# Ray Tracing. CSCI 420 Computer Graphics Lecture 15. Ray Casting Shadow Rays Reflection and Transmission [Ch ]

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1 CSCI 420 Computer Graphics Lecture 15 Ray Tracing Ray Casting Shadow Rays Reflection and Transmission [Ch ] Jernej Barbic University of Southern California 1

2 Local Illumination Object illuminations are independent No light scattering between objects No real shadows, reflection, transmission OpenGL pipeline uses this 2

3 Global Illumination Ray tracing (highlights, reflection, transmission) Radiosity (surface interreflections) Photon mapping Precomputed Radiance Transfer (PRT) 3

4 Object Space: Graphics pipeline: for each object, render Efficient pipeline architecture, real-time Difficulty: object interactions (shadows, reflections, etc.) Image Space: Ray tracing: for each pixel, determine color Pixel-level parallelism Difficulty: very intensive computation, usually off-line 4

5 First idea: Forward Ray Tracing Shoot (many) light rays from each light source Rays bounce off the objects Simulates paths of photons Problem: many rays will miss camera and not contribute to image! This algorithm is not practical 5

6 Backward Ray Tracing Shoot one ray from camera through each pixel in image plane 6

7 Generating Rays Camera is at (0,0,0) and points in the negative z-direction Must determine coordinates of image corners in 3D 7

8 Generating Rays y aspect ratio = w / h z x ray w y z x center of projection (COP) field of view angle (fov) image plane h side view frontal view 8

9 Generating Rays z y x z y x y = tan(fov/2) z = -1 COP field of view angle (fov) image plane y = 0 z = 0 f = 1 image plane side view side view y = -tan(fov/2) z = -1 9

10 Generating Rays z y x a = aspect ratio = w / h x = -a tan(fov/2) y = tan(fov/2) z = -1 x = -a tan(fov/2) y = -tan(fov/2) z = -1 h w x = 0 y = 0 z = -1 frontal view x = a tan(fov/2) y = tan(fov/2) z = -1 x = a tan(fov/2) y = -tan(fov/2) z = -1 10

11 Determining Pixel Color 1. Phong model (local as before) 2. Shadow rays 3. Specular reflection 4. Specular transmission Steps (3) and (4) require recursion. 11

12 Shadow Rays Determine if light really hits surface point Cast shadow ray from surface point to each light If shadow ray hits opaque object, no contribution from that light This is essentially improved diffuse reflection light source scene object 2 shadow ray (blocked) n ray camera image plane scene object 1 12

13 Phong Model camera If shadow ray can reach to the light, apply a standard Phong model light source shadow ray (unblocked) l ray v image plane n scene object 13

14 Where is Phong model applied in this example? Which shadow rays are blocked? 14

15 Reflection Rays For specular component of illumination Compute reflection ray (recall: backward!) Call ray tracer recursively to determine color 15

16 Angle of Reflection Recall: incoming angle = outgoing angle r = 2(l n) n l Compute only for surfaces that are reflective 16

17 Reflections Example 17

18 Transmission Rays Calculate light transmitted through surfaces Example: water, glass Compute transmission ray Call ray tracer recursively to determine color 18

19 Transmitted Light Index of refraction is speed of light, relative to speed of light in vacuum Vacuum: 1.0 (per definition) Air: (approximate to 1.0) Water: 1.33 Glass: 1.49 Compute t using Snell s law η l = index for upper material η t = index for lower material 19

20 Translucency Most real objects are not transparent, but blur the background image Scatter light on other side of surface Use stochastic sampling (called distributed ray tracing) 20

21 Transmission + Translucency Example 21

22 The Ray Casting Algorithm Simplest case of ray tracing 1. For each pixel (x,y), fire a ray from COP through (x,y) 2. For each ray & object, calculate closest intersection 3. For closest intersection point p Calculate surface normal For each light source, fire shadow ray For each unblocked shadow ray, evaluate local Phong model for that light, and add the result to pixel color Critical operations Ray-surface intersections Illumination calculation 22

23 Recursive Ray Tracing Also calculate specular component Reflect ray from eye on specular surface Transmit ray from eye through transparent surface Determine color of incoming ray by recursion Trace to fixed depth Cut off if contribution below threshold 23

24 Ray Tracing Assessment Global illumination method Image-based Pluses Relatively accurate shadows, reflections, refractions Minuses Slow (per pixel parallelism, not pipeline parallelism) Aliasing Inter-object diffuse reflections require many bounces 24

25 Raytracing Example I 25

26 Raytracing Example II 26

27 Raytracing Example III 27

28 Raytracing Example IV 28

29 Summary Ray Casting Shadow Rays and Local Phong Model Reflection Transmission Next lecture: Geometric queries 29

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