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Publicações

Publicações por CTM

2025

Model compression techniques in biometrics applications: A survey

Autores
Caldeira, E; Neto, PC; Huber, M; Damer, N; Sequeira, AF;

Publicação
INFORMATION FUSION

Abstract
The development of deep learning algorithms has extensively empowered humanity's task automatization capacity. However, the huge improvement in the performance of these models is highly correlated with their increasing level of complexity, limiting their usefulness in human-oriented applications, which are usually deployed in resource-constrained devices. This led to the development of compression techniques that drastically reduce the computational and memory costs of deep learning models without significant performance degradation. These compressed models are especially essential when implementing multi-model fusion solutions where multiple models are required to operate simultaneously. This paper aims to systematize the current literature on this topic by presenting a comprehensive survey of model compression techniques in biometrics applications, namely quantization, knowledge distillation and pruning. We conduct a critical analysis of the comparative value of these techniques, focusing on their advantages and disadvantages and presenting suggestions for future work directions that can potentially improve the current methods. Additionally, we discuss and analyze the link between model bias and model compression, highlighting the need to direct compression research toward model fairness in future works.

2025

SHAPing Latent Spaces in Facial Attribute Classification Models

Autores
Ferreira, L; Oliveira, M; Goncalves, T; Mamede, RM; Neto, PC; Sequeira, AF;

Publicação
2025 INTERNATIONAL CONFERENCE OF THE BIOMETRICS SPECIAL INTEREST GROUP, BIOSIG

Abstract
This study investigates the use of SHAP (SHapley Additive exPlanations) values as an explainable artificial intelligence (xAI) technique applied on a facial attribute classification task. We analyse the consistency of SHAP value distributions across diverse classifier architectures that share the same feature extractor, revealing that key features driving attribute classification remain stable regardless of classifier architecture. Our findings highlight the challenges in interpreting SHAP values at the individual sample level, as their reliability depends on the model's ability to learn distinct class-specific features; models exploiting inter-class correlations yield less representative SHAP explanations. Furthermore, pixel-level SHAP analysis reveals that superior classification accuracy does not necessarily equate to meaningful semantic understanding; notably, despite FaceNet exhibiting lower performance than CLIP, it demonstrated a more nuanced grasp of the underlying class attributes. Finally, we address the computational scalability of SHAP, demonstrating that KernelExplainer becomes infeasible for high-dimensional pixel data, whereas DeepExplainer and GradientExplainer offer more practical alternatives with trade-offs. Our results suggest that SHAP is most effective for small to medium feature sets, providing interpretable and computationally manageable explanations.

2025

RISC++: Towards an HLS Defined RISC-V SoC

Autores
De Oliveira, GV; Pirassoli, V; Sousa, LM; Paulino, N;

Publicação
DSD

Abstract
The relevance of heterogeneous architectures has significantly increased over the last decade due to stagnation of performance scaling. Concurrently, increased performance-energy tradeoff requirements driven by the growth of edge computing, with a large focus on Artificial Intelligence (AI) inference, further motivates efforts towards hardware customization. In this context, the open RISC-V Instruction Set Architecture (ISA) and its custom extension oriented paradigm are a relevant technology towards this specialization. However, customizing a processor is a lengthy process requiring Hardware Description Language (HDL) expertise. Furthermore, for validation and simulation purposes, implementing an Instruction Set Simulator (ISS) of the modified core may also be a necessity. This introduces the need for development of two unrelated codebases, increasing development time and effort. In this paper, we explore High-Level-Synthesis (HLS) to realize both the hardware and the respective simulator through a single codebase, which reduces design effort and simplifies specialization of a RISC-V through specification of custom instructions at high level. We present a C++ based design of a RISC-V core, and validate it as an ISS, as well as a hardware module synthesized for an AMD Zynq UltraScale+ Field Programmable Gate Array (FPGA) through HLS, which we integrated in a System-on-Chip (SoC), and functionally validated through a state-of-the-art set of unit tests.

