Depiction is a form of non-verbal representation in which two-dimensional images (pictures) are regarded as viable substitutes for things seen, remembered or imagined. Basically, a picture maps an object to a two-dimensional scheme or picture plane. Pictures are made with various materials and techniques, such as painting, drawing, or prints (including photography and movies) mosaics, tapestries, stained glass, and collages of unusual and disparate elements. Occasionally pictures may occur in simple inkblots, accidental stains, peculiar clouds or a glimpse of the moon, but these are special cases. Sculpture and performances are sometimes said to depict but this arises where depiction is taken to include all reference that is not linguistic or notational. The bulk of research in depiction however deals only in pictures. While sculpture and performance clearly represent or refer, they do not strictly picture their objects. Pictures may be factual or fictional, literal or metaphorical, realistic or idealised and in various combination. Idealised depiction is also termed schematic or stylised and extends to icons, diagrams and maps. Classes or styles of picture may abstract their objects by degrees, conversely, establish degrees of the concrete (usually called, a little confusingly, figuration or figurative, since the 'figurative' is then often quite literal). Stylisation can lead to the fully abstract picture, where reference is only to conditions for a picture plane – a severe exercise in self-reference and ultimately a sub-set of pattern.