Self-Organizing Topological Tree for Online Vector Quantization and Data Clustering Advisor : Dr. Hsu Graduate : Kuo-min Wang Authors :Pengei Xu, Chip-Hong, Senior Member,IEEE Andrew Palinski , Member, IEEE 2005 Expert Systems with Applications .
Outline • Motivation • Objective • Introduction • Structure of SOTT and Related Terminology • Training Algorithm of SOTT • Simulation Results • Conclusions • Personal Opinion
Motivation • SOM have two important operations • Vector quantization • Topology-preserving mapping • Disadvantage in its application to clustering problem 1. Clustering result is sensitive to the number of partitions 2. The clustering result of one partition provides knowledge at only one similarity level 3. Favoring “equally-sized compact spheroidal clusters” 4. Computational complexity is long.
Objective • Propose an online self-organizing topological tree (SOTT) with faster learning. • Computational complexity is O (log N) rather than O (N) as for the basic SOM • A hybrid clustering algorithm that fully exploit the online learning and multi-resolution characteristics of SOTT is devised. • A new linkage metric is proposed which can be updated online to accelerate the time consuming agglomerative hierarchical clustering stage.
Introduction • The applications of SOM • Obtain an optimal set of codebook vectors that maximizes the rate-distortion performance • Clustering which aims to segregate a chaotic mixture of patterns for the purpose of knowledge discovery and analysis • Growing SOM • overcomes the first problem • By growing the SOM to suitable number of partitions through the insertion of new neurons
Introduction (cont.) • Tree-Structured SOM  • provide a hierarchical structure to reduce the computation complexity and alleviates the first two problems simultaneously. • GHSOM  • Proposed to grow a hierarchical SOM to solve the third problem.
Introduction (cont.) • Vector Quantization • 由Y. Linde, A. Buzo, and R. M. Gray 三位學者於1980年所提出 • 將影像切割成一群大小是n × n的影像區塊 • 每個以利用事先設計好的編碼簿來處理。
Introduction (cont.) • Generalized Lloyd Algorithm (GLA)  • 從一群區塊向量(training vector)中，使用分群(clustering)的方法，去訓練一個能夠還原原影像區塊的編碼簿 • Tree search vector quantizer (TSVQ)  • 加速搜尋最鄰近碼向量的過程 • 需要較多的儲存空間 • 利用樹狀結構編碼簿所得到的影像品質，較傳統向量量化編碼簿的影像品質來的差。 • Tree-structure SOM (TS-SOM)  • Organized layers by layers • All training data are fed into the system repeatedly at every layer, taxes the system resources heavily for large database.
Introduction (cont.) • We propose a new multi-resolution self-organizing topological tree (SOTT) to accelerate the search procedure. • Globally suboptimal and fails to find the real BMU • Using multi-path to overcome • How to maintain two kinds of neighborhood relationship co-exist in the network • The inter-layer parent-child relationship • And the intra-layer sibling relationship • Using winning path to overcome
Introduction (cont.) • In hybrid clustering scheme, a low complexity partition clustering algorithm is first applied to reduce the large amount of data before the computational AHC • Linkage metric is a proximity measure used to merge subset rather than individual points in AHC SOTT AHC algorithm
Structure of SOTT and Related Terminology • A static SOTT can be viewed as a multi-layer SOM, with fixed depth and breadth. • Input vector and • L, the number of layers and Niis the number of neurons at the ith layer • The ith layer hasneurons • Two kinds of relationship • The intra-layer neighborhood • The inter-layer neighborhood
Structure of SOTT and Related Terminology • Gi is a fully connected graph by the neurons and their interconnections at the ith layer • A neuron, u is said to be in the k-distance neighborhood of the neuron v if there is a connected path from u to v and || u – v || ≦k • u is said to be a child of vthe neurons of have the same parent neuron v, are called the siblings
Butterfly Permutation for Input Randomization • Online learning causes the learning performance to be order dependent, when the training set contains a high degree of redundant information • A block based butterfly jumping sequence was used to subsample the pixels from each block to form different training sweeps by Pei and Lo.
Butterfly Permutation for Input Randomization (cont.) • A global butterfly permutation sequence is used to present the spatially correlated input data from a multidimensional coordinate system • The aim is to let the neurons learn the characteristics of the training source as early as possible to prevent the performance degraded by order dependent learning. • The butterfly permutation is defined by a mapping :an input order number to a n-dimensional coordinate system, where is a finite integer space which bounded by [0,2J-1]n.
Searching for the Winning Path • The updating of the winning neuron and its neighborhood • Until a winning path has been identified for each input • To trace the winning path, we need to search for asingle winning leaf • Uses two key parameters λκ to bias the competitiveness of some layers and emulate the positive effect of a multi path search • The idea of the algorithm is 1) find the winning child neurons progressively on each layer, until a winning child at the leaf layer is found. 2) Then path to the win_leaf is set as the winning path.
Updating of Winning Path Neurons and Their Neighborhoods • , is a monotonic decreasing gain function of the sweep time, this neighborhood taper is Neighborhood width
Updating of Winning Path Neurons and Their Neighborhoods • Maintain both the intra-layer relationship and the inter-layer relationship correct is import, the following updating rules are imposed 1. The initialization of neighborhood widths is proportional 2. the children neurons will only be updated if their parent neuron is also updated 3. the neighborhood neurons will only be updated if it is sufficiently close to the winning neuron of their layer.
Convergence Criteria • If the average square difference of the neuron weights, wLj at the leaf layer is less than 0.1, the training is terminated
SOTT AHC Hybrid Clustering Algorithm on SOTT • The main idea behind the hybrid clustering is to combine the efficiency of the partition clustering and the prowess of discrimination of AHC • To merge clusters rather than individual points the distance between individual points has to be generalized to the distance between clusters (sets of points)
Hybrid Clustering Algorithm on SOTT • A metric Bond (Ai, Aj) to assess the connectivity of two atomic clusters Ai and Aj is defined as follows: • Computational complexity is O ((ki + kj) kikj)
Simulation Results • Measure the performance of the proposed SOTT in VQ and compare it to the performance of the SOM, GLA, and TSVQ
Conclusions • The proposed SOTT hybrid clustering algorithm has demonstrated to be • Computational efficient and possesses good scalability • Overcome the clustering performance deficiencies of k-means and SOM algorithms. • The experimental results show that the computation efficiency of SOTT is much better than that of basic SOM and other vector quantizers.
Personal Opinions • Advantage • Computational complexity is faster than others. • Application • Pattern classification applications • Drawback • The structure of the paper is not good, • So it is not easy to understand.