Point clouds frequently contain noise and outliers presenting obstacles for downstream applications. In this work we introduce a novel denoising method for point clouds. By leveraging the latent space we explicitly uncover noise components allowing for the extraction of a clean latent code. This in turn facilitates the restoration of clean points via inverse transformation. A key component in our network is a new multi-level graph convolution network for capturing rich geometric structural features at various scales from local to global. These features are then integrated into the invertible neural network which bijectively maps the latent space to guide the noise disentanglement process. Additionally we employ an invertible monotone operator to model the transformation process effectively enhancing the representation of integrated geometric features. This enhancement allows our network to precisely differentiate between noise factors and the intrinsic clean points in the latent code by projecting them onto separate channels. Both qualitative and quantitative evaluations demonstrate that our method outperforms state-of-the-art methods at various noise levels. The source code is available at https://github.com/yanbiao1/PD-LTS.
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From Variance to Veracity: Unbundling and Mitigating Gradient Variance in Differentiable Bundle Adjustment Layers
PDF ↗Various pose estimation and tracking problems in robotics can be decomposed into a correspondence estimation problem (often computed using a deep network) followed by a weighted least squares optimization problem to solve for the poses. Recent work has shown that coupling the two problems by iteratively refining one conditioned on the other's output yields SOTA results across domains. However training these models has proved challenging requiring a litany of tricks to stabilize and speed up training. In this work we take the visual odometry problem as an example and identify three plausible causes: (1) flow loss interference (2) linearization errors in the bundle adjustment (BA) layer and (3) dependence of weight gradients on the BA residual. We show how these issues result in noisy and higher variance gradients potentially leading to a slow down in training and instabilities. We then propose a simple solution to reduce the gradient variance by using the weights predicted by the network in the inner optimization loop to also weight the correspondence objective in the training problem. This helps the training objective 'focus' on the more important points thereby reducing the variance and mitigating the influence of outliers. We show that the resulting method leads to faster training and can be more flexibly trained in varying training setups without sacrificing performance. In particular we show 2-2.5x training speedups over a baseline visual odometry model we modify.
Large-scale datasets have fueled recent advancements in AI-based autonomous vehicle research. However these datasets are usually collected from a single vehicle's one-time pass of a certain location lacking multiagent interactions or repeated traversals of the same place. Such information could lead to transformative enhancements in autonomous vehicles' perception prediction and planning capabilities. To bridge this gap in collaboration with the self-driving company May Mobility we present the MARS dataset which unifies scenarios that enable MultiAgent multitraveRSal and multimodal autonomous vehicle research. More specifically MARS is collected with a fleet of autonomous vehicles driving within a certain geographical area. Each vehicle has its own route and different vehicles may appear at nearby locations. Each vehicle is equipped with a LiDAR and surround-view RGB cameras. We curate two subsets in MARS: one facilitates collaborative driving with multiple vehicles simultaneously present at the same location and the other enables memory retrospection through asynchronous traversals of the same location by multiple vehicles. We conduct experiments in place recognition and neural reconstruction. More importantly MARS introduces new research opportunities and challenges such as multitraversal 3D reconstruction multiagent perception and unsupervised object discovery. Our data and codes can be found at https://ai4ce.github.io/MARS/.
Learning Spatial Adaptation and Temporal Coherence in Diffusion Models for Video Super-Resolution
PDF ↗Diffusion models are just at a tipping point for image super-resolution task. Nevertheless it is not trivial to capitalize on diffusion models for video super-resolution which necessitates not only the preservation of visual appearance from low-resolution to high-resolution videos but also the temporal consistency across video frames. In this paper we propose a novel approach pursuing Spatial Adaptation and Temporal Coherence (SATeCo) for video super-resolution. SATeCo pivots on learning spatial-temporal guidance from low-resolution videos to calibrate both latent-space high-resolution video denoising and pixel-space video reconstruction. Technically SATeCo freezes all the parameters of the pre-trained UNet and VAE and only optimizes two deliberately-designed spatial feature adaptation (SFA) and temporal feature alignment (TFA) modules in the decoder of UNet and VAE. SFA modulates frame features via adaptively estimating affine parameters for each pixel guaranteeing pixel-wise guidance for high-resolution frame synthesis. TFA delves into feature interaction within a 3D local window (tubelet) through self-attention and executes cross-attention between tubelet and its low-resolution counterpart to guide temporal feature alignment. Extensive experiments conducted on the REDS4 and Vid4 datasets demonstrate the effectiveness of our approach.
