An ambigram is a word which can be also read from a different direction. Generation of ambigrams, however, requires not only enormous work but also design skills for maintaining their readability. This paper, therefore, aims at ambigram generation for a pair of Hiragana. First, characters are thinned, and strokes are extracted and simplified to obtain feature points of the characters. Next, subgraphs are identified from the graphs with the feature points as nodes, common structures between the characters are extracted by verifying isomorphism of the subgraphs, and ambigram structure is generated by affine transformation based on the correspondence between common structures. Finally, stroke thicknesses are exchanged with brush style strokes to improve their readability and 1,081 ambigrams were generated from pairs of 46 hiragana characters. Furthermore, readability of ambigrams was evaluated. In case of a single ambigram, 325 out of 724 ambigrams which can be given by hand crafted were readable. Moreover, 93 out of 357 ambigrams which are not pointed out to be generated were readable. Words composed of 4 hiragana characters including one ambigram with a low readability also revealed that 216 out of 246 words were readable.
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