holy smoke! faster particle rendering using direct compute by gareth thomas

presentation title

Holy Smoke! Faster Particle rendering using directcomputeAmd and Microsoft developer day, june 2014, stockholmGareth thomas2nd june 2014

| fASTER PARTICLE RENDERING USING DIRECTCOMPUTE | AMD AND MICROSOFT GAME DEVELOPER DAY - june 2 2014, STOCKHOLM#

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PLAN FOR TODAYSimulation OverviewCollisionsSortingTiled RenderingConclusions


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overviewWhy use the gpu for simulation?Highly parallel workloadFree your CPU to do other cool stuffLeverage computeTake advantage of the Local Data Store (LDS)Asynchronous compute on some platforms

motivation


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OVERVIEWEmitSimulateSortRenderRasterize billboardsTiled Rendering using DirectCompute

How to build a GPU particle system

| fASTER PARTICLE RENDERING USING DIRECTCOMPUTE | AMD AND MICROSOFT GAME DEVELOPER DAY - june 2 2014, STOCKHOLM#Add pic4

SIMULATION OVERVIEWHow the simulation fits togetherSimulate Compute ShaderUpdate Particles. Add alive ones to Alive List, add dead ones to Dead List

Dead ListPersistent list of particle indicesAlive ListList of alive particle indices. Rebuilt each frame by Simulation CSEmit Compute ShaderReads free indices from dead list. Writes new particle data into global array

Particle ArrayPersistent list of particle indices


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collisionsCan no longer use CPU-side physics engine for collisionsUse depth buffer [Tchou11]Project particle into screen space and read depth bufferProject particle into view spaceTransform depth buffer value into view space and compare depthsGenerate collision responseUse G-buffer normalsOr take multiple depth samples to reconstruct the normal

A gpu-based solutionview space

P(n)P(n+1)

thicknessZ


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collisionsOnly collides against geometry in the depth bufferParticles would collide against depth buffer even if they are behind the geometryUse a thickness value to assume particles are in free space behind geometryParticles dont collide when they are off screenCauses issues when particles that are at rest on the floor have gone off-screen and have now disappearedPut particles to sleep in the simulation once they have come to restUse G-buffer to mark parts of the scene that particles can sleep on (static objects)Not Multi-GPU Friendly!Switch off depth buffer collisions in MGPU modeProblems with using the depth buffer

Fallen through world!


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73681425for( subArraySize=2; subArraySize0; compareDist/=2) { // Begin: GPU part of the sort for each element n n = selectBitonic(n, n^compareDist); // End: GPU part of the sort }}Bitonic Sort


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251468

73for( subArraySize=2; subArraySize0; compareDist/=2) // compareDist == 1 { // Begin: GPU part of the sort for each element n n = selectBitonic(n, n^compareDist); // End: GPU part of the sort }}Bitonic Sort (Pass 1)

| fASTER PARTICLE RENDERING USING DIRECTCOMPUTE | AMD AND MICROSOFT GAME DEVELOPER DAY - june 2 2014, STOCKHOLM#3786











Sorted Alive List

Vertex ShaderRead Particle Buffer

Geometry ShaderExpand one point to four. Billboard in view space.

Pixel ShaderTexturing and tinting. Depth fade for soft particles.

Particle PoolRendering


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Sorted Alive List

Vertex ShaderRead particle buffer and billboard in view space

Pixel ShaderTexturing and tinting. Depth fade for soft particles.

Particle Pool

Index Buffer

Rendering


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RenderingThe alive particle count is only available on the GPUUse Indirect APIDrawInstancedIndirect( GPU-args ) for Geometry Shader billboardsD3DPT_POINTLIST with no VB, IB or IA VertexId = Particle indexVertexCountPerInstance = NumParticlesInstanceCount = 1Geometry Shader expands the point into four vertices and a 2 triangle strip per billboardOr better still. DrawIndexedInstancedIndirect( GPU-args )D3DPT_TRIANGLELIST, use IBVertexId / 4 = Particle indexVertexId % 4 = Billboard corner indexIndexCountPerInstance = NumParticles * 6InstanceCount = 1

Rasterization for old school GPU particle systems

| fASTER PARTICLE RENDERING USING DIRECTCOMPUTE | AMD AND MICROSOFT GAME DEVELOPER DAY - june 2 2014, STOCKHOLM#RenderingOverdraw from large particles kills game performance!Get artists to throttle back on the VFX OptimizationsTightly fit polygons around texture [Persson09]Render to smaller buffer [Cantlay07]Sorting issuesLoss of fidelityProblems with Rasterization


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Tiled renderingInspired by Forward+ [Harada12]Screen-space binning of particles instead of point lights!Use a 32x32 thread group to shade a 32x32 pixel tile in screen spaceCull particles (just like Forward+)Sort particles Per pixel/threadEvaluate colour of each particleBlend togetherComposite back onto scene

overview


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Tiled rendering

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[1][1,2,3][2,3]Divide screen into tilesBuild index lists of intersecting particles per tile


