Real-time Rendering of Efficient Substitutes for Subdivision Surfaces

Implementation Tianyun Ni

Overview This course section: real-time rendering of efficient substitutes for subdivision surfaces. – Recent theoretical results – Leveraging DirectX 11 Topics: – Introduction to DirectX 11 Tessellation – Implementation on DirectX 11 pipeline – Implementation on current hardware – Practical implementation issues Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Current Authoring Pipeline Control Cage, or SubD Mesh Smooth Surface Displaced Surface Generate LODs … GPU Polygon Mesh

Direct3D 11 Pipeline For Real-time Tessellation Rendering + Displacement Control Cage Map GPU Optimally Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009 Tessellated Mesh

Direct3D 11 Tessellation Pipeline • Save memory and bandwidth - Memory is a critical bottleneck to render highly detailed surfaces = + © Bay Raitt Level 8 Level 16 Level 32 Level 64 Regular Triangle Mesh 16MB 59MB 236MB 943MB D3D11 compact 1.9MB 7.5MB 30MB 118MB representation Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Direct3D 11 Tessellation Pipeline + Displacement Control Cage Map GPU Optimally Tessellated Mesh Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

D3D11 Tessellation Process on GPU + Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Direct3D 11 Graphics Rendering Pipeline Input Assembler Resources (buffers, textures, constants) Vertex Shader Hull Shader New Direct3D 11 Tessellator Pipeline Stages Domain Shader Geometry Shader Setup/Raster Pixel Shader Programmable Output Merger Fixed function Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Direct3D 11 Tessellation Vertex Shader + Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Vertex Shader • Transforms control cage vertices Vertex Shader Hull Shader Tessellator Domain Shader Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Vertex Shader • Applications • Realistic animation: skinning, morph targets, etc Vertex Shader Hull Shader • Physical Simulation: hair simulation, particle system, soft body deformation Tessellator Domain Shader Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Vertex Shader Direct3D 11 Tessellation New Primitive Hull Shader Type: Patch + Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Hull Shader • New patch primitive type Vertex Shader - The only supported primitive when tessellation is enabled - Arbitrary vertex count (up to 32) Hull Shader - No implied topology Tessellator • Computes control points • Computes tessellation factors Domain Shader Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Adaptive Tessellation • Curvature, view-dependent Level of Detail © Pixolator @ ZBrushCentral © Bay Raitt Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Hull Shader: Control Point Construction • All control points Pj are a weighted sum of vertices: Pj = ∑ wij Vi • For each connectivity, pre-computes wij and stores them in a stencil texture • The hull shader invokes multiple threads. Each thread computes one control point. Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Hull Shader: Control Point Construction ACC_CONTROL_POINT SubDToParametricPatchHS( InputPatch<CONTROL_POINT_OUTPUT, M> p, uint tid : SV_OutputControlPointID, uint pid : SV_PrimitiveID ) { ACC_CONTROL_POINT output; int topo = getPatchConnectivityID(pid); connectivity type ID int num = getVertexCount(pid); the number of vertices in the patch primitive float3 output.pos= float3(0,0,0); for (int i=0; i< num; i++) { Compute int idx = getVertexIDinPatch(pid,i); use index global ordering one control point per int index = fetchIndexInStencil(topo, idx, tid); output.pos += p[i] * gStencil.Load(int3((index, 0,0)); thread } return output; } Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Vertex Shader Direct3D 11 Tessellation New Primitive Hull Shader Type: Patch Tessellator + Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Tessellator • Fixed function stage, but configurable • Domains: - Triangle Vertex Shader - Quad - Isolines Hull Shader • Spacing: Tessellator - Discrete - Continuous (fractional) Domain Shader - Pow2 Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Tessellator Vertex Shader Hull Shader Tessellator Level 5 Level 5.4 Level 6.6 Domain Shader Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Vertex Shader Direct3D 11 Tessellation New Primitive Hull Shader Type: Patch Domain Shader Tessellator + Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Domain Shader • One invocation per generated vertex • Evaluate surface given parametric UV{W} coordinates Vertex Shader • Apply displacements Hull Shader Tessellator Control U V {W} points coordinates Domain Shader one vertex Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Tessellation Process Regular Extraordinary + Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Patch Construction Schemes • PN Triangles by Alex Vlachos, Jörg Peters, Chas Boyd, and Jason Mitchell “Curved PN Triangles”, I3D 2001: Proceedings of the 2001 Symposium on Interactive 3D Graphics, pages 159-166, 2001. Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

