Revisiting Radial Distortion Rectification in Polar-Coordinates: A New and Efficient Learning Perspective

Abstract

Fisheye cameras can capture a large field-of-view (Fov) scene but it introduces severe radial distortion in images. Thus, distortion rectification is a crucial step for subsequent computer vision tasks using fisheye cameras. A prevalent type of method predicts the displacement field between the input and output to rectify the distorted images. However, it is challenging to estimate the accurate flow in Cartesian coordinates (both <inline-formula> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula> directions), in which the sampling strategy of the convolution kernel ignores the radial symmetry of distortion. In general, the pixel&#x2019;s distortion at the same radius from the center is the same, while the radius corresponds to one parameter in polar coordinates. Motivated by this fact, we exploit the radial symmetry of distortion to predict a more straightforward one-dimensional flow, transforming the distorted image into the polar coordinates domain instead of predicting two-dimensional flow in <inline-formula> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula> directions. Specifically, we propose a Polar coordinates Distortion Rectification Network (PCDRN), whose sampling strategy corresponds to the radial distortion characteristic so that a more accurate flow can be predicted. To eliminate the blurs and ring artifacts induced by the coordinates transformation, a Polar-To-Cartesian Appearance Enhancement Network is designed to enhance the local appearance of rectified images. Experimental results on the synthesized dataset and real-world dataset demonstrate the superiority of our approach in both quantitative and qualitative evaluations.