Generalization to new domains not seen during training is one of the long-standing challenges in deploying neural networks in real-world applications. Existing generalization techniques either necessitate external images for augmentation and/or aim at learning invariant representations by imposing various alignment constraints. Large-scale pretraining has recently shown promising generalization capabilities along with the potential of binding different modalities. For instance the advent of vision-language models like CLIP has opened the doorway for vision models to exploit the textual modality. In this paper we introduce a simple framework for generalizing semantic segmentation networks by employing language as the source of randomization. Our recipe comprises three key ingredients: (i) the preservation of the intrinsic CLIP robustness through minimal fine-tuning (ii) language-driven local style augmentation and (iii) randomization by locally mixing the source and augmented styles during training. Extensive experiments report state-of-the-art results on various generalization benchmarks.
Powered by massive curated training data Segment Anything Model (SAM) has demonstrated its impressive generalization capabilities in open-world scenarios with the guidance of prompts. However the vanilla SAM is class-agnostic and heavily relies on user-provided prompts to segment objects of interest. Adapting this method to diverse tasks is crucial for accurate target identification and to avoid suboptimal segmentation results. In this paper we propose a novel framework termed AlignSAM designed for automatic prompting for aligning SAM to an open context through reinforcement learning. Anchored by an agent AlignSAM enables the generality of the SAM model across diverse downstream tasks while keeping its parameters frozen. Specifically AlignSAM initiates a prompting agent to iteratively refine segmentation predictions by interacting with the foundational model. It integrates a reinforcement learning policy network to provide informative prompts to the foundational models. Additionally a semantic recalibration module is introduced to provide fine-grained labels of prompts enhancing the model's proficiency in handling tasks encompassing explicit and implicit semantics. Experiments conducted on various challenging segmentation tasks among existing foundation models demonstrate the superiority of the proposed AlignSAM over state-of-the-art approaches. Project page: https://github.com/Duojun-Huang/AlignSAM-CVPR2024.
The realism of digital avatars is crucial in enabling telepresence applications with self-expression and customization. While physical simulations can produce realistic motions for clothed humans they require high-quality garment assets with associated physical parameters for cloth simulations. However manually creating these assets and calibrating their parameters is labor-intensive and requires specialized expertise. Current methods focus on reconstructing geometry but don't generate complete assets for physics-based applications. To address this gap we propose DiffAvatar a novel approach that performs body and garment co-optimization using differentiable simulation. By integrating physical simulation into the optimization loop and accounting for the complex nonlinear behavior of cloth and its intricate interaction with the body our framework recovers body and garment geometry and extracts important material parameters in a physically plausible way. Our experiments demonstrate that our approach generates realistic clothing and body shape suitable for downstream applications. We provide additional insights and results on our webpage: people.csail.mit.edu/liyifei/publication/diffavatar.
Nearest is Not Dearest: Towards Practical Defense against Quantization-conditioned Backdoor Attacks
PDF ↗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.
3D instance segmentation is fundamental to geometric understanding of the world around us. Existing methods for instance segmentation of 3D scenes rely on supervision from expensive manual 3D annotations. We propose UnScene3D the first fully unsupervised 3D learning approach for class-agnostic 3D instance segmentation of indoor scans. UnScene3D first generates pseudo masks by leveraging self-supervised color and geometry features to find potential object regions. We operate on a basis of 3D segment primitives enabling efficient representation and learning on high-resolution 3D data. The coarse proposals are then refined through self-training our model on its predictions. Our approach improves over state-of-the-art unsupervised 3D instance segmentation methods by more than 300% Average Precision score demonstrating effective instance segmentation even in challenging cluttered 3D scenes.
In the realm of computer vision Neural Fields have gained prominence as a contemporary tool harnessing neural networks for signal representation. Despite the remarkable progress in adapting these networks to solve a variety of problems the field still lacks a comprehensive theoretical framework. This article aims to address this gap by delving into the intricate interplay between initialization and activation providing a foundational basis for the robust optimization of Neural Fields. Our theoretical insights reveal a deep-seated connection among network initialization architectural choices and the optimization process emphasizing the need for a holistic approach when designing cutting-edge Neural Fields.
