WetBrush: GPU-based 3D Painting Simulation at the Bristle Level Zhili Chen Byungmoon Kim Adobe Research GTC 2016
Oil painting with brush Complex interactions • Bristle-Bristle • Bristle-Paint • Paint-Paint Motivation: • Exhaustive simulation? 2 GTC 2016
Oil Painting Simulation Two-way Brush head Bristle-Paint Liquid movement Interaction User Input Brush Sim Paint Liquid Renderer 3 GTC 2016
Brush Model Previous works on brush model Natural brushes in Photoshop DiVerdi et al., 2010 D-Brush in Photoshop Chu et al., 2002 Expresii 2D Surface wrapped Individual bristles 2D Stamps around skeleton • • • No variations from No details from individual Brush shape not realistic without brush dynamics bristles with large inter-bristle interactions deformation 4 GTC 2016
Brush Model Our model Bristle vertices 𝐪 𝑗 Bristle samples 𝑚 0 𝐪 𝑗+1 Stretching 𝐪 𝑗−1 𝐪 𝑗 𝐪 𝑗+1 𝜄 Bending 50-100 bristles ~10 vertices per bristle 5 GTC 2016
Brush Model Inter-bristle interactions • Collision • Friction 6 GTC 2016
Paint Liquid Model Height field Volumetric grid Particles Overhang Chu, et al. 2010 Microsoft FreshPaint ? • Track volume and mass better • Less blurring from grid-sampling • Model paint carried by brush Multi-layer color mixing 7 GTC 2016
Hybrid Fluid Representation Adaptive Hybrid Fluid Representation based on • Distance to brush • Velocity Brush Fluid particles 8 GTC 2016
Hybrid Fluid Representation Adaptive Hybrid Fluid Representation based on • Distance to brush • Velocity Particles Brush Close to the brush Fluid OR Fast Moving particles Cover smaller region 9 GTC 2016
Hybrid Fluid Representation Adaptive Hybrid Fluid Representation based on • Distance to brush • Velocity Particles Brush Close to the brush Fluid OR Fast Moving particles Cover smaller region Density Grid Further away from the brush AND Slow moving Cover larger region 10 GTC 2016
Fluid Conversions Grid & Particles Only Particles Visualized Visualized Particle Fluid • • Convert and delete Sample new stationary particles particles Grid Fluid FLIP • Particles sample from velocity grid • Avoid expensive neighbor searching 11 GTC 2016
Hybrid Fluid • Less volume loss • Better surface tracking • Sharper surface detail • Sharper color mixing Grid Only Grid+Particle 12 GTC 2016
Brush-Paint Interaction • Brush emit new pigment particles • Brush carries and pushes existing fluid particles Particle Fluid Brush Sim • Brush pick up color from paint on canvas • Thick paint affect brush motion and shape Grid Fluid 13 GTC 2016
Implementation CUDA implementation • Both brush and paint liquid in CUDA • CUDA+OpenGL interop. for rendering Rendering • Ray casting on paint density grid and pigment grid • Screen space particle rendering 14 GTC 2016
Implementation Forward Brush Grid Fluid Forward FLIP k_AdvectPaintDensityGrid Simulation k_ForwardPosition k_Advection_Particles k_FindConvertParticlesToPaintDensity Particles k_VoxelizeBrush Convert inactive Voxelize particles to density Brush Rasterize Hash Brush Pickup Pigment grid k_FilterBrushVoxelization k_BrushPickupPigment Inter-Bristle k_RasterizeParticleTrilinear Particles k_ExtendPigment Collision onto Velocity SolveSelfCollision Find Brush Grid Sample New Particles k_NormalizeRasterization SpawnNewPaticles FindBrushBoundingB ox Bounding Box from Density Grid k_ProjectConstraints Bristle Save current SaveActiveDomain Diffusion Stretching Diffusion and window back to full- k_BrushDistanceMap Create Brush Dissapation k_BrushPaintNodesDistanceToPaintSurface and Bending canvas texture k_UpdateFinalSim PostProcessVelocity Brush Vertices emit Distance Field new Paint Particles k_PaintNodeEmitNewParticlesBasedOnDistanceToPaint Update Brush k_UpdateSorted Attributes for k_MixBrushVelocity Mix with Rendering k_UpdateRendering Brush Resample FLIP Particle k_PIC Velocities Velocity Field k_InterpolatePaintNo Update Brush des Paint Nodes that holds k_UpdatePaintNodesRendering pigment 15 GTC 2016
Performance GTX Titan X Benchmark sequence: 46 fps, 92K Particles (~2M at maximum) Grid-particle transfer 24% (5.2ms) Bristle-particle transfer (1.6ms) 8% Rendering 9% 24% (2.0ms) 5% Brush simulation Particle-based (1.1ms) liquid simulation (5.2ms) 30% Grid-based liquid simulation (6.5ms) 16 GTC 2016
