Adding reference and context to PointsRenderer

Summary: User confusion (#1579) about how zbuf is used for alpha compositing. Added small description and reference to paper to help give some context.

Reviewed By: bottler

Differential Revision: D51374933

fbshipit-source-id: 8c489a5b5d0a81f0d936c1348b9ade6787c39c9a

Author: dsjohns2

pytorch3d/renderer/points/rasterizer.py

@@ -16,8 +16,30 @@
 from .rasterize_points import rasterize_points
 
 
-# Class to store the outputs of point rasterization
 class PointFragments(NamedTuple):
+    """
+    Class to store the outputs of point rasterization
+
+    Members:
+        idx: int32 Tensor of shape (N, image_size, image_size, points_per_pixel)
+            giving the indices of the nearest points at each pixel, in ascending
+            z-order. Concretely `idx[n, y, x, k] = p` means that `points[p]` is the kth
+            closest point (along the z-direction) to pixel (y, x) - note that points
+            represents the packed points of shape (P, 3).
+            Pixels that are hit by fewer than points_per_pixel are padded with -1.
+        zbuf: Tensor of shape (N, image_size, image_size, points_per_pixel)
+            giving the z-coordinates of the nearest points at each pixel, sorted in
+            z-order. Concretely, if `idx[n, y, x, k] = p` then
+            `zbuf[n, y, x, k] = points[n, p, 2]`. Pixels hit by fewer than
+            points_per_pixel are padded with -1.
+        dists: Tensor of shape (N, image_size, image_size, points_per_pixel)
+            giving the squared Euclidean distance (in NDC units) in the x/y plane
+            for each point closest to the pixel. Concretely if `idx[n, y, x, k] = p`
+            then `dists[n, y, x, k]` is the squared distance between the pixel (y, x)
+            and the point `(points[n, p, 0], points[n, p, 1])`. Pixels hit with fewer
+            than points_per_pixel are padded with -1.
+    """
+
     idx: torch.Tensor
     zbuf: torch.Tensor
     dists: torch.Tensor

pytorch3d/renderer/points/renderer.py

@@ -30,6 +30,12 @@ class PointsRenderer(nn.Module):
     A class for rendering a batch of points. The class should
     be initialized with a rasterizer and compositor class which each have a forward
     function.
+
+    The points are rendered with with varying alpha (weights) values depending on
+    the distance of the pixel center to the true point in the xy plane. The purpose
+    of this is to soften the hard decision boundary, for differentiability.
+    See Section 3.2 of "SynSin: End-to-end View Synthesis from a Single Image"
+    (https://arxiv.org/pdf/1912.08804.pdf) for more details.
     """
 
     def __init__(self, rasterizer, compositor) -> None: