Manga (Japanese comic) is a globally popular content. In recent years, sales of e-comics that converted to electronic data from paper-based manga are increasing because of the widespread use of electronic terminals. Against this background, it has been proposed to improve the accessibility of e-comics by tagging manga images with metadata. In order to allocate metadata more efficiently, technology that automatically extracts elements such as character and speech is required. One way to classify characters is to get image features from the character's faces and cluster them. Previous research has shown that using the intermediate output of CNN which fine-tuned with character face images is effective for character face recognition. We proposed a clustering method using Density-Based Spatial Clustering of Applications with Noise (DBSCAN) to classify character face images without specifying the number of clusters. However, DBSCAN is greatly affected by the hyperparameter. The purpose of this study is to automatically classify character face images without complicated hyperparameter setting. We examine the application of Ordering Points to Identify the Clustering Structure (OPTICS) and Hierarchical DBSCAN (HDBSCAN), which are density-based clustering algorithms that extend DBSCAN. OPTICS is an algorithm for finding clusters in spatial data, and HDBSCAN is an algorithm extracts flat partition from hierarchical cluster data. We also verify the effective CNN model as the feature extractor of face images. Experimental results showed that HDBSCAN is effective for character face image clustering.
Manga (Japanese comic) is popular content worldwide. In Japan, e-comic accounts for about 80% of e-book market. In recent years, metadata extraction from manga image has been studied for providing e-comic service. Manga character is one of the important contents for story understanding. In conventional research, some character identification methods are proposed those classify characters’ face images using k-means clustering. However, there are two problems. First, kmeans method needs to specify the number of clusters, however the number of characters in target manga images is commonly unknown. Second, manga includes characters with few appearing, so it is difficult to classify characters with high purity. To solve these problems, we propose clustering method using DBSCAN which decides number of clusters automatically and is robust to noise data. In our prior research, it is experimented that character face clustering using DBSCAN and general CNN features. However, general CNN model is difficult to capture detailed features of manga characters. In this paper, we apply DBSCAN to fine-tuned CNN with manga character faces to improve the clustering accuracy. We also compare the optimal parameter determination method of DBSCAN. Experimental results showed that the dimensional reduction using Kernel PCA and UMAP is effective. In addition, we confirmed the validity of proposed method that determining the parameters of DBSCAN based on the slope changing of k-distance graph.
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