CVPR 2024 Papers

Our understanding of the generalization capabilities of neural networks NNs is still incomplete. Prevailing explanations are based on implicit biases of gradient descent GD but they cannot account for the capabilities of models from gradientfree methods nor the simplicity bias recently observed in untrained networks This paper seeks other sources of generalization in NNs. To understand the inductive biases provided by architectures independently from GD we examine untrained randomweight networks Even simple MLPs show strong inductive biases uniform sampling in weight space yields a very biased distribution of functions in terms of complexity But unlike common wisdom NNs do not have an inherent simplicity bias This property depends on components such as ReLUs residual connections and layer normalizations Alternative architectures can be built with a bias for any level of complexity. Transformers also inherit all these properties from their building blocks. We provide a fresh explanation for the success of deep learning independent from gradientbased training It points at promising avenues for controlling the solutions implemented by trained models.

Controllable generation of 3D assets is important for many practical applications like content creation in movies games and engineering as well as in AR/VR. Recently diffusion models have shown remarkable results in generation quality of 3D objects. However none of the existing models enable disentangled generation to control the shape and appearance separately. For the first time we present a suitable representation for 3D diffusion models to enable such disentanglement by introducing a hybrid point cloud and neural radiance field approach. We model a diffusion process over point positions jointly with a high-dimensional feature space for a local density and radiance decoder. While the point positions represent the coarse shape of the object the point features allow modeling the geometry and appearance details. This disentanglement enables us to sample both independently and therefore to control both separately. Our approach sets a new state of the art in generation compared to previous disentanglement-capable methods by reduced FID scores of 30-90% and is on-par with other non-disentanglement-capable state-of-the art methods.

Reconstructing dynamic objects from monocular videos is a severely underconstrained and challenging problem and recent work has approached it in various directions. However owing to the ill-posed nature of this problem there has been no solution that can provide consistent high-quality novel views from camera positions that are significantly different from the training views. In this work we introduce Neural Parametric Gaussians (NPGs) to take on this challenge by imposing a two-stage approach: first we fit a low-rank neural deformation model which then is used as regularization for non-rigid reconstruction in the second stage. The first stage learns the object's deformations such that it preserves consistency in novel views. The second stage obtains high reconstruction quality by optimizing 3D Gaussians that are driven by the coarse model. To this end we introduce a local 3D Gaussian representation where temporally shared Gaussians are anchored in and deformed by local oriented volumes. The resulting combined model can be rendered as radiance fields resulting in high-quality photo-realistic reconstructions of the non-rigidly deforming objects. We demonstrate that NPGs achieve superior results compared to previous works especially in challenging scenarios with few multi-view cues.

We propose a self-supervised approach for learning physics-based subspaces for real-time simulation. Existing learning-based methods construct subspaces by approximating pre-defined simulation data in a purely geometric way. However this approach tends to produce high-energy configurations leads to entangled latent space dimensions and generalizes poorly beyond the training set. To overcome these limitations we propose a self-supervised approach that directly minimizes the system's mechanical energy during training. We show that our method leads to learned subspaces that reflect physical equilibrium constraints resolve overfitting issues of previous methods and offer interpretable latent space parameters.

Stereo matching is a core task for many computer vision and robotics applications. Despite their dominance in traditional stereo methods the hand-crafted Markov Random Field (MRF) models lack sufficient modeling accuracy compared to end-to-end deep models. While deep learning representations have greatly improved the unary terms of the MRF models the overall accuracy is still severely limited by the hand-crafted pairwise terms and message passing. To address these issues we propose a neural MRF model where both potential functions and message passing are designed using data-driven neural networks. Our fully data-driven model is built on the foundation of variational inference theory to prevent convergence issues and retain stereo MRF's graph inductive bias. To make the inference tractable and scale well to high-resolution images we also propose a Disparity Proposal Network (DPN) to adaptively prune the search space of disparity. The proposed approach ranks 1^ st on both KITTI 2012 and 2015 leaderboards among all published methods while running faster than 100 ms. This approach significantly outperforms prior global methods e.g. lowering D1 metric by more than 50% on KITTI 2015. In addition our method exhibits strong cross-domain generalization and can recover sharp edges. The codes at https://github.com/aeolusguan/NMRF.

