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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 Yao2010.3.25

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Outlines Introduction Related work Imperfect shadow maps

Scene preprocessing VPL generation Point based depth maps Pull push Shading

Results Conclusion

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Outlines Introduction Related work Imperfect shadow maps

Scene preprocessing VPL generation Point based depth maps Pull push Shading

Results Conclusion

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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

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Main steps

1. Scene preprocessing2. VPL generation3. Point-based depth maps (ISMs)4. Pull-push to fill holes of ISMs5. Shading

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Outlines Introduction Related work Imperfect shadow maps

Scene preprocessing VPL generation Point based depth maps Pull push Shading

Results Conclusion

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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]

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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]

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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]

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Real-Time Global Illumination Approximate visibility by ambient occlusion

[Bunnell 2005]

Iterative process using antiradiance (negative light)[Dachsbacher et al. 2007]

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Instant Radiosity

Direct light

Virtual point light

VPL

VPL

VPL

Indirectlight

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Instant Radiosity bottleneck

Virtual point light

1024 VPLs 100k 3D model 32x32 depth map ~300M transforms 100x overdraw

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Reflective Shadow Maps

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Outlines Introduction Related work Imperfect shadow maps

Scene preprocessing VPL generation Point based depth maps Pull push Shading

Results Conclusion

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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

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Outlines Introduction Related work Imperfect shadow maps

Scene preprocessing VPL generation Point based depth maps Pull push Shading

Results Conclusion

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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 Nvpl VPLs by importance sampling

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Direct light VPLs

Direct lightVPLs! Li

ght

Norm

al

Posit

ion

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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

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Outlines Introduction Related work Imperfect shadow maps

Scene preprocessing VPL generation Point based depth maps Pull push Shading

Results Conclusion

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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

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Point based depth maps (ISMs) Non-uniform point distributions

Many low-resolution ISMs are stored in a single, large texture

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Point based depth maps (ISMs)

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ISM compared with classic shadow map

ImperfectClassic ImperfectSmaller points

Less points

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In dynamic scenes Deform the point distributions according to the

deformation of the corresponding trianglesFrame t Frame t+1

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Outlines Introduction Related work Imperfect shadow maps

Scene preprocessing VPL generation Point based depth maps Pull push Shading

Results Conclusion

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Pull-push approach Use a sparse set of points → holes in the depth map

Without pull-pushClassic

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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

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Depth maps comparison

3D2D

Without pull-pushClassic With pull-push

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Depth maps comparison

With pull-pushWithout pull-pushClassic

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Depth maps comparison

With pull-pushWithout pull-push

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Outlines Introduction Related work Imperfect shadow maps

Scene preprocessing VPL generation Point based depth maps Pull push Shading

Results Conclusion

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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

Direct + IndirectDirect only Indirect only

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G-buffer applying Reduce the rendering cost → use G-buffer to gather from VPLs (indirect illumination)

Interleaved sampling With geometry aware blur

Simple blur Edge-ware

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G-buffer applying

Using G-BufferIndirect only (origin)

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Outlines Introduction Related work Imperfect shadow maps

Scene preprocessing VPL generation Point based depth maps Pull push Shading

Results Conclusion

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Improvement with pull-push

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With geometrically complex scenes

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Multiple bounces

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With BRDF changing

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With area lights

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With environment maps

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Error comparison with different parameters

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Performance comparison

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Performance comparison

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Demo video

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Outlines Introduction Related work Imperfect shadow maps

Scene preprocessing VPL generation Point based depth maps Pull push Shading

Results Conclusion

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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

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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

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