Real-Time Rendering Paper Presentation Imperfect Shadow Maps for Efficient Computation of Indirect Illumination T. Ritschel T. Grosch M. H. Kim H.-P. Seidel C. Dachsbacher J. Kautz Presented by Bo-Yin Yao

Outlines Introduction Related work Imperfect shadow maps Scene preprocessing VPL generation Point based depth maps Pull push Shading Results Conclusion 2

Outlines Introduction Related work Imperfect shadow maps Scene preprocessing VPL generation Point based depth maps Pull push Shading Results Conclusion 3

Introduction Interactive compute the indirect illumination in large and fully dynamic scenes Approximate visibility for indirect illumination with imperfect shadow maps (ISMs) Low-resolution Rendered from crude point-based representation of the scene Use ISMs with a global illumination algorithm based on virtual point lights (VPLs) – instant-radiosity 4

Main steps 1. Scene preprocessing 2. VPL generation 3. Point-based depth maps (ISMs) 4. Pull-push to fill holes of ISMs 5. Shading 5

Outlines Introduction Related work Imperfect shadow maps Scene preprocessing VPL generation Point based depth maps Pull push Shading Results Conclusion 6

Global Illumination Methods Use accurate visibility By intersecting rays with the scene geometry Path tracing Photon mapping Ray-tracing Through shadow volumes / maps or ray-casting (Based on VPLs) Instant radiosity [Keller 1997] Instant global illumination [Wald et al. 2002] 7

Global Illumination Methods Use visibility approximations Lightcuts method [Walter et al. 2005] Incident lighting from nearby geometry is integrated without computing visibility [Arikan et al. 2005] Reduce the geometric complexity [Rushmeier et al. 1993; Christensen et al. 2003; Tabellion and Lamorlette 2004] 8

Real-Time Global Illumination Precomputed radiance transfer (PRT) Only support lighting changes, restricted to static scene [Sloan et al. 2002] Only allow movement of rigid objects [Iwasaki et al. 2007; Wang et al. 2007] Neglect the visibility for indirect illumination, allow dynamic models [Dachsbacher and Stamminger 2005; Dachsbacher and Stamminger 2006] 9

Real-Time Global Illumination Approximate visibility by ambient occlusion [Bunnell 2005] Iterative process using antiradiance (negative light) [Dachsbacher et al. 2007] 10

Instant Radiosity 11 Direct light Virtual point light VPL Indirect light

Instant Radiosity bottleneck 12 Virtual point light 1024 VPLs 100k 3D model 32x32 depth map ~300M transforms 100x overdraw 32

Reflective Shadow Maps 13

Outlines Introduction Related work Imperfect shadow maps Scene preprocessing VPL generation Point based depth maps Pull push Shading Results Conclusion 14

Point-based representation of the scene Approximate the 3D scene by a set of points Each point is created by randomly selecting a triangle with probability proportional to its area, and then pick a random location on it Also store the barycentric coordinate To support dynamic scenes without recomputing the point representation To retrieve the normal and reflectance 15

Outlines Introduction Related work Imperfect shadow maps Scene preprocessing VPL generation Point based depth maps Pull push Shading Results Conclusion 16

Direct light VPLs The 3D position of each VPL is determined by rendering a cube map from the viewpoint of the direct point light source Select N vpl VPLs by importance sampling 17

Direct light VPLs 18 Direct light VPLs! Light Normal Position

Indirect light VPLs (for multiple bounces) Generalize ISMs to imperfect reflective shadow maps (IRSMs) Instead of rendering the shaded geometry, render shaded points into the IRSMs Each paraboloid map stores its own indirect illumination (instead of depth values) Use importance sampling to generate the second bounce VPLs 19

Outlines Introduction Related work Imperfect shadow maps Scene preprocessing VPL generation Point based depth maps Pull push Shading Results Conclusion 20

Point based depth maps (ISMs) ISM is created by splatting the point representation into the depth buffer Box splatting kernel Point splat size is base on its squared distance to the VPL position Use parabolic maps Oriented along the normal of the surface VPLs emit light over a hemisphere → need to cover a full hemisphere of depth information 21

Point based depth maps (ISMs) Non-uniform point distributions Many low-resolution ISMs are stored in a single, large texture 22

Point based depth maps (ISMs) 23

ISM compared with classic shadow map 24 ImperfectClassicImperfect Smaller points Less points

In dynamic scenes Deform the point distributions according to the deformation of the corresponding triangles 25 Frame tFrame t+1

Outlines Introduction Related work Imperfect shadow maps Scene preprocessing VPL generation Point based depth maps Pull push Shading Results Conclusion 26

Pull-push approach Use a sparse set of points → holes in the depth map 27 Without pull-pushClassic

Pull-push approach Pull phase Create an image pyramid where the image is downsampled by a factor of two (mipmap) Only valid pixels are used for averaging the pixels in the coarser level Only combine depth values that are close to each other Push phase Fill the holes top-down by interpolating the pixels from the coarser level Only replace depth values that are far from the coarse depth values pushed down 28

Depth maps comparison 29 3D 2D Without pull-pushClassicWith pull-push

Depth maps comparison 30 With pull-pushWithout pull-pushClassic

Depth maps comparison 31 With pull-pushWithout pull-push

Outlines Introduction Related work Imperfect shadow maps Scene preprocessing VPL generation Point based depth maps Pull push Shading Results Conclusion 32

Direct and indirect illumination with ISMs Separate direct and indirect, both use deferred shading Sum up the contribution of all VPLs Take into account shadowing 33 Direct + Indirect Direct onlyIndirect only

G-buffer applying Reduce the rendering cost → use G-buffer to gather from VPLs (indirect illumination) Interleaved sampling With geometry aware blur 34 Simple blurEdge-ware

G-buffer applying 35 Using G-BufferIndirect only (origin)

Outlines Introduction Related work Imperfect shadow maps Scene preprocessing VPL generation Point based depth maps Pull push Shading Results Conclusion 36

Improvement with pull-push 37

With geometrically complex scenes 38

Multiple bounces 39

With BRDF changing 40

With area lights 41

With environment maps 42

Error comparison with different parameters 43

Performance comparison 44

Performance comparison 45

Demo video 46

Outlines Introduction Related work Imperfect shadow maps Scene preprocessing VPL generation Point based depth maps Pull push Shading Results Conclusion 47

Advantage Interactive (real-time?) global illumination in large and fully dynamic scenes allowing for light, geometry, and material changes Sacrifice a little accuracy to gain a significant performance enhancing Can be used for direct illumination in certain cases, such as textured area lights or environment map illumination 48

Disadvantage Indirect illumination is restricted to point and spotlight illumination (with using reflective shadow maps to generate VPLs) Low-resolution ISMs cannot resolve indirect shadows from small geometry Without sufficient number of VPLs and point samples Temporal flickering Light leaking due to shadow biasing Not fully automatic – parameters need to be chosen by the user 49

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