Previous unsupervised monocular depth estimation methods mainly focus on the day-time scenario, and their frameworks are driven by warped photometric consistency. While in some challenging environments, like night, rainy night or snowy winter, the photometry of the same pixel on different frames is inconsistent because of the complex lighting and reflection, so that the day-time unsupervised frameworks cannot be directly applied to these complex scenarios. In this paper, we investigate the problem of unsupervised monocular depth estimation in certain highly complex scenarios. We address this challenging problem by using domain adaptation, and a unified image transfer-based adaptation framework is proposed based on monocular videos in this paper. The depth model trained on day-time scenarios is adapted to different complex scenarios. Instead of adapting the whole depth network, we just consider the encoder network for lower computational complexity. The depth models adapted by the proposed framework to different scenarios share the same decoder, which is practical. Constraints on both feature space and output space promote the framework to learn the key features for depth decoding, and the smoothness loss is introduced into the adaptation framework for better depth estimation performance. Extensive experiments show the effectiveness of the proposed unsupervised framework in estimating the dense depth map from the night-time, rainy night-time and snowy winter images.
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