Given a well-behaved neural network is possible to identify its parent based on which it was tuned? In this paper we introduce a novel task known as neural lineage detection aiming at discovering lineage relationships between parent and child models. Specifically from a set of parent models neural lineage detection predicts which parent model a child model has been fine-tuned from. We propose two approaches to address this task. (1) For practical convenience we introduce a learning-free approach which integrates an approximation of the finetuning process into the neural network representation similarity metrics leading to a similarity-based lineage detection scheme. (2) For the pursuit of accuracy we introduce a learning-based lineage detector comprising encoders and a transformer detector. Through experimentation we have validated that our proposed learning-free and learning-based methods outperform the baseline in various learning settings and are adaptable to a variety of visual models. Moreover they also exhibit the ability to trace cross-generational lineage identifying not only parent models but also their ancestors.

Face morphing is a problem in computer graphics with numerous artistic and forensic applications. It is challenging due to variations in pose lighting gender and ethnicity. This task consists of a warping for feature alignment and a blending for a seamless transition between the warped images. We propose to leverage coord-based neural networks to represent such warpings and blendings of face images. During training we exploit the smoothness and flexibility of such networks by combining energy functionals employed in classical approaches without discretizations. Additionally our method is time-dependent allowing a continuous warping/blending of the images. During morphing inference we need both direct and inverse transformations of the time-dependent warping. The first (second) is responsible for warping the target (source) image into the source (target) image. Our neural warping stores those maps in a single network dismissing the need for inverting them. The results of our experiments indicate that our method is competitive with both classical and generative models under the lens of image quality and face-morphing detectors. Aesthetically the resulting images present a seamless blending of diverse faces not yet usual in the literature.

Neural fields have emerged as a powerful and broadly applicable method for representing signals. However in contrast to classical discrete digital signal processing the portfolio of tools to process such representations is still severely limited and restricted to Euclidean domains. In this paper we address this problem by showing how a probabilistic re-interpretation of neural fields can enable their training and inference processes to become "filter-aware". The formulation we propose not only merges training and filtering in an efficient way but also generalizes beyond the familiar Euclidean coordinate spaces to the more general set of smooth manifolds and convolutions induced by the actions of Lie groups. We demonstrate how this framework can enable novel integrations of signal processing techniques for neural field applications on both Euclidean domains such as images and audio as well as non-Euclidean domains such as rotations and rays. A noteworthy benefit of our method is its applicability. Our method can be summarized as primarily a modification of the loss function and in most cases does not require changes to the network architecture or the inference process.

Computer vision in unconstrained outdoor scenarios must tackle challenging high dynamic range (HDR) scenes and rapidly changing illumination conditions. Existing methods address this problem with multi-capture HDR sensors and a hardware image signal processor (ISP) that produces a single fused image as input to a downstream neural network. The output of the HDR sensor is a set of low dynamic range (LDR) exposures and the fusion in the ISP is performed in image space and typically optimized for human perception on a display. Preferring tonemapped content with smooth transition regions over detail (and noise) in the resulting image this image fusion does typically not preserve all information from the LDR exposures that may be essential for downstream computer vision tasks. In this work we depart from conventional HDR image fusion and propose a learned task-driven fusion in the feature domain. Instead of using a single companded image we introduce a novel local cross-attention fusion mechanism that exploits semantic features from all exposures learned in an end-to-end fashion with supervision from downstream detection losses. The proposed method outperforms all tested conventional HDR exposure fusion and auto-exposure methods in challenging automotive HDR scenarios.

