A new visual problem: phenomenic folding.

It is possible to produce outline drawings that are perceived as representations of sheets or plates folded over themselves. However, only some of the many possible representations are immediately and necessarily perceived as such. Investigations were carried out to find out which elements must be included in a drawing if a subject is to perceive folding. Four necessary, though not individually sufficient, factors were detected. Other factors which are not necessary but which can intensify the perception of folding were also found. The four necessary factors are: (i) the existence of two phenomenically overlapping figures; (ii) at least one side of the upper figure must perfectly coincide with one side of the lower figure, this common side being defined as the folding line; (iii) the two phenomenically overlapping areas must be on the same side of the folding line; (iv) three segments must converge at the ends of the folding line. Some cognitive processes which appear to be involved in the phenomenon are also discussed.