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Tiled renderingView space asymmetric frustum generated per tileUse cameras near planeUse cameras far planeOr calculate far plane from depth buffer

Tile0Tile1Tile2Tile3


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Tiled rendering


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Tiled renderingnumthreads[ 32,32,1]Culling 1024 particles in parallelAdd to LDS index listWrite out to memoryParticle countParticle indicesThread group view


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Tiled renderingTile complexity


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Tiled renderingCannot sort global list of particlesBecause 1024 particles get culled in parallel they get added to visible list in arbitrary orderNeed to sort particles per-tileThis is a good thing!Only need to sort a subset of the global listSorting particles in single pass in LDS vs main memory and in multiple passes

Per tile bitonic sort


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Tiled renderingnumthreads[ 32, 32, 1 ] 1 thread = 1 pixel in screen spaceSet accumulation colour to float4( 0, 0, 0, 0 )For each particle in tile (back to front)Evaluate particle contributionUV generation & radius checkTexture lookupNormal generation and lightingManually blendColour = ( srcA x srcCol ) + ( invSrcA x destCol )Alpha = srcA + ( invSrcA x destA )Write result to screen size UAV

Evaluating tile colour


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Tiled renderingnumthreads[ 32, 32, 1 ] 1 thread = 1 pixel in screen spaceSet accumulation colour to float4( 0, 0, 0, 0 )For each particle in tile (front to back)Evaluate particle contributionUV generation & radius checkTexture lookupNormal generation and lightingManually blend [Bavoil08]Colour = ( invDestA x srcA x srcCol ) + destColAlpha = srcA + ( invSrcA x destA )if ( accumulation alpha > threshold )accumulation alpha = 1 and bailWrite result to screen size UAV

Evaluating tile colour improved!!!


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Tiled renderingBin particles into 8x8 gridFor each particleFor each binTest particle against binAdd particle if visibleUAV0 for particle indices (size = 8 x 8 x maxparticles)Array split into 64 bins using offsetsUAV1 for storing particle count per bin (size = 8 x 8)1 element per binUse InterlockedAdd() to bump bins counter

Coarse culling


Tiled renderingCompute shader setup

Per-bin particle indices

Per-tile sorted particle indices

Screen space colour buffer

Per-bin frustum planes

Per-tile particle indices and distances

Particle data (position, radius, colour etc)Compute ShadersLDSShader Output

Updated particle data

Simulationnumthreads[256, 1, 1], 1 thread per particle

Coarse Cullingnumthreads[256, 1, 1], 1 thread per particle

Tile Culling and Sortingnumthreads[32, 32, 1], 1 thread per particle

Tile Renderingnumthreads[32, 32, 1], 1 thread per pixel


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modeframe time (ms)*Rasterization5.2Tiled3.4

*AMD Radeon R9 290X @ 1080p

Breakdownframe time (ms)*Simulation0.50Coarse Culling0.06Tile Culling and Sorting0.37Tiled Rendering1.86

PERFORMAnce resultsDefault View, ~35K particles


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modeframe time (ms)*Rasterization27.3Tiled6.2

*AMD Radeon R9 290X @ 1080p

PERFORMAnce resultsIn Smoke View, ~35K particles


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conclusionsDepth buffer collisionsGreat bang-for-buckNot perfect!Bitonic sortGood fit for sorting on the GPUTiled RenderingFaster than rasterizationGreat for combatting heavy overdrawMore predictable behaviourFuture workAdd arbitrary geometry for OITVolume tracing


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Questions?Demo with full source coming soonhttp://developer.amd.com/tools/graphics-development/amd-radeon-sdk/

| fASTER PARTICLE RENDERING USING DIRECTCOMPUTE | AMD AND MICROSOFT GAME DEVELOPER DAY - june 2 2014, STOCKHOLM#References[Tchou11] Chris Tchou, Halo Reach Effects Tech, GDC 2011[Persson09] Emil Persson, http://www.humus.name/index.php?page=News&ID=266[Cantlay07] Iain Cantlay, High-Speed, Off-Screen Particles, GPU Gems 3 2007[Harada12] Takahiro Harada et al, Forward+: Bringing Deferred Lighting to the Next Level, Short Papers, Eurographics 2012[Bavoil08] Louis Bavoil et al, Order Independent Transparency with Dual Depth Peeling, 2008

| fASTER PARTICLE RENDERING USING DIRECTCOMPUTE | AMD AND MICROSOFT GAME DEVELOPER DAY - june 2 2014, STOCKHOLM#Disclaimer & AttributionThe information presented in this document is for informational purposes only and may contain technical inaccuracies, omissions and typographical errors.

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holy smoke! faster particle rendering using direct compute by gareth thomas

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