Advanced Ray Tracing

1. Advanced Ray Tracing Mani Thomas CISC 440/640 Computer Graphics

2. Review • Ray tracing • Compute 3D ray into the scene for each 2D image pixel • Compute 3D intersection point of ray with nearest object in scene • Test each primitive in the scene for intersection • Find nearest intersection • Recursively spawn rays from the point of intersection • Shadow Rays • Reflected rays • Transmitted rays • Accumulate the color from each of the spawned rays at the point of intersection

3. Review • Ray object intersection • Intersection with a plane • Implicit form • Intersection • Intersection with a sphere • Implicit form • Intersection

4. Review • “Shadow feelers” • Spawn a ray from P to the light sources • If there is an intersection of the shadow ray with any object then P is in shadow • Reflection • Angle of incidence = angle of Reflection

5. Review • Refraction • Ray passing through media of different refractive indices bend towards/away from the normal • Snell’s Law • ni and nr are the refractive indices of the two media • Transmitted ray

6. Road map • Super sampling • Acceleration techniques • Monte Carlo methods • Distributed ray tracing • Bidirectional ray tracing • Caustics • POV-ray

7. Anti aliasing • Ray tracing gives a color for every possible point in the image • But a square pixel contains an infinite number of points • These points may not all have the same color • Sampling: choose the color of one point (center of pixel) • This leads to aliasing • jaggies • moire patterns • Aliasing means one frequency (high) masquerading as another (low) • e.g. wagon wheel effect

8. Super-sampling • Ray tracing is a point-sampling process • Take discrete looks at the scene along individual rays passing through each pixel • Reduce aliasing due to this discrete signal sampling Courtesy F.S. Hill, “Computer Graphics using OpenGL”

9. Adaptive Super-sampling • Instead of shooting one ray per pixel, shoot four rays through the corners of a pixel • Color at the pixel is the average of the colors at each corners • Adaptive super-sampling (Whitted’s approach) • Compute the intensity variation between the four corners with the average • Shoot more rays through corners with higher intensity variation • Compute final color as a weighted average rather than the regular average Courtesy of C. Rasmussen, CISC 640

10. Stochastic Super sampling • Visible aliasing is possible even with adaptive super-sampling • sampling grid interacts with regular structures • objects aligned with sampling grid • Stochastic sampling • instead of a regular grid, subsample randomly • keep taking samples until the color estimates converge • jittering: perturb a regular grid Courtesy of C. Rasmussen, CISC 640

11. Using Extents • Ray tracing is slow, performing the same functions. • Most of the time is spent in computing intersections • Each ray should be intersected with every object in the scene • Each ray, spawns out reflected/transmitted rays which have to be interested with the objects in the scene

12. Using Extents • Extent of an object is a shape that encloses the object • Compute complicated intersections if and only if the ray hits the extent • Two shapes most commonly used as extents • Sphere – specified by a center and radius (C , r) • Box – specified by sides aligned to the coordinate axis

13. Distributed Ray Tracing • Distributed ray tracing is NOT ray tracing on a distributed system. • Distributed ray tracing is a ray tracing method based on randomly distributed oversampling to reduce aliasing artifacts in rendered images (Allen Martin, http://www.cs.wpi.edu/~matt/courses/cs563/talks/dist_ray/dist.html)

14. Distributed Ray Tracing • Developed by Cook, et. al. (“Distributed Ray Tracing”, Computer Graphics, vol. 18, no. 3, pp 137-145, 1984) • Stochastic Oversampling (http://www.cs.virginia.edu/~cs551dl/lecture11/sld016.htm) • Pixel for antialiasing • Light source for soft shadows • Reflection function for soft (glossy) reflections • Time for motion blur • Lens for depth of field

15. Distributed Ray tracing • Gloss (fuzzy reflections) • Partially reflecting surfaces • Traditional ray tracing • reflections look identical to the scene they are reflecting • reflections are always sharp • Randomly distributing the rays reflected by the surface • Send out a packet of rays around the reflecting direction. • The actual value of reflectance is the statistical mean of the values returned by each of these rays

16. Distributed Ray tracing • Distributing a set of reflection rays by randomly perturbing the ideal specular reflection ray. • The spread of the distribution determines the glossiness where a wider distribution spread models a rougher surface.   taken from http://www.cs.wpi.edu/~emmanuel/courses/cs563/write_ups/zackw/realistic_raytracing.html

17. Distributed Ray tracing • first image is from the traditional ray tracer • second one uses 16 rays in place of the single reflected ray • third image uses 64 rays taken from http://www.uwm.edu/People/dtstrock/graphics/mcrt.html

18. Distributed Ray tracing taken from http://www.cs.wpi.edu/~emmanuel/courses/cs563/write_ups/zackw/realistic_raytracing.html

19. Distributed Ray tracing • Fuzzy translucency • Same as glossy reflections, but you jitter the refracted ray • Analytical function similar to the shading • A transmission function is used instead of a reflectance function • Light is gathered from the other side of the surface. • Cast randomly distributed rays in the general direction of the transmitted ray from traditional ray tracing. • The average value computed from each of these rays the true translucent component.

