1 Perception, Illusion and VR HNRS 299, Spring 2008 Lecture 2 Introduction, Light Course webpage:

2 What's the Big Deal? Vision seems easy. It is effortless for us. Building machine vision systems is hard. Machines still cannot see. Understanding how the brain processes visual information is hard. We still understand only the most basic computations. To understand why it is so difficult, we must examine the problem.

3 Light behaves like both particles and waves Light sometimes acts as individual particles: Photons are individual particles of light. Photons are detected by photoreceptor cells in the retina. Light sometimes behaves as a wave: Light travels in waves, that have a frequency or wavelength. The wavelength of light affects the perceived color. The electromagnetic spectrum

4 The properties of light Light is emitted from one or more sources. These may be point sources or more distributed sources of light. The light hits surfaces and interacts with them, with some being reflected, some absorbed and some transmitted. The reflected light may bounce off multiple surfaces before reaching the eye. Some of the light rays will eventually reach the eye and be focused on the retina. We will diagram this in class.

5 The Eye as a Pinhole Camera We can approximate the image formation performed by the lens of the eye as a pinhole camera. Light rays from an object project through a single point (the center of projection) onto the retina (or image plane). This is perspective projection. We will diagram this in class. Objects that are more distant form smaller images. We will work out the equation in class.

6 Ill-posed problems It is not so hard to compute a 2D image from a 3D scene. It is very difficult to compute the original 3D scene from the 2D image. Many aspects of a 2D scene are consistent with multiple (or even infinite) possible arrangements of the 3D scene. Because there is no single solution, the problem is ill-posed.

7 Visual Difficulties An image of a given size can represent a small, nearby object or a large, distant object. Reflectance changes occur at corners of a room or of objects. How do we see the color as constant? How do we know which edges belong to which objects? When you move your eyes, the image moves. How do you know the world is not moving?

8 How do we solve it? To solve ill-posed problems, we must make assumptions about the scene and the objects in it. These assumptions are also known as heuristics or constraints. We can determine which assumptions are made by the human visual system by performing psychophysical experiments. Psychophysical experiments use a known physical stimulus to test what information is important for perception and what the limits of the visual system are.

9 Height in the Visual Field In normal images, things that are farther away often have images that are higher in the visual field. The visual system uses this height in the visual field as a cue to distance. We then adjust our interpretation of the size of the object based on this distance.

10 A small man When we remove the height cue, the second man looks small indeed!

11 Assumption of Right Angles The visual system has a tendency to interpret angles as right angles. We see this as a right angle This allows us to construct interesting visual illusions such as...

12 The Ames Room Ever wonder how the hobbits are made to look so short in "The Lord of the Rings" movies?