Novel-view synthesis of specular objects like shiny metals or glossy paints remains a significant challenge. Not only the glossy appearance but also global illumination effects including reflections of other objects in the environment are critical components to faithfully reproduce a scene. In this paper we present Neural Directional Encoding (NDE) a view-dependent appearance encoding of neural radiance fields (NeRF) for rendering specular objects. NDE transfers the concept of feature-grid-based spatial encoding to the angular domain significantly improving the ability to model high-frequency angular signals. In contrast to previous methods that use encoding functions with only angular input we additionally cone-trace spatial features to obtain a spatially varying directional encoding which addresses the challenging interreflection effects. Extensive experiments on both synthetic and real datasets show that a NeRF model with NDE (1) outperforms the state of the art on view synthesis of specular objects and (2) works with small networks to allow fast (real-time) inference. The source code is available at: https://github.com/lwwu2/nde

We investigate a fundamental aspect of machine vision: the measurement of features by revisiting clustering one of the most classic approaches in machine learning and data analysis. Existing visual feature extractors including ConvNets ViTs and MLPs represent an image as rectangular regions. Though prevalent such a grid-style paradigm is built upon engineering practice and lacks explicit modeling of data distribution. In this work we propose feature extraction with clustering (FEC) a conceptually elegant yet surprisingly ad-hoc interpretable neural clustering framework which views feature extraction as a process of selecting representatives from data and thus automatically captures the underlying data distribution. Given an image FEC alternates between grouping pixels into individual clusters to abstract representatives and updating the deep features of pixels with current representatives. Such an iterative working mechanism is implemented in the form of several neural layers and the final representatives can be used for downstream tasks. The cluster assignments across layers which can be viewed and inspected by humans make the forward process of FEC fully transparent and empower it with promising ad-hoc interpretability. Extensive experiments on various visual recognition models and tasks verify the effectiveness generality and interpretability of FEC. We expect this work will provoke a rethink of the current de facto grid-style paradigm.

We present Neural 3D Strokes a novel technique to generate stylized images of a 3D scene at arbitrary novel views from multi-view 2D images. Different from existing methods which apply stylization to trained neural radiance fields at the voxel level our approach draws inspiration from image-to-painting methods simulating the progressive painting process of human artwork with vector strokes. We develop a palette of stylized 3D strokes from basic primitives and splines and consider the 3D scene stylization task as a multi-view reconstruction process based on these 3D stroke primitives. Instead of directly searching for the parameters of these 3D strokes which would be too costly we introduce a differentiable renderer that allows optimizing stroke parameters using gradient descent and propose a training scheme to alleviate the vanishing gradient issue. The extensive evaluation demonstrates that our approach effectively synthesizes 3D scenes with significant geometric and aesthetic stylization while maintaining a consistent appearance across different views. Our method can be further integrated with style loss and image-text contrastive models to extend its applications including color transfer and text-driven 3D scene drawing. Results and code are available at http://buaavrcg.github.io/Neural3DStrokes.

Neural radiance fields (NeRFs) have gained popularity in the autonomous driving (AD) community. Recent methods show NeRFs' potential for closed-loop simulation enabling testing of AD systems and as an advanced training data augmentation technique. However existing methods often require long training times dense semantic supervision or lack generalizability. This in turn hinders the application of NeRFs for AD at scale. In this paper we propose \modelname a robust novel view synthesis method tailored to dynamic AD data. Our method features simple network design extensive sensor modeling for both camera and lidar -- including rolling shutter beam divergence and ray dropping -- and is applicable to multiple datasets out of the box. We verify its performance on five popular AD datasets achieving state-of-the-art performance across the board. To encourage further development we openly release the NeuRAD source code at https://github.com/georghess/NeuRAD.

The complex dynamicity of open-world objects presents non-negligible challenges for multi-object tracking (MOT) often manifested as severe deformations fast motion and occlusions. Most methods that solely depend on coarse-grained object cues such as boxes and the overall appearance of the object are susceptible to degradation due to distorted internal relationships of dynamic objects. To address this problem this work proposes NetTrack an efficient generic and affordable tracking framework to introduce fine-grained learning that is robust to dynamicity. Specifically NetTrack constructs a dynamicity-aware association with a fine-grained Net leveraging point-level visual cues. Correspondingly a fine-grained sampler and matching method have been incorporated. Furthermore NetTrack learns object-text correspondence for fine-grained localization. To evaluate MOT in extremely dynamic open-world scenarios a bird flock tracking (BFT) dataset is constructed which exhibits high dynamicity with diverse species and open-world scenarios. Comprehensive evaluation on BFT validates the effectiveness of fine-grained learning on object dynamicity and thorough transfer experiments on challenging open-world benchmarks i.e. TAO TAO-OW AnimalTrack and GMOT-40 validate the strong generalization ability of NetTrack even without finetuning.

