Pictures are surfaces. Pictures show surfaces. But what is a theory of perception of surfaces? Surface perception was first mentioned in experimental psychology by Metzger in Ganzfeld experiments in the 1930s. However, it was first offered as a serious concept in perception theory by Alhazen in his Book of Optics (1039). Remarkably, almost no contemporary theory of perception uses the term. To rectify this omission, a theory of surfaces is presented here, suggesting that surface perception occurs in all 8 of vision's modes. Optical information for the shapes of surfaces is given by the ratio of azimuth to elevation. Flat surfaces such as the ground have a linear to quadratic ratio. Increase the ratio and hills are seen. Decrease it and the surrounds are a bowl. Sudden changes in the ratio indicate changes in slant. Sudden changes in density without changes in the ratio indicate a drop-off. The theory is applied to outline drawing and to the fact that pictures provide two surfaces (the real surface of the picture and the depicted surface). The two surfaces create illusions. Features on the picture surface cannot be seen correctly. The importance of surface perception is its breadth of application. The theory of surface perception shows why pictures taken on the Moon or Mars are as intelligible as terrestrial pictures. Surfaces allow control of action even for creatures that fly in 3D without touching surfaces during flight, such as bats and birds.