A shape in art is a closed line that is limited to two directions: width and length. It is defined by other elements of art: form, space and value. A shape can be geometric, which means it is mathematically determined, or organic, which means it is created by nature.
The simplest way to outline the shape of an object is its shadow. Recognizable shapes are called naturalistic or realistic. Shapes that are difficult to identify are abstract.
Shapes can be positive and negative. A positive shape is the one formed by the object itself. The one formed by the space around the object is the negative shape. The combination of positive and negative shapes gives people the image of what they see.
Artists use this capability of the human brain to create optical illusions. For example, an object on a two-dimensional surface seems to exist in three dimensions, and a white and black drawing can seem colourful. Also, it can be difficult to distinguish between a figure and the ground in an image because both make sense and are recognizable objects. M. C. Escher, P. Picasso, H. Matisse and K. Walker are the most famous artists for creating dual shapes in their works.
The boundaries of an artwork are important for perception. For instance, a painting of a tree on a background of a distant mountain gives an impression of open air and vast space. If the same tree and mountains are painted as if in a window frame, the natural elements seem more distant, which does not offer a feeling of open space.