Low Overhead Probe- based Global Illumination Using Ray

Low Overhead Probe- based Global Illumination Using Ray Tracing Jens Fursund

2 @PowerVRInsider │ #idc15 facebook.com/imgtec

Why Global Illumination (GI)?


Why Global Illumination (GI)?


What makes GI challenging?


Mobile real-time GI?


How do we do GI today?

High cost

Low dynamism

Path tracing

Light mapping

Probes

Voxel-based techniques

Screen-space GI Low fidelity


Path-tracing


Screen-space GI

[SSDO]


Lightmapping


Voxels

[VCT]


Probes

[LPT]


Storing light directions


Storing light directions


Spherical Harmonics (SH)

[SHT]


Using SH in games

[LPT] [SAS]


SH Probes in Real-time?


Baking probes


Baking probes


Classic SH Baking


Direct to SH


Real-time Probe Baking

color = colorprevious⋅ weight + colorcurrent ⋅ ( 1 - weight )


Shadowed probes


Probe interpolation


Tetrahedral interpolation

[LPT]


Tetrahedral interpolation

[LPT]


Inverse distance weighted interpolation


Inverse distance weighted interpolation

DEMO


Indirect occlusion


Indirect occlusion


Probe occlusion


Probe occlusion


Reducing ray count


Bilateral upscaling


Bilateral upscaling


Bilateral upscaling


Temporal reprojection filter

Previous camera

Current camera

[REP][STF]



Previous camera

Current camera

[REP][STF]



Previous camera

Current camera

[REP][STF]


Bilateral blur

[FRI]


Pipeline overview

Sample probes

Bilateral blur

Temporal filter

Bilateral blur

Bilateral upscale

Progressive sub-realtime probe-rendering

[FRI]

DEMO


Render Stats ▪ 6 passes total ▪ 1-pixel * 9 SH per probe ▪ Low-res buffer size is multiplied by the amount of SH coefficients (so e.g.

9 SH gives 192x192 * 9 in low-res buffer ) ▪ Multiple accumulation targets for storing SH coefficients ( no need to

render more than once )


Render Stats In Sponza scene: ▪ 2K rays per probe (48 probes in this case, one bounce, one light), can be

amortized across multiple frames ▪ 4.5 rays per low-res buffer pixel ▪ In total a 2K resolution GI render is approximately 0.8 rays per pixel


Future improvements ▪ Culling probe update based on body centered LOD ▪ Automatic placement of probes ▪ Better bilateral upsampling with normals ▪ Better probe occlusion method ▪ Trace against lower LOD ▪ Extracting SH dominant light direction for specular ▪ Using dominant light direction for indirect occlusion


Next-gen mobile GI


Thank you!

Jens Fursund [email protected]

@jensfursund


www.imgtec.com/idc


References [SHT] Spherical Harmonic Lighting: The Gritty Details, Robin Green [FRI] Tech Feature: SSAO and Temporal Blur, Frictional Games [LPT] Light Probe Interpolation Using Tetrahedral Tessellations, Robert Cupisz [VCT] Interactive Indirect Illumination Using Voxel Cone Tracing, Crassin et al [SSDO] Approximating Dynamic Global Illumination in Image Space, Ritschel et al [SAS] Screen Space Spherical Harmonic Lighting, Steve Anichini [REP] Accelerating Real-Time Shading with Reverse Reprojection Caching, Nehab et al [STF] Stable SSAO in Battlefield 3 with Selective Temporal Filtering, Bavoil et al

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Low Overhead Probe- based Global Illumination Using Ray