The Evolution of Monocular Depth Estimation:From Spatial Regression to GenerativeFoundations and the Reliability Gaps
Monocular Depth Estimation (MDE) is one of the most rigorously studied problems in modern computer vision, yet it is fundamentally ill-posed. Recovering absolute three-dimensional geometry from a single two-dimensional projection is mathematically impossible without strong inductive priors. This paper presents a structurally organized review of MDE's evolution spanning from 2005 to 2026. We trace the trajectory from handcrafted Markov random fields to brute-force pixel-wise regression with convolutional neural networks (CNNs), and then to photometric self-supervision, which liberated the field from expensive LiDAR sensor suites. We further analyzed the vision transformers (ViTs) and generative diffusion priors have achieved unprecedented zero-shot metric generalization - with models such as Metric3D v2 attaining an Absolute Relative Error (AbsRel) as low as 0.039 on the KITTI benchmark. We also show that the standard self supervised baseline Monodepth2 degrades catastrophically to an AbsRel of 1.185 under nighttime conditions in NuScenes-Night dataset, which is a massive performance collapse from its clear-weather baseline. Physical-prior models such as PhysDepth recover this to 0.118 AbsRel by embedding Rayleigh scattering theory directly into the network. At the efficiency frontier, architectures such as LEDepth achieve 0.101 AbsRel at 5.7 ms inference time with only 3.1M parameters. We conclude that the future of depth estimation lies in embedding rigid physical and spatiotemporal priors into foundational latent spaces to ensure unyielding reliability in the physical world.