20. Distributed Ray tracing • first image is obtained from a traditional ray tracer • Second image uses 10 rays for the transmitted ray • third image uses 20 rays taken from http://www.cs.wpi.edu/~matt/courses/cs563/talks/dist_ray/dist_trans_fuzzy_20.html

21. Distributed Ray tracing • first image is from the traditional ray tracer • second one uses 16 rays in place of the single reflected ray • third images uses 64 rays taken from http://www.uwm.edu/People/dtstrock/graphics/mcrt.html

22. Distributed Ray tracing • Penumbras (soft shadows) • Traditional ray tracing shadows are discrete • Shadow feelers used to check if a point is in shadow with respect to a point light source • Incorrect for large light sources and/or light sources that are close to the object • The transition from fully shadowed to partially shadowed is gradual. • Due to the finite area of real light sources, and scattering of light of other surfaces

23. Distributed Ray tracing • Penumbras (soft shadows) • A set of rays are cast about the projected area of the light source. • The projected area helps tackle the large area light source • The amount of light transmitted by the ratio of the number of rays that hit the source to the number of rays cast

24. Distributed Ray tracing • In case of a point source, the occluder would create a sharp shadow boundary • In an area light source or if the light source is closer to the object • Creation of a penumbra region • Sending out shadow feelers to capture the penumbra region taken from http://www.cs.wpi.edu/~emmanuel/courses/cs563/write_ups/zackw/realistic_raytracing.html

25. taken from http://www.cs.wpi.edu/~emmanuel/courses/cs563/write_ups/zackw/realistic_raytracing.html

26. Distributed Ray tracing taken from http://www.cs.unc.edu/~andrewz/comp238/hw2/

27. Distributed Ray tracing • Depth of field - the distance that objects appear in focus • Objects that are too far away or two close will appear unfocused and blurry • pinhole camera model does not truly mimic the real world situation • Pinhole assumed to be infinitely small • Changing focal length change field of view but does not change focus

28. Distributed Ray tracing • Distributed ray tracing creates depth of field by placing an artificial lens in front of the view plane. • Randomly distributed rays are used once again to simulate the blurring of depth of field. • The first ray cast is not modified by the lens. • focal point of the lens is at a fixed distance along this ray • Rest of the rays sent out for the same pixel will be scattered about the surface of the lens • Points in the scene that are close to the focal point of the lens will be in sharp focus. • Points closer or further away will be blurred

29. taken from http://www-courses.cs.uiuc.edu/~cs419/mp2/gallery-sp04/

30. Distributed Ray tracing taken from http://www-courses.cs.uiuc.edu/~cs419/mp2/mp2-gallery-sp05/

31. Distributed Ray tracing • Motion blur • Temporal sampling rather than spatial sampling • A Frame represents an average of the scene during the time that the camera shutter is open • Before each ray is cast, objects are translated or rotated to their correct position for that frame. • The rays are averaged to give the actual value. • Objects with the most motion will have the most blurring in the rendered image.

32. Distributed Ray tracing taken from http://www-courses.cs.uiuc.edu/~cs419/mp2/mp2-gallery-sp05/

33. Distributed Ray tracing taken from http://www-courses.cs.uiuc.edu/~cs419/mp2/mp2-gallery-sp05/

34. Bidirectional Ray tracing caustic Created by H. Wann Jensen

35. Bidirectional Ray tracing • Caustic - (Concentrated) specular reflection/refraction onto a diffuse surface • Standard ray tracing cannot handle caustics caustic Created by H. Wann Jensen

36. Light Paths • Interactions of the light ray can be expressed using regular expressions • L is the light source • E is the eye/camera • D is a diffuse surface • S is a specular surface from Sillion & Puech

37. Light Paths • Direct visualization of the light: LE • Local illumination: LDE, LSE • Ray tracing: LS*E, LDS*E • Caustics: LS+DE from Sillion & Puech Taken from cisc 440/640 – Fall 2005

38. Diffuse Surfaces • Uncertainty in the direction that a photon will take for diffuse surfaces • For specular surfaces, the BRDF (probability that incoming photon will leave in a particular direction) has a thin profile • We can predict the direction of the outgoing photon • For an ideal diffuse surfaces, the BRDF would be spherical • The photon can travel along any of the direction with equal probability from Sillion & Puech

39. Bidirectional Ray tracing from P. Heckbert • Idea: Trace forward light rays into scene as well as backward eye rays • At diffuse surfaces, light rays additively “deposit” photons in radiosity textures, or “rexes”, where they are picked up by eye rays • The rays of the forward and backward pass "meet in the middle" to exchange information. Paul S. Heckbert, “Adaptive radiosity textures for bidirectional ray tracing “, SIGGRAPH 1990

40. Radiosity • Handling cases such as LD*E • “Color Bleeding” courtesy of Cornell

41. Softwares • Two beautiful rendering/modeling softwares • POV-ray (http://www.povray.org/) • Persistence of Vision - ray tracer • A free rendering tool (not a modeling tool) • Uses a text based scene description language (SDL) • Available on Windows/linux/MAC OS • Blender (http://www.blender3d.org) • Modeling, Animation, rendering tool • Especially useful in 3D game creation • Available for Windows, Linux, Irix, Sun Solaris, FreeBSD or Mac OS X under GPL

42. POV-ray created using POV-ray, http://www.povray.org/

43. POV-ray created using POV-ray, http://www.povray.org/

44. POV-ray created using POV-ray, http://www.povray.org/

45. POV-ray created using POV-ray, http://www.povray.org/

46. Conclusion • Traditional Ray tracing • Shadow feelers • Reflection • Refraction • Distributed Ray tracing • Jittered sampling • Bidirectional Ray tracing • Caustics

47. Thank you