# Chapter 11 Global Illumination. Part 1 Ray Tracing. Reading: Angel s Interactive Computer Graphics (6 th ed.) Sections 11.1, 11.2, 11.

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1 Chapter 11 Global Illumination Part 1 Ray Tracing Reading: Angel s Interactive Computer Graphics (6 th ed.) Sections 11.1, 11.2, 11.3 CG(U), Chap.11 Part 1:Ray Tracing 1

2 Can pipeline graphics renders images like these? Picture from 2

3 Global Illumination Local (direct) lighting The calculations depend on only the surface material properties, surface s local geometry, and parameters of the light sources Can be added to a fast pipeline graphics architecture No shadow, indirect lighting (reflection, refraction) 3

4 Global Illumination Global (indirect) lighting Evaluate the light reflected from a surface point taking into account all illumination that arrives at the point, including Direct light, indirect light reflected or refracted from other object Lighting is a recursive process and amounts to an integral equation, the rendering equation 4

5 Global Illumination Global (indirect) lighting Ray tracing Specular interreflection/refraction Hard shadow Ok for dynamic scenes Radiosity Diffuse interreflection between polygons Soft shadow Good for static scenes Need to compute the form factor between polygon pairs and radiosity for each polygon» Costly, done in preprocessing Render using pipeline 5

6 Forward vs. backward ray tracing Starting at the light Starting at the eye 6

7 Recursive ray tracing HSR ray tracing or ray casting (one pass) Computes the visible point for each pixel and then determine the local illumination. Recursive ray tracing Incorporate the following in a single framework HSR Global specular interaction by simulating indirect reflection and refraction of light Hard-shadow calculation 7

8 Recursive ray tracing Examples -1 by J. L. Lin 8

9 Recursive ray tracing Examples -2 by J. L. Lin 9

10 Recursive ray tracing Examples -3 by J. L. Lin 10

11 Recursive ray tracing Examples -4 by J. L. Lin 11

12 Recursive ray tracing Another example 12

13 Recursive ray tracing -1 Ray: a ray from eye through a pixel Recursive ray tracing At each ray-object intersection, a reflected and a refracted ray are fired to trace the light ray propagation. L R L V R V T 13

14 Recursive ray tracing -2 Ray termination Termination of recursive ray tracing Always terminate if a ray intersects a diffuse surface. Can terminate when a pre-set depth of trace has been reached. Can terminate when the energy of the ray has dropped below a threshold. 14

15 Recursive ray tracing -3 Ray tracing tree 15

16 Recursive ray tracing -4 Intensity calculation Pixel s RGB is the intensity at the first ray-object intersection. at each ray-object intersection, the RGB at that point is obtained by I I local k rg I reflected k tg I refracted 16

17 Recursive ray tracing -5 Main program Select a COP and window on the view plane for each scan line in the window do for each pixel p in the scan line do begin TraceRay(COP, p-cop, 1, pixel_color) end 17

18 Recursive ray tracing -6 TraceRay() TraceRay(start, direction, depth, var color: colors) begin end if depth > max_depth then color:=black else begin Intersect ray with all objects and find intersection point p that is closest to start; if no intersection then color:=background_color; else begin local_color:=. end Compute reflection direction r TraceRay(p, r, depth+1, reflected_color); Compute refraction direction t TraceRay(p, t, depth+1, refracted_color); Combine(color, local_color, k_local, end reflected_color, k_reflection, refracted_color, k_refraction); 18

19 Recursive ray tracing -8 Shadow Fire a shadow ray at a visible ray-object intersection. If the shadow ray is interrupted, the point is under shadow Shadow effect Assume the objects are wholly opaque Reduce the local term to the ambient value Produces sharp shadow boundary 19

20 Recursive ray tracing -8 Shadow using light buffer -1 Light buffer A cube of six faces with the point light source as center, and whose faces are partitioned to cells Set-up All polygons are projected to each face using the light position as the COP Each cell contains a list of polygons that can be seen from the light source through the cell The polygon are sorted in ascending order of the depth 20

21 Recursive ray tracing -8 Shadow using light buffer -2 Shadow testing Find the cell that the shadow ray passes Check to see if there is a polygon on the cell s sorted list that occludes the point Polygon may partially cover the cell 21

22 Speed-Up Methods Adaptive depth control Bounding volume and hierarchical BV First-hit speed up Spatial scene coherence Uniform partition Octree Binary Space Partition KD tree 22

23 Speed-Up Methods Bounding volume Bounding box aims to avoid unnecessary intersection computations. Efficiency gained depends on how small the void area is. Examples of bounding volume Bounding sphere Bounding box Axis-aligned bounding box (AABB) Oriented bounding box (OBB) 23

24 Speed-Up Methods Bounding volume Sphere AABB OBB 24

25 Speed-Up Methods Hierarchical bounding volume Hierarchical bounding volume provides an even better void area ratio, and a hierarchical testing steps. Hierarchical bounding sphere Hierarchical axis-aligned bounding box Hierarchical oriented bounding box 25

