1 Perception and VR MONT 104S, Spring 2008 Lecture 22 Other Graphics Considerations Review

2 More Complex Objects Complex objects are made up of small polygons, usually triangles (sometimes 1000's of them). Specific algorithms allow the creation of curved surfaces from polygons. More polygons give smoother surfaces, but are slower for rendering. Speeding things up for real-time applications often means using somewhat blocky looking objects.

3 Lighting and Shading Most real-time graphics use simple models of lighting and shading to generate shaded objects quickly. The models compute the approximate shaded color of each surface pixel based on the following properties: Simulated position of the light source Orientation of the surface relative to the position of the light source The color of the light The material properties of the surface: Color and amount of diffuse reflection (dull surface) Color and amount of specular reflection (shiny surface) Color and amount of ambient lighting. OpenGL allows specification of all of these properties.

4 More realistic lighting and shading More realistic shading comes with a time cost. The techniques are not generally used for Virtual Reality because they're slow. Ray Tracing: In ray tracing, the light rays are traced from the simulated eye position, through each pixel on the screen, back to the object. If necessary, the reflected light ray is projected to the light source. Radiosity: In this technique, the light energy is computed as it reflects off multiple surfaces. This is the slowest technique.

5 Texture Mapping Rendering scenes with lots of detail can take too much time. Imagine drawing all the individual blades of grass in a field. We can give the appearance of detail by mapping an image onto a surface. This is known as texture mapping. For example, we can map a picture of grass onto a flat rectangular floor. This is a fast, easy way to generate detailed looking scenes.

6 Collision Detection When objects are moving, we need to detect when one object collides with another, so they can interact appropriately. Example: Pong The ball bounces off the paddles or the walls and changes direction with each bounce. The computer must keep track of the position of the boundaries of the ball and test for when the ball touches a surface. For complex objects, we can use a bounding box to calculate the position of the boundaries.

7 User Interface Interactivity is essential to Virtual Reality. The user must be able to interact with objects within the virtual world. Simple Interactivity: We can use basic computer tools to interact: Use the mouse to select and drag Use menus to perform actions Use the keyboard to enter commands. Slightly more complex: Use a game-controller to interact. Most Complicated: Track the user's position. Track the position of a data glove. Allow the user to interact with objects using the glove.

8 Review--Essay questions 1.a) What are the steps for computing depth from binocular stereo? b) What is the correspondence problem in stereo vision? What do random dot stereograms tell us about the brain's ability to solve the correspondence problem? 2. What is the critical period in development? Give an example of a visual ability that is affected by the critical period. Explain how this is known.

9 Review--Essay questions 3. Consider the Muller-Lyer illusion (shown below). Describe the illusion and give an explanation of why it appears the way it does. Justify your answer based on what we have learned about visual perception. 4. What is change blindness? What are the implications of this for how we view movies?

10 Review--Essay questions 5. In the painting shown, list 3 cues to depth used by the artist. Briefly describe each. 6. Describe how Spatial summation is used by artists.

11 Binary, Hexadecimal, Decimal 7. a) Convert the following binary number into hexadecimal: b) Convert the following binary number into decimal: c) Convert the following hexadecimal number into decimal: 25B

12 Colors in computer graphics 8. a) What is indexed color? b) What color is represented by #AA00EF? c) How many colors can be represented using a bit depth of 10? d) If we are using 32 bit color, and our screen has a resolution of 850 x 1000, how many bytes are needed to store the color values for the entire screen?

13 Computer Graphics 9. What 4 stages are performed by a computer graphics system in the imaging process? Briefly describe each one.

14 Transformations 10. a) Given the following three points, graph the triangle they describe. P1 = (50, 10, 0) P2 = (75, 60, 0) P3 = (100, 10, 0) b) Compute the new positions of each of the points after rotating by 90deg about the Z axis. Use the following equations: x' = xcos  - ysin  y' = xsin  + ycos  cos(90) = 0; sin(90) = 1 Graph the triangle in the new position