We developed a method for inputting characters by thumb gestures. The method is optimized to be used while holding the left and right ends of a tablet PC with hands. In this paper, one stroke means a movement of a certain distance to either direction of the up, down, left or right. A gesture is defined by connecting the strokes, however, connection to the same direction is prohibited. In addition, each gesture is designed inside the range of one stroke in vertical and horizontal from its start position. For this reason, all gestures are completed within the area in which the fingertip of thumb can be reached without difficulty. Hiragana, alphanumeric and symbols are entered with the dominant hand. Gestures carried out in the opposite hand are assigned to some control codes that are needed to kana-to-kanji conversion and document modification. The guide window is displayed to assist the character input. Since the image of the guide is refreshed each time when one stroke is recognized, a user can enter any character by moving his thumb to the direction of the character shown on the guide, even if he or she does not memorize its gesture. The experimental result showed that beginners may understand the system by 20 minutes training. Input speed at that time was 18.2 character per minute (cpm) in average. The speed of the person using for four months was about 59.4 cpm.
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