From eb517dd70b20c8431a1cfc46e47b4c5c78529c5b Mon Sep 17 00:00:00 2001
From: Amitav Baruah <amitav@fb.com>
Date: Thu, 10 Sep 2020 12:52:13 -0700
Subject: [PATCH] Fix look_at corner case

Summary: When the camera is vertically oriented, calculating the look_at x-axis (also known as the "right" vector) does not succeed, resulting in the x-axis being placed at the origin. Adds a check to correctly calculate the x-axis if this case occurs.

Reviewed By: nikhilaravi, sbranson

Differential Revision: D23511859

fbshipit-source-id: ee5145cdbecdbe2f7c7d288588bd0899480cb327
---
 pytorch3d/renderer/cameras.py |  6 ++++++
 tests/test_cameras.py         | 14 ++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/pytorch3d/renderer/cameras.py b/pytorch3d/renderer/cameras.py
index 8ea5d4d3..3549b355 100644
--- a/pytorch3d/renderer/cameras.py
+++ b/pytorch3d/renderer/cameras.py
@@ -1226,6 +1226,12 @@ def look_at_rotation(
     z_axis = F.normalize(at - camera_position, eps=1e-5)
     x_axis = F.normalize(torch.cross(up, z_axis, dim=1), eps=1e-5)
     y_axis = F.normalize(torch.cross(z_axis, x_axis, dim=1), eps=1e-5)
+    is_close = torch.isclose(x_axis, torch.tensor(0.0), atol=5e-3).all(
+        dim=1, keepdim=True
+    )
+    if is_close.any():
+        replacement = F.normalize(torch.cross(y_axis, z_axis, dim=1), eps=1e-5)
+        x_axis = torch.where(is_close, replacement, x_axis)
     R = torch.cat((x_axis[:, None, :], y_axis[:, None, :], z_axis[:, None, :]), dim=1)
     return R.transpose(1, 2)
 
diff --git a/tests/test_cameras.py b/tests/test_cameras.py
index b74cd74d..298301d2 100644
--- a/tests/test_cameras.py
+++ b/tests/test_cameras.py
@@ -433,6 +433,20 @@ class TestCameraHelpers(TestCaseMixin, unittest.TestCase):
         RT = get_world_to_view_transform(R=R, T=T)
         self.assertTrue(isinstance(RT, Transform3d))
 
+    def test_look_at_view_transform_corner_case(self):
+        dist = 2.7
+        elev = 90
+        azim = 90
+        expected_position = torch.tensor([0.0, 2.7, 0.0], dtype=torch.float32).view(
+            1, 3
+        )
+        position = camera_position_from_spherical_angles(dist, elev, azim)
+        self.assertClose(position, expected_position, atol=2e-7)
+        R, _ = look_at_view_transform(eye=position)
+        x_axis = R[:, :, 0]
+        expected_x_axis = torch.tensor([0.0, 0.0, -1.0], dtype=torch.float32).view(1, 3)
+        self.assertClose(x_axis, expected_x_axis, atol=5e-3)
+
 
 class TestCamerasCommon(TestCaseMixin, unittest.TestCase):
     def test_view_transform_class_method(self):