Providing a natural mapping between multi-touch gestures and manipulations of digital content is important for user-friendly interfaces. Although there are some guidelines for 2D digital content available in the literature, a guideline for manipulation of 3D content has yet to be developed. In this research, two sets of gestures were developed for experiments in the ease of manipulating 3D content on a touchscreen. As there typically are large differences between age groups in the ease of learning new interfaces, we compared a group of adults with a group of children. Each person carried out three tasks linked to rotating the digital model of a green turtle to inspect major characteristics of its body. Task completion time, subjective evaluations, and gesture changing frequency were measured. Results showed that using the conventional gestures for 2D object rotation was not appropriate in the 3D environment. Gestures that required multiple touch points hampered the real-time visibility of rotational effects on a large screen. While the cumulative effects of 3D rotations became complicated after intensive operations, simpler gestures facilitated the mapping between 2D control movements and 3D content displays. For rotation in Cartesian coordinates, moving one fingertip horizontally or vertically on a 2D touchscreen corresponded to the rotation angles of two axes for 3D content, while the relative movement between two fingertips was used to control the rotation angleof the third axis. Based on behavior analysis, adults and children differed in the diversity of gesture types and in the touch points with respect to the object's contours. Offering a robust mechanism for gestural inputs is necessary for universal control of such a system.

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