Title: HRDFuse: Monocular 360$^\circ$ Depth Estimation by Collaboratively Learning Holistic-with-Regional Depth Distributions. Abstract: Depth estimation from a monocular 360$^\circ$ image is a burgeoning problem as a 360$^\circ$ image provides holistic sensing of a scene with a wide field of view. Recently, some methods, \eg, OmniFusion, have applied the tangent projection (TP) to represent a 360$^\circ$ image and predicted depth values via patch-wise regressions, which are merged to get a depth map with equirectangular projection (ERP) format. However, these methods suffer from 1) non-trivial process of merging a large number of patches; 2) less smooth and accurate depth results caused by ignoring the holistic contextual information contained only in the ERP image and directly regressing the depth value of each pixel. In this paper, we propose a novel framework, HRDFuse, that subtly combines the potential of convolutional neural networks (CNNs) and transformers by collaboratively learning the holistic contextual information from the ERP and the regional structural information from the TP.  Firstly, we propose a spatial feature alignment (SFA) module that learns feature similarities between the TP and ERP to aggregate the TP features into a complete ERP feature map in a pixel-wise manner. Secondly, we propose a collaborative depth distribution classification CDDC module that learns the holistic-with-regional histograms capturing the ERP and TP depth distributions. As such, the final depth values can be predicted as a linear combination of histogram bin centers. Lastly, we adaptively combine the depth predictions from two projections to obtain the final depth map. Extensive experiments on three benchmark datasets show that our method achieves more smooth and accurate depth results while favorably surpassing the SOTA methods by a significant margin.