1 Perception, Illusion and VR HNRS 299, Spring 2008 Lecture 10 Vision and Art Thanks to John Krantz at Hanover College:

2 Artists Use Multiple Depth Cues For realistic paintings, artists use multiple cues to depth: Height in the visual field Interposition Relative Size Texture Gradients Shading Aerial perspective Linear Perspective

3 Height in the Visual Field "The Coast of Protrieux," by Eugene Boudin

4 Interposition Depth order can be determined when one object appears to be covering part of another. This is known as interposition. "Madonna of the magnificat," by Sandro Botticelli

5 Relative Size "Sculls" by Gustave Caillebotte

6 Texture Gradient Paris Street: A Rainy Day by Gustave Caillebotte

7 Shading "Rock Art in the British Landscape", Photographs by Gus van Veen & Jan Brouwer

8 Aerial Perspective Because of light scattering by the atmosphere, things in the distance have a bluish tinge and are blurrier. Photograph of Grand Canyon

9 More Aerial perspective "Near Salt Lake City," by Albert Bierstadt

10 Linear Perspective Paris Street: A Rainy Day by Gustave Caillebotte

11 Size Constancy When 3D scenes are projected onto the retina, the images of distant objects are smaller than those of nearby objects. We don't perceive these objects as smaller. Our visual system corrects our perception of size, based on its estimate of the distance to the item. Because depth cues in paintings are not as strong as for a real 3D scene (e.g. there is no stereo or motion parallax), our estimate of distance is not as good. Thus, the visual system doesn't correct for size constancy as well. (E.g. distant objects in a photograph look too small). Painters can correct for this, by using less linear perspective. (i.e. object sizes don't decrease as much with distance in the painting).

12 Spatial Summation When images are too close together for the visual system to resolve, it sums the light information from the images. For example, the dots that form a TV image are all red, green or blue. We don't see the individual dots, but add them together to see color.

13 Spatial Summation "Sunday Afternoon on the Island of Grand Jatte," by Georges Seurat

14 Figure-Ground Face-Vase "Moebius with birds," by M.C. Escher The visual system distinguishes objects (the figure) from their background (the ground). In these images, the figure-ground distinction is ambiguous.

15 Impossible Figures The impossible trident. In this figure the depth cues at one end of the trident are incompatible with the depth cues at the other end. Artists can use visual cues to fool the visual system into seeing figures that are not possible in real scenes.

16 M.C. Escher Climbing and DescendingWaterfall

17 The Ames Room The angles aren't right angles. The image projected on the retina looks rectangular.

18 The Ames Window The Ames Window is a trapezoid shape, so that it appears to slant back in depth. When put into motion, it causes some interesting illusions.