The authors have been developing the computer assisted instruction system for machine drawing exercises that presents three-dimensional (3D) models based on projection drawing images taken with smartphones, reflecting mistakes as faith-fully as possible. In this study, a function to assist in correcting the projections by displaying auxiliary edges that indicate the existence of highly probable but not reflected edges in the model was developed. The system checks the correspondence of each segment and vertex between any two projections to generate the 3D model, and then the auxiliary edges are displayed in the case that visible outlines or hidden outlines are only drawn in one projection. So that auxiliary edges could suggest missing lines in the projections. The usefulness of the auxiliary edges in providing feedback toward proper correction to the learner was demonstrated through examples.
YUKI et al. (Wed,) studied this question.