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Epipolar Transformers (2005.04551v1)

Published 10 May 2020 in cs.CV

Abstract: A common approach to localize 3D human joints in a synchronized and calibrated multi-view setup consists of two-steps: (1) apply a 2D detector separately on each view to localize joints in 2D, and (2) perform robust triangulation on 2D detections from each view to acquire the 3D joint locations. However, in step 1, the 2D detector is limited to solving challenging cases which could potentially be better resolved in 3D, such as occlusions and oblique viewing angles, purely in 2D without leveraging any 3D information. Therefore, we propose the differentiable "epipolar transformer", which enables the 2D detector to leverage 3D-aware features to improve 2D pose estimation. The intuition is: given a 2D location p in the current view, we would like to first find its corresponding point p' in a neighboring view, and then combine the features at p' with the features at p, thus leading to a 3D-aware feature at p. Inspired by stereo matching, the epipolar transformer leverages epipolar constraints and feature matching to approximate the features at p'. Experiments on InterHand and Human3.6M show that our approach has consistent improvements over the baselines. Specifically, in the condition where no external data is used, our Human3.6M model trained with ResNet-50 backbone and image size 256 x 256 outperforms state-of-the-art by 4.23 mm and achieves MPJPE 26.9 mm.

Citations (156)

Summary

  • The paper introduces the Epipolar Transformer module that fuses 2D and 3D geometric features to achieve more accurate multi-view 3D pose estimation.
  • It employs epipolar sampling to constrain feature matching along geometrically consistent lines and integrates these features using advanced fusion methods.
  • Experimental results show a reduced MPJPE of 26.9 mm on Human3.6M, outperforming previous state-of-the-art techniques in 3D pose estimation.

Analysis of the "Epipolar Transformers" Paper

This paper introduces the "Epipolar Transformer" module, which enhances 2D pose detection by incorporating 3D-aware features, thus improving multi-view 3D pose estimation. The authors critique the conventional two-step process of estimating 3D poses through independent 2D detections followed by triangulation. They propose a differentiable technique that allows 2D detectors to exploit interconnected 3D information, potentially addressing issues such as occlusions and oblique viewing angles that are challenging to detect using solely 2D approaches.

Key Concept and Methodology

The central idea of the epipolar transformer is to exploit the geometrical relationship dictated by the epipolar geometry to enhance feature matching and fusion across different camera views. The epipolar transformer enhances the 2D pose detector's features by integrating information from neighboring views, leading to more accurate 3D contextual understanding at any given pixel. The paper adopts epipolar constraints borrowed from stereo vision systems to achieve this.

Epipolar Transformer Module:

  1. Epipolar Sampling: For a given point in the reference view, the module samples features along the related epipolar line in the source view. This effectively constrains the potential matching points to be along the line defined by epipolar geometry, which is computationally efficient.
  2. Feature Fusion: Once the features are sampled from the source view, they are integrated with features from the reference view. The paper explores multiple fusion methods, inspired by established architectures like non-local networks.

Experimental Validation and Results

Experimental results on datasets such as InterHand and Human3.6M demonstrate the proposed method's efficacy. On the Human3.6M dataset, the epipolar transformer reduced the mean per-joint position error (MPJPE) to 26.9 mm, outperforming previous state-of-the-art techniques like Qiu et al.'s method by 4.23 mm. This improvement is significant, considering the tighter integration of 3D information into the 2D detection process.

Implications

Practical Implications:

The proposed method suggests ways to improve existing multi-camera setups for 3D pose estimation without needing significant overhauls. It indicates that camera calibration information, if available, can be utilized effectively to enhance pose estimations. This could impact areas like augmented reality and human-computer interaction, where precise pose estimation is critical.

Theoretical Implications:

The paper contributes to the understanding of how 2D feature maps can be enhanced using 3D information through geometry-consistent correspondences. This method, leveraging deep learning with geometric constraints, paves the way for further research into hybrid methods that combine the strengths of classical vision techniques with modern deep learning paradigms.

Future Prospects

The paper outlines potential expansions of the epipolar transformer concept, notably its applicability in other domains requiring 3D understanding from 2D inputs, such as deep multi-view stereo. Future research could involve experimenting with more complex 3D learning tasks, where dynamics and interactions between multiple objects or body parts are considered.

Overall, the epipolar transformer stands as a promising tool for bridging the gap between traditional 2D detection methods and comprehensive 3D pose estimations, providing a vehicle for enhanced interpretability and accuracy in multi-view setups. The discussion on practical generalizations to any multi-camera setup, contingent on proper calibration, presents an exciting avenue for future work in computer vision and machine learning communities.

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