density_noise_std=density_noise_std,
         )
         output.prev_stage = prev_stage

Diff for: pytorch3d/implicitron/models/renderer/ray_point_refiner.py

@@ -78,29 +78,42 @@ def forward(
 
         """
 
-        z_vals = input_ray_bundle.lengths
         with torch.no_grad():
             if self.blurpool_weights:
                 ray_weights = apply_blurpool_on_weights(ray_weights)
 
-            z_vals_mid = torch.lerp(z_vals[..., 1:], z_vals[..., :-1], 0.5)
+            n_pts_per_ray = self.n_pts_per_ray
+            ray_weights = ray_weights.view(-1, ray_weights.shape[-1])
+            if input_ray_bundle.bins is None:
+                z_vals: torch.Tensor = input_ray_bundle.lengths
+                ray_weights = ray_weights[..., 1:-1]
+                bins = torch.lerp(z_vals[..., 1:], z_vals[..., :-1], 0.5)
+            else:
+                z_vals = input_ray_bundle.bins
+                n_pts_per_ray += 1
+                bins = z_vals
             z_samples = sample_pdf(
-                z_vals_mid.view(-1, z_vals_mid.shape[-1]),
-                ray_weights.view(-1, ray_weights.shape[-1])[..., 1:-1],
-                self.n_pts_per_ray,
+                bins.view(-1, bins.shape[-1]),
+                ray_weights,
+                n_pts_per_ray,
                 det=not self.random_sampling,
                 eps=self.sample_pdf_eps,
-            ).view(*z_vals.shape[:-1], self.n_pts_per_ray)
+            ).view(*z_vals.shape[:-1], n_pts_per_ray)
+
         if self.add_input_samples:
             z_vals = torch.cat((z_vals, z_samples), dim=-1)
         else:
             z_vals = z_samples
         # Resort by depth.
         z_vals, _ = torch.sort(z_vals, dim=-1)
 
-        new_bundle = ImplicitronRayBundle(**vars(input_ray_bundle))
-        new_bundle.lengths = z_vals
-        return new_bundle
+        kwargs_ray = dict(vars(input_ray_bundle))
+        if input_ray_bundle.bins is None:
+            kwargs_ray["lengths"] = z_vals
+            return ImplicitronRayBundle(**kwargs_ray)
+        kwargs_ray["bins"] = z_vals
+        del kwargs_ray["lengths"]
+        return ImplicitronRayBundle.from_bins(**kwargs_ray)
 
 
 def apply_blurpool_on_weights(weights) -> torch.Tensor:

Diff for: pytorch3d/implicitron/models/renderer/raymarcher.py

@@ -4,7 +4,7 @@
 # This source code is licensed under the BSD-style license found in the
 # LICENSE file in the root directory of this source tree.
 
-from typing import Any, Callable, Dict, Tuple
+from typing import Any, Callable, Dict, Optional, Tuple
 
 import torch
 from pytorch3d.implicitron.models.renderer.base import RendererOutput
@@ -119,6 +119,7 @@ def forward(
         rays_features: torch.Tensor,
         aux: Dict[str, Any],
         ray_lengths: torch.Tensor,
+        ray_deltas: Optional[torch.Tensor] = None,
         density_noise_std: float = 0.0,
         **kwargs,
     ) -> RendererOutput:
@@ -131,6 +132,9 @@ def forward(
             aux: a dictionary with extra information.
             ray_lengths: Per-ray depth values represented with a tensor
                 of shape `(..., n_points_per_ray, feature_dim)`.
+            ray_deltas: Optional differences between consecutive elements along the ray bundle
+                represented with a tensor of shape `(..., n_points_per_ray)`. If None,
+                these differences are computed from ray_lengths.
             density_noise_std: the magnitude of the noise added to densities.
 