2025

SIMD Acceleration of Matrix-Vector Operations on RISC-V for Variable Precision Neural Networks

Autores
Salinas, G; Sequeira, G; Rodriguez, A; Bispo, J; Paulino, N;

Publicação
2025 IEEE INTERNATIONAL PARALLEL AND DISTRIBUTED PROCESSING SYMPOSIUM WORKSHOPS, IPDPSW

Abstract
The rapid proliferation of Edge AI applications demands efficient, low-power computing architectures tailored to specific workloads. The RISC-V ecosystem is a promising solution, and has led to a fast growth of implementations based on custom instructions extensions, but with varying degrees of functionality and support which may hinder easy adoption. In this paper, we extensively review existing RISC-V extensions targeting primarily the AI domain and respective compilation flows, highlighting challenges in deployment, usability, and compatibility. We further implement and provide usable containerized environments for two of these works. To address the identified challenges, we then propose an approach for lightweight early validation of custom instructions via source-to-source transformations, without need of compiler modifications. We target our own Single Instruction Multiple Data (SIMD) accelerator, which we integrate into a CORE-V cv32e40px baseline core through custom instructions, and versus which we achieve up to 11.9x speedup for matrix-vector operations.

2025

Acceleration of C/C plus plus Kernels and ONNX Models on CGRAs with MLIR-Based Compilation

Autores
Gallego, J; Ferreira, J; Alves, L; Vázquez, D; Bispo, J; Rodríguez, A; Paulino, N; Otero, A;

Publicação
2025 40TH CONFERENCE ON DESIGN OF CIRCUITS AND INTEGRATED SYSTEMS, DCIS

Abstract
Executing Artificial Intelligence (AI) at the edge is challenging due to tight energy and computational constraints. Heterogeneous platforms, particularly those incorporating Coarse-Grained Reconfigurable Arrays (CGRAs), offer a compelling trade-off between hardware specialization and programmability, supporting spatially distributed and energy-efficient computation. Despite their potential, the deployment of applications on CGRA accelerators remains limited by the lack of practical toolchains and methodologies. In this work, we propose a compilation flow based on MLIR to enable the seamless integration of both C/C++ kernels and ONNX-based AI models into a RISC-V system augmented with a CGRA accelerator. Our approach extracts the underlying Data Flow Graph (DFG) from the high-level representation. It maps it onto the CGRA using an Integer Linear Programming (ILP) mapper that accounts for the accelerator's architectural constraints. A custom backend completes the toolchain by generating the necessary binaries for coordinated execution across the RISC-V processor and the CGRA. This framework enables the practical deployment of heterogeneous edge workloads, combining the flexibility of software execution with the efficiency of hardware acceleration.

2025

A case study on phishing detection with a machine learning net

Autores
Bezerra, A; Pereira, I; Rebelo, MA; Coelho, D; de Oliveira, DA; Costa, JFP; Cruz, RPM;

Publicação
INTERNATIONAL JOURNAL OF DATA SCIENCE AND ANALYTICS

Abstract
Phishing attacks aims to steal sensitive information and, unfortunately, are becoming a common practice on the web. Email phishing is one of the most common types of attacks on the web and can have a big impact on individuals and enterprises. There is still a gap in prevention when it comes to detecting phishing emails, as new attacks are usually not detected. The goal of this work was to develop a model capable of identifying phishing emails based on machine learning approaches. The work was performed in collaboration with E-goi, a multi-channel marketing automation company. The data consisted of emails collected from the E-goi servers in the electronic mail format. The problem consisted of a classification problem with unbalanced classes, with the minority class corresponding to the phishing emails and having less than 1% of the total emails. Several models were evaluated after careful data selection and feature extraction based on the email content and the literature regarding these types of problems. Due to the imbalance present in the data, several sampling methods based on under-sampling techniques were tested to see their impact on the model's ability to detect phishing emails. The final model consisted of a neural network able to detect more than 80% of phishing emails without compromising the remaining emails sent by E-goi clients.

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