Which type of optical illusion involves a two-dimensional figure perceived as three-dimensional?

The type of optical illusion that involves a two-dimensional figure being perceived as three-dimensional is known as the impossible figure. This type of illusion typically presents a scene or object that cannot exist in three-dimensional space, and our perception tries to interpret it as if it were real, leading to confusion and intrigue.

Impossible figures, such as the Penrose triangle or the waterfall illusion, compel observers to engage with the artwork or image in a way that challenges their understanding of space and depth. These illusions take advantage of the brain's attempts to construct a three-dimensional perception from two-dimensional cues, creating an experience that is both visually captivating and logically paradoxical.

Other types of illusions mentioned, such as ambiguous figures and perspective illusions, focus on different aspects of perception. Ambiguous figures can be seen in multiple ways, allowing for different interpretations but not necessarily conveying an impossible three-dimensional structure. Perspective illusions often rely on depth cues to create a sense of three-dimensionality in an image, but they do so in a way that is coherent and achievable in reality. Illusory contours refer to visual perception phenomena where we perceive edges or shapes despite their absence in the actual image.