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Depiction is reference conveyed through pictures. Basically a picture refers to its object through a non-linguistic two-dimensional scheme. A picture is not writing or notation. A depictive two-dimensional scheme is called a picture plane and may be constructed according to descriptive geometry where they are usually divided between projections (orthogonal and various oblique angles) and perspectives (according to number of vanishing points). 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, picture-like features may be recognised in simple inkblots, accidental stains, peculiar clouds or a glimpse of the moon, but these are special cases, and it is controversial whether they count as genuine instances of depiction. Similarly, sculpture and theatrical performances are sometimes said to depict, but this requires a broad understanding of 'depict', as simply designating a form of representation that is not linguistic or notational. The bulk of studies of depiction however deal only with pictures. While sculpture and performance clearly represent or refer, they do not strictly picture their objects.
Objects pictured 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. altes 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.
But just how pictures function remains controversial. Philosophers, art historians and critics, perceptual psychologists and other researchers in the arts and social sciences have contributed to the debate and many of the most influential contributions have been interdisciplinary. Some key positions are briefly surveyed below.