The subject/shape might be depicted with a simple background, or it might be made most prominent by making it much larger, more colorful, better lit, harder edged, and more detailed than any other subjects/shapes. An artist might design a piece this way in order to compel the viewer to identify with the prominently featured subject, or the artist might simply consider the single subject/shape to be worthy of prolonged attention.

Featured: The Fife Player by Édouard Manet.