The term Convention is applied to decoration in which there is distinct evidence of artistic restraint, which may be purely æsthetic or due to technical conditions.
The rendering of any form in outline is probably the simplest form of convention, which is generally accepted through tradition as representation. Though the objects so depicted really depend on light, shade, and local colour for their appearance.
Such outline drawings may be in other respects realistic, but a further degree of convention is the desirable elimination of perspective where it is unsuitable to the effect desired. For instance, in silhouette, profile renderings only are intelligible, and in delicate bas-relief modelling any foreshortening should be avoided if confusing to the effect.
In direct personal work, such as drawing or painting, when craft conditions other than that of the medium employed are not involved, convention is purely a matter of discretion and consideration of the nature and object of the work; but it is essential in design when the material and method of production have to be considered. In painting the artist may employ the full resources of his palette and be as realistic in his effect as is in his power, but if the purpose be mural decoration restraint is at once imposed.
Under certain conditions realism would be out of place, and any attempt at illusion would fail to convince. The one time fashion of painting ceilings with sprawling deities of either sex, which cannot be seen without a painful crick in the neck, or worse still to suggest sky with floating amorini, occasionally framed by marble balustrading in wonderful perspective is deplorable.
Such decoration, if it can be so termed, is not only stagey but is foredoomed to failure in effect, as the ordinary interior lighting is not adequate. Furthermore, it displays a lack of appreciation of fitness, and that the purpose of a ceiling is to convey a sense of shelter.
Realism, though desirable in portraiture, either of individuals, places or events, is not necessarily of the greatest interest except to those concerned. In mural decoration realism should give place to convention, and the whole considered as a design with regard to balance of form and colour, and recognition of the surface to which the decoration is applied.