Performance Faster stroke requires higher framerate • Faster stroke => more stroke samples per unit time • Maximum distance between stroke sample ~= 3-5 pixels Stroke Sample distance 30px Stroke Sample distance 2px 17 GTC 2016
Optimization 18 GTC 2016
Optimization Adaptive simulation window (vs fixed simulation window) thrust::minmax_element() Axis-aligned bounding box for brush Active simulation window 46 fps-> 75 fps (63%) For grid fluid simulation 19 GTC 2016
Optimization Particle insertion Particle Array AtomicInc(Num_Particles) Num_Particles 20 GTC 2016
Optimization Optimized Particle insertion New Particle Array Marker Array • • Pre-allocate maximum # 1 => a new particle is • No more than 27 particles within one grid cell particle space for each cell added for the element • No more than one particle in one subcell Cell 𝑗 − 1 1 1 0 0 0 0 0 Cell 𝑗 0 1 0 1 1 0 0 0 Cell 𝑗 + 1 1 0 21 GTC 2016
Optimization Optimized Particle insertion New Particle Array Marker Array Compact New Particle Array • • Pre-allocate maximum # 1 => a new particle is Particle Array particle space for each cell added for the element Cell 𝑗 − 1 1 1 0 0 0 0 0 Cell 𝑗 0 1 0 1 thrust::remove_if() Append 1 0 0 0 Cell 𝑗 + 1 1 0 75 fps-> 92 fps (22%) Shrink List 22 GTC 2016
Kernel Pipeline k_AdvectPaintDensityGrid k_ForwardPosition k_Advection_Particles k_FindConvertParticlesToPaintDensity k_VoxelizeBrush Hash k_FilterBrushVoxelization k_BrushPickupPigment k_RasterizeParticleTrilinear k_ExtendPigment SolveSelfCollision k_NormalizeRasterization SpawnNewPaticles FindBrushBoundingB ox k_ProjectConstraints SaveActiveDomain Diffusion k_BrushPaintNodesDistanceToPaintSurface k_BrushDistanceMap k_UpdateFinalSim PostProcessVelocity k_PaintNodeEmitNewParticlesBasedOnDistanceToPaint k_UpdateSorted k_MixBrushVelocity k_UpdateRendering k_PIC k_InterpolatePaintNo des k_UpdatePaintNodesRendering 23 GTC 2016
Kernel Pipeline k_AdvectPaintDensityGrid k_ForwardPosition k_Advection_Particles k_FindConvertParticlesToPaintDensity k_VoxelizeBrush Hash k_FilterBrushVoxelization k_BrushPickupPigment k_RasterizeParticleTrilinear k_ExtendPigment SolveSelfCollision k_NormalizeRasterization SpawnNewPaticles FindBrushBoundingB ox k_ProjectConstraints SaveActiveDomain Diffusion k_BrushPaintNodesDistanceToPaintSurface k_BrushDistanceMap k_UpdateFinalSim PostProcessVelocity k_PaintNodeEmitNewParticlesBasedOnDistanceToPaint k_UpdateSorted k_MixBrushVelocity k_UpdateRendering k_PIC k_InterpolatePaintNo des k_UpdatePaintNodesRendering Low occupancy, GPU almost idle 24 GTC 2016
Dependencies k_ForwardPosition Hash SolveSelfCollision k_ProjectConstraints SaveActiveDomain k_UpdateFinalSim k_ExtendPigment k_FindConvertParticlesToPaintDensity FindBrushBoundingB k_BrushDistanceMap k_UpdateSorted ox k_PIC k_UpdateRendering k_InterpolatePaintNo k_VoxelizeBrush k_FilterBrushVoxelization des SpawnNewPaticles k_PaintNodeEmitNewParticlesBasedOnDistanceToPaint k_UpdatePaintNodesRendering k_Advection_Particles k_RasterizeParticleTrilinear k_MixBrushVelocity k_AdvectPaintDensityGrid k_BrushPaintNodesDistanceToPaintSurface k_NormalizeRasterization Diffusion k_BrushPickupPigment PostProcessVelocity 25 GTC 2016
Dependencies k_ForwardPosition • Brush # Vertices << Grid Dimension, Particle # Hash • Low occupancy SolveSelfCollision k_ProjectConstraints SaveActiveDomain k_UpdateFinalSim k_ExtendPigment k_FindConvertParticlesToPaintDensity FindBrushBoundingB k_BrushDistanceMap k_BrushDistanceMap k_UpdateSorted ox k_PIC k_UpdateRendering k_InterpolatePaintNo k_VoxelizeBrush k_FilterBrushVoxelization des SpawnNewPaticles k_PaintNodeEmitNewParticlesBasedOnDistanceToPaint k_UpdatePaintNodesRendering k_Advection_Particles k_RasterizeParticleTrilinear k_MixBrushVelocity k_AdvectPaintDensityGrid k_BrushPaintNodesDistanceToPaintSurface k_NormalizeRasterization Diffusion k_BrushPickupPigment • First half of fluid simulation do not depend on brush PostProcessVelocity 26 GTC 2016
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