         Returns:
@@ -152,14 +156,17 @@ def forward(
             density_1d=True,
         )
 
-        ray_lengths_diffs = ray_lengths[..., 1:] - ray_lengths[..., :-1]
-        if self.replicate_last_interval:
-            last_interval = ray_lengths_diffs[..., -1:]
+        if ray_deltas is None:
+            ray_lengths_diffs = torch.diff(ray_lengths, dim=-1)
+            if self.replicate_last_interval:
+                last_interval = ray_lengths_diffs[..., -1:]
+            else:
+                last_interval = torch.full_like(
+                    ray_lengths[..., :1], self.background_opacity
+                )
+            deltas = torch.cat((ray_lengths_diffs, last_interval), dim=-1)
         else:
-            last_interval = torch.full_like(
-                ray_lengths[..., :1], self.background_opacity
-            )
-        deltas = torch.cat((ray_lengths_diffs, last_interval), dim=-1)
+            deltas = ray_deltas
 
         rays_densities = rays_densities[..., 0]
 

Diff for: pytorch3d/renderer/implicit/harmonic_embedding.py

@@ -24,7 +24,7 @@ def __init__(
         and the integrated position encoding in
         `MIP-NeRF <https://arxiv.org/abs/2103.13415>`_.
 
-        During, the inference you can provide the extra argument `diag_cov`.
+        During the inference you can provide the extra argument `diag_cov`.
 
         If `diag_cov is None`, it converts
         rays parametrized with a `ray_bundle` to 3D points by

Diff for: tests/implicitron/test_ray_point_refiner.py

@@ -70,6 +70,71 @@ def test_simple(self):
                 (lengths_random[..., 1:] - lengths_random[..., :-1] > 0).all()
             )
 
+    def test_simple_use_bins(self):
+        """
+        Same spirit than test_simple but use bins in the ImplicitronRayBunle.
+        It has been duplicated to avoid cognitive overload while reading the
+        test (lot of if else).
+        """
+        length = 15
+        n_pts_per_ray = 10
+
+        for add_input_samples, use_blurpool in product([False, True], [False, True]):
+            ray_point_refiner = RayPointRefiner(
+                n_pts_per_ray=n_pts_per_ray,
+                random_sampling=False,
+                add_input_samples=add_input_samples,
+            )
+
+            bundle = ImplicitronRayBundle(
+                lengths=None,
+                bins=torch.arange(length + 1, dtype=torch.float32).expand(
+                    3, 25, length + 1
+                ),
+                origins=None,
+                directions=None,
+                xys=None,
+                camera_ids=None,
+                camera_counts=None,
+            )
+            weights = torch.ones(3, 25, length)
+            refined = ray_point_refiner(bundle, weights, blurpool_weights=use_blurpool)
+
+            self.assertIsNone(refined.directions)
+            self.assertIsNone(refined.origins)
+            self.assertIsNone(refined.xys)
+            expected_bins = torch.linspace(0, length, n_pts_per_ray + 1)
+            expected_bins = expected_bins.expand(3, 25, n_pts_per_ray + 1)
+            if add_input_samples:
+                expected_bins = torch.cat((bundle.bins, expected_bins), dim=-1).sort()[
+                    0
+                ]
+            full_expected = torch.lerp(
+                expected_bins[..., :-1], expected_bins[..., 1:], 0.5
+            )
+
+            self.assertClose(refined.lengths, full_expected)
+
+            ray_point_refiner_random = RayPointRefiner(
+                n_pts_per_ray=n_pts_per_ray,
+                random_sampling=True,
+                add_input_samples=add_input_samples,
+            )
+
+            refined_random = ray_point_refiner_random(
+                bundle, weights, blurpool_weights=use_blurpool
+            )
+            lengths_random = refined_random.lengths
+            self.assertEqual(lengths_random.shape, full_expected.shape)
+            if not add_input_samples:
+                self.assertGreater(lengths_random.min().item(), 0)
+                self.assertLess(lengths_random.max().item(), length)
+
+            # Check sorted
+            self.assertTrue(
+                (lengths_random[..., 1:] - lengths_random[..., :-1] > 0).all()
+            )
+
     def test_apply_blurpool_on_weights(self):
         weights = torch.tensor(
             [