  In this paper, we present a spatial rectifier to estimate surface normals of
tilted images. Tilted images are of particular interest as more visual data are
captured by arbitrarily oriented sensors such as body-/robot-mounted cameras.
Existing approaches exhibit bounded performance on predicting surface normals
because they were trained using gravity-aligned images. Our two main hypotheses
are: (1) visual scene layout is indicative of the gravity direction; and (2)
not all surfaces are equally represented by a learned estimator due to the
structured distribution of the training data, thus, there exists a
transformation for each tilted image that is more responsive to the learned
estimator than others. We design a spatial rectifier that is learned to
transform the surface normal distribution of a tilted image to the rectified
one that matches the gravity-aligned training data distribution. Along with the
spatial rectifier, we propose a novel truncated angular loss that offers a
stronger gradient at smaller angular errors and robustness to outliers. The
resulting estimator outperforms the state-of-the-art methods including data
augmentation baselines not only on ScanNet and NYUv2 but also on a new dataset
called Tilt-RGBD that includes considerable roll and pitch camera motion.
