Painting has always been conceived primarily in three dimensions or primarily in two according to the degree of concreteness that the painters have sought. Line is perhaps the most direct vehicle of thought and feeling. A rendering in two dimensions tends to be general. Line can suggest a third dimension, but cannot show it. When form is modeled up in light and shade, the rendering becomes correspondingly more specific, more concrete.
Symbols turn into facts, and tend to be seen with the eyes instead of with the mind.
The tendencies of modern architecture, too, contribute to the growing popularity of these media. The power of oil painting lies, to some degree, in its natural low key, its wide range of value, coupled with depth and richness in the darks. To preserve this power a glossy surface is optically necessary. The water media, with their natural high key and matte surface, together with a certain crispness natural to them and foreign to oil paint, fit the decorative requirements of modern architecture better than the oils and varnishes of past generations. If a painting in tempera is varnished, the surface may be flatted without injury to the effect.