Add PyTorch3D->OpenCV camera parameter conversion.

Summary: This diff implements the inverse of D28992470 (https://github.com/facebookresearch/pytorch3d/commit/8006842f2a5ab1546a90797a6394f875adce045c): a function to extract OpenCV convention camera parameters from a PyTorch3D `PerspectiveCameras` object. This is the first part of the new PyTorch3d<>OpenCV<>Pulsar conversion functions.

Reviewed By: patricklabatut

Differential Revision: D29278411

fbshipit-source-id: 68d4555b508dbe8685d8239443f839d194cc2484

pytorch3d/utils/__init__.py

@@ -4,7 +4,10 @@
 # This source code is licensed under the BSD-style license found in the
 # LICENSE file in the root directory of this source tree.
 
-from .camera_conversions import cameras_from_opencv_projection
+from .camera_conversions import (
+    cameras_from_opencv_projection,
+    opencv_from_cameras_projection,
+)
 from .ico_sphere import ico_sphere
 from .torus import torus
 

pytorch3d/utils/camera_conversions.py

@@ -4,10 +4,12 @@
 # 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 Tuple
+
 import torch
 
 from ..renderer import PerspectiveCameras
-from ..transforms import so3_exponential_map
+from ..transforms import so3_exponential_map, so3_log_map
 
 
 def cameras_from_opencv_projection(
@@ -35,7 +37,7 @@ def cameras_from_opencv_projection(
     followed by the homogenization of `x_screen_opencv`.
 
     Note:
-        The parameters `rvec, tvec, camera_matrix` correspond e.g. to the inputs
+        The parameters `rvec, tvec, camera_matrix` correspond, e.g., to the inputs
         of `cv2.projectPoints`, or to the ouputs of `cv2.calibrateCamera`.
 
     Args:
@@ -74,3 +76,51 @@ def cameras_from_opencv_projection(
         focal_length=focal_pytorch3d,
         principal_point=p0_pytorch3d,
     )
+
+
+def opencv_from_cameras_projection(
+    cameras: PerspectiveCameras,
+    image_size: torch.Tensor,
+) -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor]:
+    """
+    Converts a batch of `PerspectiveCameras` into OpenCV-convention
+    axis-angle rotation vectors `rvec`, translation vectors `tvec`, and the camera
+    calibration matrices `camera_matrix`. This operation is exactly the inverse
+    of `cameras_from_opencv_projection`.
+
+    Note:
+        The parameters `rvec, tvec, camera_matrix` correspond, e.g., to the inputs
+        of `cv2.projectPoints`, or to the ouputs of `cv2.calibrateCamera`.
+
+    Args:
+        cameras: A batch of `N` cameras in the PyTorch3D convention.
+        image_size: A tensor of shape `(N, 2)` containing the sizes of the images
+            (height, width) attached to each camera.
+
+    Returns:
+        rvec: A batch of axis-angle rotation vectors of shape `(N, 3)`.
+        tvec: A batch of translation vectors of shape `(N, 3)`.
+        camera_matrix: A batch of camera calibration matrices of shape `(N, 3, 3)`.
+    """
+    R_pytorch3d = cameras.R
+    T_pytorch3d = cameras.T
+    focal_pytorch3d = cameras.focal_length
+    p0_pytorch3d = cameras.principal_point
+    T_pytorch3d[:, :2] *= -1  # pyre-ignore
+    R_pytorch3d[:, :, :2] *= -1  # pyre-ignore
+    tvec = T_pytorch3d.clone()  # pyre-ignore
+    R = R_pytorch3d.permute(0, 2, 1)  # pyre-ignore
+
+    # Retype the image_size correctly and flip to width, height.
+    image_size_wh = image_size.to(R).flip(dims=(1,))
+
+    principal_point = (-p0_pytorch3d + 1.0) * (0.5 * image_size_wh)  # pyre-ignore
+    focal_length = focal_pytorch3d * (0.5 * image_size_wh)
+
+    camera_matrix = torch.zeros_like(R)
+    camera_matrix[:, :2, 2] = principal_point
+    camera_matrix[:, 2, 2] = 1.0
+    camera_matrix[:, 0, 0] = focal_length[:, 0]
+    camera_matrix[:, 1, 1] = focal_length[:, 1]
+    rvec = so3_log_map(R)
+    return rvec, tvec, camera_matrix

tests/test_camera_conversions.py

@@ -15,6 +15,7 @@ from pytorch3d.ops import eyes
 from pytorch3d.transforms import so3_exponential_map, so3_log_map
 from pytorch3d.utils import (
     cameras_from_opencv_projection,
+    opencv_from_cameras_projection,
 )
 
 DATA_DIR = get_tests_dir() / "data"
@@ -151,3 +152,11 @@ class TestCameraConversions(TestCaseMixin, unittest.TestCase):
         self.assertClose(
             pts_proj_opencv_in_pytorch3d_screen, pts_proj_pytorch3d, atol=1e-5
         )
+
+        # Check the inverse.
+        rvec_i, tvec_i, camera_matrix_i = opencv_from_cameras_projection(
+            cameras_opencv_to_pytorch3d, image_size
+        )
+        self.assertClose(rvec, rvec_i)
+        self.assertClose(tvec, tvec_i)
+        self.assertClose(camera_matrix, camera_matrix_i)