Part-aware panoptic segmentation (PPS) requires (a) that each foreground object and background region in an image is segmented and classified and (b) that all parts within foreground objects are segmented classified and linked to their parent object. Existing methods approach PPS by separately conducting object-level and part-level segmentation. However their part-level predictions are not linked to individual parent objects. Therefore their learning objective is not aligned with the PPS task objective which harms the PPS performance. To solve this and make more accurate PPS predictions we propose Task-Aligned Part-aware Panoptic Segmentation (TAPPS). This method uses a set of shared queries to jointly predict (a) object-level segments and (b) the part-level segments within those same objects. As a result TAPPS learns to predict part-level segments that are linked to individual parent objects aligning the learning objective with the task objective and allowing TAPPS to leverage joint object-part representations. With experiments we show that TAPPS considerably outperforms methods that predict objects and parts separately and achieves new state-of-the-art PPS results.
Nighttime conditions pose a significant challenge to color constancy due to the diversity of lighting conditions and the presence of substantial low-light noise. Existing color constancy methods struggle with nighttime scenes frequently leading to imprecise light color estimations. To tackle nighttime color constancy we propose a novel unsupervised domain adaptation approach that utilizes labeled daytime data to facilitate learning on unlabeled nighttime images. To specifically address the unique lighting conditions of nighttime and ensure the robustness of pseudo labels we propose adaptive channel masking and light uncertainty. By selectively masking channels that are less sensitive to lighting conditions adaptive channel masking directs the model to progressively focus on features less affected by variations in light colors and noise. Additionally our model leverages light uncertainty to provide a pixel-wise uncertainty estimation regarding light color prediction which helps avoid learning from incorrect labels. Our model demonstrates a significant improvement in accuracy achieving 21.5% lower Mean Angular Error (MAE) compared to the state-of-the-art method on our nighttime dataset.
We present a generative approach to forecast long-term future human behavior in 3D requiring only weak supervision from readily available 2D human action data. This is a fundamental task enabling many downstream applications. The required ground-truth data is hard to capture in 3D (mocap suits expensive setups) but easy to acquire in 2D (simple RGB cameras). Thus we design our method to only require 2D RGB data at inference time while being able to generate 3D human motion sequences. We use a differentiable 2D projection scheme in an autoregressive manner for weak supervision and an adversarial loss for 3D regularization. Our method predicts long and complex human behavior sequences (e.g. cooking assembly) consisting of multiple sub-actions. We tackle this in a semantically hierarchical manner jointly predicting high-level coarse action labels together with their low-level fine-grained realizations as characteristic 3D human poses. We observe that these two action representations are coupled in nature and joint prediction benefits both action and pose forecasting. Our experiments demonstrate the complementary nature of joint action and 3D pose prediction: our joint approach outperforms each task treated individually enables robust longer-term sequence prediction and improves over alternative approaches to forecast actions and characteristic 3D poses.
Contrastive Language-Image Pre-training (CLIP) plays an essential role in extracting valuable content information from images across diverse tasks. It aligns textual and visual modalities to comprehend the entire image including all the details even those irrelevant to specific tasks. However for a finer understanding and controlled editing of images it becomes crucial to focus on specific regions of interest which can be indicated as points masks or boxes by humans or perception models. To fulfill the requirements we introduce Alpha-CLIP an enhanced version of CLIP with an auxiliary alpha channel to suggest attentive regions and fine-tuned with constructed millions of RGBA region-text pairs. Alpha-CLIP not only preserves the visual recognition ability of CLIP but also enables precise control over the emphasis of image contents. It demonstrates effectiveness in various tasks including but not limited to open-world recognition multimodal large language models and conditional 2D / 3D generation. It has a strong potential to serve as a versatile tool for image-related tasks.
Binary neural networks utilize 1-bit quantized weights and activations to reduce both the model's storage demands and computational burden. However advanced binary architectures still incorporate millions of inefficient and nonhardware-friendly full-precision multiplication operations. A&B BNN is proposed to directly remove part of the multiplication operations in a traditional BNN and replace the rest with an equal number of bit operations introducing the mask layer and the quantized RPReLU structure based on the normalizer-free network architecture. The mask layer can be removed during inference by leveraging the intrinsic characteristics of BNN with straightforward mathematical transformations to avoid the associated multiplication operations. The quantized RPReLU structure enables more efficient bit operations by constraining its slope to be integer powers of 2. Experimental results achieved 92.30% 69.35% and 66.89% on the CIFAR-10 CIFAR-100 and ImageNet datasets respectively which are competitive with the state-of-the-art. Ablation studies have verified the efficacy of the quantized RPReLU structure leading to a 1.14% enhancement on the ImageNet compared to using a fixed slope RLeakyReLU. The proposed add&bit-operation-only BNN offers an innovative approach for hardware-friendly network architecture.