26 Speed-Up Methods Bounding volume hierarchy Objects in close spatial proximity form a cluster, and recursive clustering form a BV hierarchy. Difficulties Potential clustering and depth of the hierarchy depend on the nature of the scene. 26

27 Speed-Up Methods Spatial partition Scene space is subdivided into regions such that each region either is empty or contains only a small set of objects. Only regions intersected by the ray are checked and in a near-to-far order. The checking stops once an intersection is found. Rendering time can be relatively constant and independent on the scene complexity. 27

28 Speed-Up Methods Uniform partition Easy to implement, Easier for next-region search Hard to set a good cell-size More space requirement Poor results for inhomogeneous scenes See 28

29 Speed-Up Methods Uniform partition Grid traversal Requires enumeration of voxel along ray, by, ex., 3D-DDA Simple and hardware-friendly See 29

30 Speed-Up Methods Uniform partition Objects in multiple voxels Store only references Original mailbox uses ray-id stored with each triangle Simple, but likely to destroy CPU caches Use mailboxing to avoid multiple intersection computations Store (ray, object)-tuple in small cache Do not do intersect test if found in cache 30

31 Speed-Up Methods Uniform partition 31

32 Speed-Up Methods Uniform partition Object A: tested only once in voxel 1 and the intersection need to be cached as a mailbox. Note in voxel 1: The voxel tracking process can only be terminated when the closest intersection in contained within the current voxel. 32

33 Speed-Up Methods Octree Adjust to object distribution in the scene Start with bounding box of entire scene Recursively subdivide voxels into 8 equal subvoxels Stopping criteria Too many triangles & maximum depth Problems Each leaf of the octree Contains no object, or Contains objects less than a preset number, or Contains more objects than the preset n umber but has size less than a preset size. 33

34 Speed-Up Methods Octree 34

35 Speed-Up Methods Tracing with octree Tracking a ray in octree Starts from the cell first intersected by the ray The ray is tested for intersections with objects lie over this cell The first intersection encountered is the one Tracks to the next cell if there is no intersections. 35

36 Speed-Up Methods Find the next cell A naive way Derive a point on the ray a short distance into the next cell and search the cell containing the point by a upwardly and downwardly traversal on the octree. Glassner s hash table Derive a hash table contains an entry for each cell Point s coordinate as a code to access the table Fujimoto s SEADS SEADS structure + 3D-DDA algorithm 36

37 Speed-Up Methods SEADS and 3D-DDA SEADS (Spatially Enumerated Auxiliary Data Structure) A 2x2x2 uniform grid by dividing the cell into equally sized cells. Enable very fast tracking using 3D-DDA Octree + SEADS Standard octree with each leaf a SEADS Traversal in octree Internal nodes: standard octree traversal Within leaf (SEADS): 3D-DDA 37

38 Speed-Up Methods BSP Binary space partitioning tree (BSP tree) Even more flexible than octree In 3-D, split with cutting plane Useful for Painter s algorithm Sorts the polygons into a back-to-front order Ray tracing uses axis-aligned cutting plane for easier next-cell search 38

39 Speed-Up Methods BSP 39

40 Scene spatial coherence BSP tree Each internal node representing a single partition plane that partition the occupied space into two A leaf node represents a cell that contains a small set of objects (1 or 2) The partition planes are usually axisaligned this simplifies the next-cell search Position of the partition plane depends on the object s distribution within the scene 40

41 Speed-Up Methods K-D tree k-dimensional tree (k-d tree) Is a binary tree, split at arbitrary interior point Split one dimension at a time Good for neighborhood search 41

42 RRT s problems in quality Point light source vs. area light source Aliasing (due to point sampling) Area sampling : Cone tracing, beam tracing, pencil tracing Stochastic sampling: Distributed ray tracing Specular-to-specular interaction Perfect reflection/refraction, No specular spreading Beam tracing, pencil tracing Distributed ray tracing Specular-to-diffuse interaction Backward ray/beam tracing Photon mapping Diffuse-to-diffuse interaction Combining ray tracing and radiosity 42

43 Quality improvements Pinhole cameras No depth of field; everything in focus Instantaneous shutters No motion blur; objects complete still 43

44 Recursive ray tracing -13 Problems RRT can not handle Specular highlight Some light propagations Diffuse-to-diffuse propagation Specular-to-diffuse propagation 44

45 4 types of light transport 45

46 Why ray tracing looks fake Hard shadow vs. soft shadow 46

47 Why ray tracing looks fake Sharp mirrors vs. blurry reflection 47

48 Why ray tracing looks fake Sharp mirrors vs. blurry reflection [Interactive DRT, Boulos et al.] 48

49 Why ray tracing looks fake Without/with depth of field 49

50 Why ray tracing looks fake Without/with depth of field [Interactive DRT, Boulos et al.] 50

51 Why ray tracing looks fake Motion blur 51

52 Why ray tracing looks fake Motion blur [Interactive DRT, Boulos et al.] 52

53 Distributed Ray-Tracing[Cook`84] Stochastic sampling Pixel: Anti-Aliasing Lens: Depth-of-Field BRDF: Glossy reflections Illumination: extended light sources Time: motion-blur 53

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