AABB Conflict FIx

[Anthony-Tafoya]

This PR relates to issue #3405. Credit to hoanle for finding the issue.

Background:
OccGridEstimator uses different levels of AABBs to determine where to sample the points from with varying levels of detail.

        # multiple levels of aabbs
        aabbs = torch.stack(
            [_enlarge_aabb(roi_aabb, 2**i) for i in range(levels)], dim=0
        )

But those samples outside the original AABB are not used anyway if we disable the scene contraction:

        if self.spatial_distortion is not None:
            positions = ray_samples.frustums.get_positions()
            positions = self.spatial_distortion(positions)
            positions = (positions + 2.0) / 4.0
        else:
            positions = SceneBox.get_normalized_positions(
                ray_samples.frustums.get_positions(), self.aabb
            )

Since the positions (if distortion is disabled) are taken from a normalized standpoint until the self.aabb parameter is passed in.

Problem:
The problem is that if we begin with the scene_aabb, the outermost AABB, we will not use any of the other resolutions in our enlargement calculations. Therefore, we need to start from the innermost AABB depending on the number of levels.

[jb-ye]

would you want to rebase your PR?

[Anthony-Tafoya]

would you want to rebase your PR?

I can rebase no worries

[Anthony-Tafoya]

I am unsure at the moment of how to test this properly, so open to thoughts. Will look at other PR's and try to create something

[hoanhle]

I'm pretty sure that this would uniformly shrink or grow the scene_aabb from the origin, without considering the center.

A correct approach would be to first implement something like this in SceneBox

    def get_scaled_scene_box(self, scale_factor: Union[float, torch.Tensor] = 1.5):
        """Returns a new box that has been scaled by the given factor while
        maintaining its center.

        Args:
            scale_factor: How much to scale the bounding box by.

        Returns:
            SceneBox: A new SceneBox with the scaled bounding box.
        """
        center = self.get_center()
        scaled_aabb = (self.aabb - center) * scale_factor + center
        return SceneBox(aabb=scaled_aabb)

and then , call this function where needed.

[hoanhle]

It would be great if you could include this. I can review the PR once it's up. Also, I think it would be a good idea to disable scene contraction by default in instant-ngp.