Text-to-image (T2I) generative models have recently emerged as a powerful tool enabling the creation of photo-realistic images and giving rise to a multitude of applications. However the effective integration of T2I models into fundamental image classification tasks remains an open question. A prevalent strategy to bolster image classification performance is through augmenting the training set with synthetic images generated by T2I models. In this study we scrutinize the shortcomings of both current generative and conventional data augmentation techniques. Our analysis reveals that these methods struggle to produce images that are both faithful (in terms of foreground objects) and diverse (in terms of background contexts) for domain-specific concepts. To tackle this challenge we introduce an innovative inter-class data augmentation method known as Diff-Mix (\href https://github.com/Zhicaiwww/Diff-Mix) https://github.com/Zhicaiwww/Diff-Mix which enriches the dataset by performing image translations between classes. Our empirical results demonstrate that Diff-Mix achieves a better balance between faithfulness and diversity leading to a marked improvement in performance across diverse image classification scenarios including few-shot conventional and long-tail classifications for domain-specific datasets.
In magnetic resonance imaging (MRI) slice-to-volume reconstruction (SVR) refers to computational reconstruction of an unknown 3D magnetic resonance volume from stacks of 2D slices corrupted by motion. While promising current SVR methods require multiple slice stacks for accurate 3D reconstruction leading to long scans and limiting their use in time-sensitive applications such as fetal fMRI. Here we propose a SVR method that overcomes the shortcomings of previous work and produces state-of-the-art reconstructions in the presence of extreme inter-slice motion. Inspired by the recent success of single-view depth estimation methods we formulate SVR as a single-stack motion estimation task and train a fully convolutional network to predict a motion stack for a given slice stack producing a 3D reconstruction as a byproduct of the predicted motion. Extensive experiments on the SVR of adult and fetal brains demonstrate that our fully convolutional method is twice as accurate as previous SVR methods. Our code is available at github.com/seannz/svr.
Recent advances in generative diffusion models have enabled the previously unfeasible capability of generating 3D assets from a single input image or a text prompt. In this work we aim to enhance the quality and functionality of these models for the task of creating controllable photorealistic human avatars. We achieve this by integrating a 3D morphable model into the state-of-the-art multi-view-consistent diffusion approach. We demonstrate that accurate conditioning of a generative pipeline on the articulated 3D model enhances the baseline model performance on the task of novel view synthesis from a single image. More importantly this integration facilitates a seamless and accurate incorporation of facial expression and body pose control into the generation process. To the best of our knowledge our proposed framework is the first diffusion model to enable the creation of fully 3D-consistent animatable and photorealistic human avatars from a single image of an unseen subject; extensive quantitative and qualitative evaluations demonstrate the advantages of our approach over existing state-of-the-art avatar creation models on both novel view and novel expression synthesis tasks. The code for our project is publicly available.
We present MM-Narrator a novel system leveraging GPT-4 with multimodal in-context learning for the generation of audio descriptions (AD). Unlike previous methods that primarily focused on downstream fine-tuning with short video clips MM-Narrator excels in generating precise audio descriptions for videos of extensive lengths even beyond hours in an autoregressive manner. This capability is made possible by the proposed memory-augmented generation process which effectively utilizes both the short-term textual context and long-term visual memory through an efficient register-and-recall mechanism. These contextual memories compile pertinent past information including storylines and character identities ensuring an accurate tracking and depicting of story-coherent and character-centric audio descriptions. Maintaining the training-free design of MM-Narrator we further propose a complexity-based demonstration selection strategy to largely enhance its multi-step reasoning capability via few-shot multimodal in-context learning (MM-ICL). Experimental results on MAD-eval dataset demonstrate that MM-Narrator consistently outperforms both the existing fine-tuning-based approaches and LLM-based approaches in most scenarios as measured by standard evaluation metrics. Additionally we introduce the first segment-based evaluator for recurrent text generation. Empowered by GPT-4 this evaluator comprehensively reasons and marks AD generation performance in various extendable dimensions.
Creating high-dynamic videos such as motion-rich actions and sophisticated visual effects poses a significant challenge in the field of artificial intelligence. Unfortunately current state-of-the-art video generation methods primarily focusing on text-to-video generation tend to produce video clips with minimal motions despite maintaining high fidelity. We argue that relying solely on text instructions is insufficient and suboptimal for video generation. In this paper we introduce PixelDance a novel approach based on diffusion models that incorporates image instructions for both the first and last frames in conjunction with text instructions for video generation. Comprehensive experimental results demonstrate that PixelDance trained with public data exhibits significantly better proficiency in synthesizing videos with complex scenes and intricate motions setting a new standard for video generation.