Systematic Evaluation of Depth Backbones and Semantic Cues for Monocular Pseudo-LiDAR 3D Detection

TL;DR

This paper systematically evaluates how depth estimation methods and feature engineering influence monocular Pseudo-LiDAR 3D detection, revealing that depth backbone choice and geometric accuracy are crucial for performance.

Contribution

It provides a comprehensive comparison of depth backbones and feature cues, demonstrating the importance of depth accuracy over semantic features in monocular 3D detection.

Findings

NeWCRFs outperforms Depth Anything V2 in 3D detection accuracy.

Semantic and appearance cues offer marginal improvements, sometimes degrading performance.

Depth accuracy and geometric fidelity are more critical than secondary feature injection.

Abstract

Monocular 3D object detection offers a low-cost alternative to LiDAR, yet remains less accurate due to the difficulty of estimating metric depth from a single image. We systematically evaluate how depth backbones and feature engineering affect a monocular Pseudo-LiDAR pipeline on the KITTI validation split. Specifically, we compare NeWCRFs (supervised metric depth) against Depth Anything V2 Metric-Outdoor (Base) under an identical pseudo-LiDAR generation and PointRCNN detection protocol. NeWCRFs yields stronger downstream 3D detection, achieving 10.50\% AP$_{3D}$ at IoU$=0.7$ on the Moderate split using grayscale intensity (Exp~2). We further test point-cloud augmentations using appearance cues (grayscale intensity) and semantic cues (instance segmentation confidence). Contrary to the expectation that semantics would substantially close the gap, these features provide only marginal gains, and mask-based sampling can degrade performance by removing contextual geometry. Finally, we report a depth-accuracy-versus-distance diagnostic using ground-truth 2D boxes (including Ped/Cyc), highlighting that coarse depth correctness does not fully predict strict 3D IoU. Overall, under an off-the-shelf LiDAR detector, depth-backbone choice and geometric fidelity dominate performance, outweighing secondary feature injection.