The handwritten signature has been identified as one of the most popular biometric means of human consent and/or presence for transactions held by any number of physical or legal entities. Automated signature verification (ASV), merge popular scientific branches such as computer vision, pattern recognition and/or data-driven machine learning algorithms. Up to now, several metric learning approaches for designing a writer-independent signature verifier, have been developed within a Euclidean framework by means of having their operations closed with respect to real vector spaces. In this work, we propose, for the first time in the ASV literature, the use of a meta-learning framework in the space of the Symmetric Positive Definite (SPD) manifold as a means to learn a pairwise similarity metric for writer-independent ASV. To begin, pairs of handwritten signatures are converted into a multidimensional distance vector with elements corresponding SPD distances between spatial segments of corresponding covariance pairs. We propose a novel meta-learning approach which explores the structure of the input gradients of the SPD manifold by means of a recurrent model, constrained by the geometry of the SPD manifold. The experimental protocols utilize two popular signature datasets of Western and Asian origin in two blind-intra and blind-inter (or cross-lingual) transfer learning approach. It also provide evidence of the discriminating nature of the proposed framework at least when summarized against other State-of-the-Art models, typically realized under a framework of Euclidean, or vector space, nature.

Identifying the existence or approval of a human in a number of past, recent and present day activities with the use of a handwritten signature is a captivating biometric challenge. Several engineering branches such as computer vision, pattern recognition and quite recently data-driven machine learning algorithms are combined in a multi-disciplined signature verification framework in order to deliver an equivalent and efficient e-assistance to manually executed duties, which usually demand knowledge and skills. In this work, we propose, for the first time, the use of a learnable Symmetric Positive Definite manifold distance framework in offline signature verification literature in order to build a global writer-independent signature verification classifier. The key building block of the framework relies on the use of regional covariance matrices of handwritten signature images as visual descriptors, which maps them into the Symmetric Positive Definite manifold. The learning and verification protocol explores both blind intra and blind inter transfer learning frameworks with the use of four popular signature datasets of Western and Asian origin. Experiments strongly indicate that the learnable SPD manifold similarity distance can be highly efficient for offline writer independent signature verification.

In spite of the overwhelming high-tech marvels and applications that rule our digital lives, the use of the handwritten signature is still recognized worldwide in government, personal and legal entities to be the most important behavioral biometric trait. A number of notable research approaches provide advanced results up to a certain point which allow us to assert with confidence that the performance attained by signature verification (SV) systems is comparable to those provided by any other biometric modality. Up to now, the mainstream trend for offline SV is shared between standard -or handcrafted- feature extraction methods and popular machine learning techniques, with typical examples ranging from sparse representation to Deep Learning. Recent progress in graph mining algorithms provide us with the prospect to re-evaluate the opportunity of utilizing graph representations by exploring corresponding graph features for offline SV. In this paper, inspired by the recent use of image visibility graphs for mapping images into networks, we introduce for the first time in offline SV literature their use as a parameter free, agnostic representation for exploring global as well as local information. Global properties of the sparsely located content of the shape of the signature image are encoded with topological information of the whole graph. In addition, local pixel patches are encoded by sequential visibility motifs-subgraphs of size four, to a low six dimensional motif profile vector. A number of pooling functions operate on the motif codes in a spatial pyramid context in order to create the final feature vector. The effectiveness of the proposed method is evaluated with the use of two popular datasets. The local visibility graph features are considered to be highly informative for SV; this is sustained by the corresponding results which are at least comparable with other classic state-of-the-art approaches.

The handwritten signature is perhaps the most accustomed way for the acknowledgement of the consent of an individual or the authentication of the identity of a person in numerous transactions. In addition, the authenticity of a questioned offline or static handwritten signature still poses a case of interest, especially in forensic related applications. A common approach in offline signature verification system is to apply several predetermined image analysis models. Consequently, any offline signature sample which originates from either authentic persons or forgers, utilizes a fixed feature extraction base. In this proposed study, the feature space and the corresponding projection values depend on the training samples only; thus the proposed method can be found useful in forensic cases. In order to do so, we reenter a groundbreaking unsupervised learning method named archetypal analysis, which is connected to effective data analysis approaches such as sparse coding. Due to the fact that until recently there was no efficient implementation publicly available, archetypal analysis had only few cases of use. However, a fast optimization scheme using an active set strategy is now available. The main goal of this work is to introduce archetypal analysis for offline signature verification. The output of the archetypal analysis of few reference samples is a set of archetypes which are used to form the base of the feature space. Then, given a set of archetypes and a signature sample under examination archetypal analysis and average pooling provides the corresponding features. The promising performance of the proposed approach is demonstrated with the use of an evaluation method which employs the popular CEDAR and MCYT75 signature datasets.

This work introduces a novel representation for the off-line handwritten signature by measuring its structural characteristics with a set of partially ordered grid features. Thirty-two binary symbols are delineated within the five-by-five pixel window and considered to be the alphabet of a probabilistic source. Subsequently and for practical purposes the whole set is appropriately organized into subsets of four symbols each. The new arrangement is used to detect the presence of simple or compound symbols in the signature image. The utilization of the partially ordered set or poset notion, intuitively arranges the binary feature extraction masks into first order chains, creating in this way a first order probabilistic description of the signature's structure that is characteristic of the motoric signature generating process. In order to estimate these probabilities, the first order searching strategy is limited to pixels neighbors having their grids centered to a predetermined Chebyshev distance of two. The utilized verification strategy relies on an SVM based classifier and the equal error rate figure. Our experimental scheme handles skilled forgery as well and derives verification results for two datasets, the GPDS300 and a proprietary one.

In this work, a feature extraction method for off-line signature recognition and verification is proposed, described and validated. This approach is based on the exploitation of the relative pixel distribution over predetermined two and three-step paths along the signature trace. The proposed procedure can be regarded as a model for estimating the transitional probabilities of the signature stroke, arcs and angles. Partitioning the signature image with respect to its center of gravity is applied to the two-step part of the feature extraction algorithm, while an enhanced three-step algorithm utilizes the entire signature image. Fusion at feature level generates a multidimensional vector which encodes the spatial details of each writer. The classifier model is composed of the combination of a first stage similarity score along with a continuous SVM output. Results based on the estimation of the EER on domestic signature datasets and well known international corpuses demonstrate the high efficiency of the proposed methodology.

In this work the most important morphological granulometry, i.e. the pattern spectrum, is modeled, for the first time in the literature, as a first order Markov process. In addition, each of the terms of the process is shown to be normally distributed. The classification procedure followed for this specific application is based on modeling each separate class as a Markov process and making extensive use of the chain rule. Experimental results support the proposed classification procedure as quite promising, especially when compared to conventional classification techniques.