BITPNet: Unsupervised Bio-Inspired Two-Path Network for Nighttime Traffic Image Enhancement

Due to the low luminance in nighttime traffic images, image features are not salient, making tasks in intelligent transportation systems such as nighttime vehicle detection challenging. Recently, convolutional neural network based methods have been developed for low-light image enhancement. Most of these methods are supervised and require high-light reference images at the same scenes. However, reference images are difficult to be obtained in nighttime traffic scenes because vehicles always move. In the early visual system the input signals are processed by two parallel visual paths in the retina: one path has small receptive fields (RFs) to process the high frequency information and another path has large RFs to deal with the low frequency information. Inspired by this, we design a novel bio-inspired two-path convolutional neural network (BITPNet) for nighttime traffic image enhancement. The high-frequency path with small convolution kernel size is designed to suppress noises and preserve the details. The low-frequency path with large convolution kernel size is used to enhance the luminance of images. Each path includes an encoder-to-decoder network followed by a new multi-level attention module to combine features of levels with different RFs. The outputs of the two paths are summed by learnt weights for generating the final image enhancement result. Several no-reference image quality metrics are utilized to design a new loss function, resulting in an unsupervised approach. The proposed BITPNet is trained on one nighttime traffic image dataset and evaluated on another nighttime dataset. Experimental results demonstrate that the proposed BITPNet outperforms several state-of-the-art low-light image enhancement methods in terms of visual quality and three no-reference image quality metrics. In addition, when the proposed BITPNet is used as pre-processing for the nighttime multi-class vehicle detection task, it achieves higher detection rate (97.18%) than other methods.