Video scene detection aims to temporally link shots for obtaining semantically compact scenes. It is essential for this task to capture scene-distinguishable affinity among shots by similarity assessment. However most methods relies on ordinary shot-to-shot similarities which may inveigle similar shots into being linked even though they are from different scenes and meanwhile hinder dissimilar shots from being blended into a complete scene. In this paper we propose NeighborNet to inject shot contexts into shot-to-shot similarities through carefully exploring the relations between semantic/temporal neighbors of shots over a local time period. In this way shot-to-shot similarities are remeasured as semantic/temporal neighbor-aware similarities so that NeighborNet can learn context embedding into shot features using graph convolutional network. As a result not only do the learned shot features suppress the affinity among similar shots from different scenes but they also promote the affinity among dissimilar shots in the same scene. Experimental results on public benchmark datasets show that our proposed NeighborNet yields substantial improvements in video scene detection especially outperforms released state-of-the-arts by at least 6% in Average Precision (AP). The code is available at https://github.com/ExMorgan-Alter/NeighborNet.

Model quantization is widely used to compress and accelerate deep neural networks. However recent studies have revealed the feasibility of weaponizing model quantization via implanting quantization-conditioned backdoors (QCBs). These special backdoors stay dormant on released full-precision models but will come into effect after standard quantization. Due to the peculiarity of QCBs existing defenses have minor effects on reducing their threats or are even infeasible. In this paper we conduct the first in-depth analysis of QCBs. We reveal that the activation of existing QCBs primarily stems from the nearest rounding operation and is closely related to the norms of neuron-wise truncation errors (i.e. the difference between the continuous fullprecision weights and its quantized version). Motivated by these insights we propose Error-guided Flipped Rounding with Activation Preservation (EFRAP) an effective and practical defense against QCBs. Specifically EFRAP learns a non-nearest rounding strategy with neuron-wise error norm and layer-wise activation preservation guidance flipping the rounding strategies of neurons crucial for backdoor effects but with minimal impact on clean accuracy. Extensive evaluations on benchmark datasets demonstrate that our EFRAP can defeat state-of-the-art QCB attacks under various settings. Code is available here.

We present NeRSP a Neural 3D reconstruction technique for Reflective surfaces with Sparse Polarized images. Reflective surface reconstruction is extremely challenging as specular reflections are view-dependent and thus violate the multiview consistency for multiview stereo. On the other hand sparse image inputs as a practical capture setting commonly cause incomplete or distorted results due to the lack of correspondence matching. This paper jointly handles the challenges from sparse inputs and reflective surfaces by leveraging polarized images. We derive photometric and geometric cues from the polarimetric image formation model and multiview azimuth consistency which jointly optimize the surface geometry modeled via implicit neural representation. Based on the experiments on our synthetic and real datasets we achieve the state-of-the-art surface reconstruction results with only 6 views as input.

We propose NeRFiller an approach that completes missing portions of a 3D capture via generative 3D inpainting using off-the-shelf 2D visual generative models. Often parts of a captured 3D scene or object are missing due to mesh reconstruction failures or a lack of observations (e.g. contact regions such as the bottom of objects or hard-to-reach areas). We approach this challenging 3D inpainting problem by leveraging a 2D inpainting diffusion model. We identify a surprising behavior of these models where they generate more 3D consistent inpaints when images form a 2x2 grid and show how to generalize this behavior to more than four images. We then present an iterative framework to distill these inpainted regions into a single consistent 3D scene. In contrast to related works we focus on completing scenes rather than deleting foreground objects and our approach does not require tight 2D object masks or text. We compare our approach to relevant baselines adapted to our setting on a variety of scenes where NeRFiller creates the most 3D consistent and plausible scene completions. Our project page is at https://ethanweber.me/nerfiller/.