Hardware-accelerated global illumination by image space photon mapping

Morgan McGuire & David Luebke

The Authors

• Morgan McGuire PhD.– Prof at Williams College/visiting prof. at NVIDIA

research• David Luebke PhD: Researcher NVIDIA Corp.

• Paper: 2009 ACM New Orleans, LA

Motivation

Dynamic global illumination in which lights and objects can move freely - in real time, at high resolution

Goal: Dynamic Global IlluminationGoal: Global Illumination

a

Goal: Dynamic HD Global Illumination

a

1920x1080, 20fps

Goal: Dynamic Global Illumination

Contribution 1

Photon Mapping Time (seconds)e.g., [Jensen 96, 01, Pharr and Humphreys 04]

1st Bounce 2nd 3rd 4th… Build Tree

Image Space Photon Mapping (milliseconds)

Last Bounce(“Estimate Radiance”)

…

Data Transfer

GPU Bounce Map GPU Photon VolumesCPU Trace

Pseudocode1. For each emitter:

(a) Render shadow map.(b) Render G-buffer from the emitter’s view.(c) Emit photons, bounce once, and store in a bounce map.(d) (CPU)

Continue tracing bounce map photons through the scene until absorption, storing them before each bounce in the photon map.

2. Render G-buffer from the eye’s view.

3.Compute direct illumination using shadow maps and deferred shading of the eye G-buffer.

4. Render indirect illumination by scattering photon volumes.

5. ender translucent surfaces back-to-front with direct, mirror,and refracted illumination only.

Steps

1. Rasterize the scene from the eye to create a screen space deferred-shading G-buffer. 2. For each light, rasterize a bounce map in light space. 3. Advance bounced photons by world-space ray tracing. 4. Scatter the final photons in screen space, invoking illumination on the G-buffer by rasterizing photon volumes.

SystemCPU Trace

Last Bounce: Photon Volumes

Direct + Shadows1st Bounce:Bounce Map

Bounce Map

11

Position

Outgoing DirectionOutgoing Power Refractive Index,A Priori Differential Probability

Normal Material Parameters (BSDF)

…

Radiance Estimate

INSERT r = f(rho)

13

Radiance Estimate• Traditional photon mapping: gather

• Per pixel• k-NN search in k-d tree• World-space (3D)

• Image-space photon mapping: scatter• Per photon• Hardware rasterization using photon volumes• Image space (2D)

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photon

• Invoke an illumination contribution on all pixels for which a photon might be a valid estimate of incident radiance

• Not virtual point lights (a.k.a. instant radiosity)• Not 2D splatting

Photon Volumes

16

Results

All at 1920 x 1080No LODLit from scratch every frame

Direct Only

Indirect

Direct + Indirect

Photons

Direct + Ambient

22

Performance Details

25

– Point light sources.– Pinhole camera

Limitations:

- Utilized the concept of image space to eliminate the need to adapting kd-Tree to GPU architecture.

Advantages

Assumptions

-- Clipping at near plane – ignorable/avoidable: Accurate light/volume rep resulting in problems when the camera gets inside the volume-- 4x more expensive than direct illumination (heavy fill consumption)-- Consistent, but biased (like photon mapping)-- Performance is limited by CPU trace

- Algorithm was simplified enabling implementation of the original photon map without loss of quality

- Capable of rendering full featured offline renders without considerable amount of noise.