Author name cluster

Alejandro Sztrajman

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4 papers

2 author rows

AAAI Conference 2026 Conference Paper

M3ashy: Multi-Modal Material Synthesis via Hyperdiffusion

Chenliang Zhou
Zheyuan Hu
Alejandro Sztrajman
Yancheng Cai
Yaru Liu
Cengiz Oztireli

High-quality material synthesis is essential for replicating complex surface properties to create realistic scenes. Despite advances in the generation of material appearance based on analytic models, the synthesis of real-world measured BRDFs remains largely unexplored. To address this challenge, we propose M^3ashy, a novel multi-modal material synthesis framework based on hyperdiffusion. M^3ashy enables high-quality reconstruction of complex real-world materials by leveraging neural fields as a compact continuous representation of BRDFs. Furthermore, our multi-modal conditional hyperdiffusion model allows for flexible material synthesis conditioned on material type, natural language descriptions, or reference images, providing greater user control over material generation. To support future research, we contribute two new material datasets and introduce two BRDF distributional metrics for more rigorous evaluation. We demonstrate the effectiveness of M^3ashy through extensive experiments, including a novel statistics-based constrained synthesis, which enables the generation of materials of desired categories.

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ICML Conference 2025 Conference Paper

LSCD: Lomb-Scargle Conditioned Diffusion for Time series Imputation

Elizabeth Fons
Alejandro Sztrajman
Yousef El-Laham
Luciana Ferrer
Svitlana Vyetrenko
Manuela Veloso

Time series with missing or irregularly sampled data are a persistent challenge in machine learning. Many methods operate on the frequency-domain, relying on the Fast Fourier Transform (FFT) which assumes uniform sampling, therefore requiring prior interpolation that can distort the spectra. To address this limitation, we introduce a differentiable Lomb–Scargle layer that enables a reliable computation of the power spectrum of irregularly sampled data. We integrate this layer into a novel score-based diffusion model (LSCD) for time series imputation conditioned on the entire signal spectrum. Experiments on synthetic and real-world benchmarks demonstrate that our method recovers missing data more accurately than purely time-domain baselines, while simultaneously producing consistent frequency estimates. Crucially, our method can be easily integrated into learning frameworks, enabling broader adoption of spectral guidance in machine learning approaches involving incomplete or irregular data.

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TMLR Journal 2024 Journal Article

iHyperTime: Interpretable Time Series Generation with Implicit Neural Representations

Elizabeth Fons
Alejandro Sztrajman
Yousef El-Laham
Andrea Coletta
Alexandros Iosifidis
Svitlana Vyetrenko

Implicit neural representations (INRs) have emerged as a powerful tool that provides an accurate and resolution-independent encoding of data. Their robustness as general approximators has been shown across diverse data modalities, such as images, video, audio, and 3D scenes. However, little attention has been given to leveraging these architectures for time series data. Addressing this gap, we propose an approach for time series generation based on two novel architectures: TSNet, an INR network for interpretable trend-seasonality time series representation, and iHyperTime, a hypernetwork architecture that leverages TSNet for time series generalization and synthesis. Through evaluations of fidelity and usefulness metrics, we demonstrate that iHyperTime outperforms current state-of-the-art methods in challenging scenarios that involve long or irregularly sampled time series, while performing on par on regularly sampled data. Furthermore, we showcase iHyperTime fast training speed, comparable to the fastest existing methods for short sequences and significantly superior for longer ones. Finally, we empirically validate the quality of the model's unsupervised trend-seasonality decomposition by comparing against the well-established STL method.

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NeurIPS Conference 2023 Conference Paper

Neural Fields with Hard Constraints of Arbitrary Differential Order

Fangcheng Zhong
Kyle Fogarty
Param Hanji
Tianhao Wu
Alejandro Sztrajman
Andrew Spielberg
Andrea Tagliasacchi
Petra Bosilj

While deep learning techniques have become extremely popular for solving a broad range of optimization problems, methods to enforce hard constraints during optimization, particularly on deep neural networks, remain underdeveloped. Inspired by the rich literature on meshless interpolation and its extension to spectral collocation methods in scientific computing, we develop a series of approaches for enforcing hard constraints on neural fields, which we refer to as Constrained Neural Fields (CNF). The constraints can be specified as a linear operator applied to the neural field and its derivatives. We also design specific model representations and training strategies for problems where standard models may encounter difficulties, such as conditioning of the system, memory consumption, and capacity of the network when being constrained. Our approaches are demonstrated in a wide range of real-world applications. Additionally, we develop a framework that enables highly efficient model and constraint specification, which can be readily applied to any downstream task where hard constraints need to be explicitly satisfied during optimization.

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