PN Triangles on Direct3D 11 Pipeline HS input: Hull Shader Tessellation factors Tessellator control points Domain Shader DS Input from Tessellator: • uvw coordinates for one vertex DS Output: • one tessellated vertex Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Patch Construction Schemes • ACC Patches by Charles Loop and Scott Schaefer “Approximating Catmull-Clark Subdivision Surfaces with Bicubic Patches” , ACM Transactions on Graphics, Vol. 27 No. 1 Article 8 March 2008. Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

ACC Patch on Direct3D 11 Pipeline HS input: Hull Shader Tessellation factors Tessellator control points Domain Shader DS Input from Tessellator: • uv coordinates for one vertex Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Patch Construction Schemes • Pm Patches by Ashish Myles, Tianyun Ni and Jörg Peters “Fast Parallel Construction of Smooth Surfaces from Meshes with Tri/Quad/Pent Facets”, Symposium on Geometry Processing (SGP), Copenhagen, Denmark, January 2 - 4, 2008. m Pm-sectors m=3 m=4 … Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Pm – Patch on Direct3D 11 Pipeline HS input: Hull Shader Tessellation factors Tessellator control points Domain Shader DS Input from Tessellator: • uv, or uvw coordinates for one vertex Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Instanced Tessellation on Current Hardware Input Assembler Vertex Shader Vertex Shader Pass1: Animation Hull Shader Vertex Shader Pass 2: Patch Construction Tessellator Geometry Shader Domain Shader Pass 3: Vertex Shader Compute TF Geometry Shader Setup/Raster Pixel Shader Output Merger Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Instanced Tessellation on Current Hardware Input Assembler Vertex Shader Hull Shader • Define a generic pattern Tessellator • Replicate the patterns multiple times • Draw instanced geometry Domain Shader • Adaptive Tessellation Geometry Shader • per object • per patch Setup/Raster Pixel Shader Output Merger Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Instanced Tessellation on Current Hardware Input Assembler Patch index buffer Patch Factor Vertex Shader 2 4 5 0 2 Hull Shader 1 2 One Bucket 2 1 Patch index buffer Tessellator 3 4 0 1 Domain Shader 4 1 Geometry Shader 5 1 Patch index buffer 6 4 Setup/Raster 3 6 Pixel Shader Output Merger Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Adaptive Tessellation on Current Hardware • Generic Mesh Refinement On GPU http://iparla.labri.fr/publications/2005/BS05/GenericMeshRefinementOnGPU.pdf • Generic Adaptive Mesh Refinement http://http.developer.nvidia.com/GPUGems3/gpugems3_ch05.html • Semi-uniform Adaptive Patch Tessellation http://sintef.org/project/Heterogeneous%20Computing/preprints/topofix-draft.pdf Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Instanced Tessellation on Current Hardware Input Assembler Vertex Shader Vertex Shader Pass1: Animation Hull Shader Vertex Shader Pass 2: Patch Construction Tessellator Geometry Shader Pass 3: Domain Shader Vertex Shader Compute TF Geometry Shader Vertex Shader Setup/Raster Pass 4: Setup/Raster Surface Pixel Shader Evaluation Pixel Shader +DM Output Merger Output Merger Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Instanced Tessellation on Current Hardware Input Assembler Vertex Shader • In the vertex shader - Load patch index from Hull Shader bucket’s patch list Pass 4: Surface - Load edge tessellation Tessellator Evaluation level to stitch boundaries +DM Domain Shader - Load control points to evaluate surface Vertex Shader Geometry Shader - Apply Displacement Setup/Raster Mapping Setup/Raster Pixel Shader Pixel Shader Output Merger Output Merger Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Watertight Tessellation • Floating point precision issue - Addition is neither commutative nor associative - Special care has to be taken to obtain watertight results to prevent cracks Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Watertight Patch Construction • Control Points Construction - Control points type: corner, edge, face - In all cases we can compute a control point as weighted sum: Pj = ∑ wij Vi - Needs consistent ordering of the summation V8 V7 V6 V9 V5 V0 V1 V3 V4 V2 Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Watertight Surface Evaluation Inconsistent Consistent Positions: orientations P(u,v) = ∑ bi Bi(u,v) Problem: a+b+c != c+b+a Solution 1: All computations need to done in consistent parametric orientation Solution 2: Symmetric evaluation Courtesy of Valve Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Watertight Displacement • Displacement Mapping - Sample displacement value from a texture - Displace vertex along its normal Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Watertight Displacement • Watertight normals - Cross product of a pair of tangent, bitangent vectors - All three vectors should be co-planar - Problem: cross(tanU,tanV) ≠cross(tanV, tanU) - Solution: Define corner and edge ownership tanV tanU tanU Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Watertight Displacement Problem: Non-watertight Texture Seams due to - Bilinear discontinuities - Varying floating point precision on different regions of the texture map - Seamless parameterization removes bilinear artifacts, but does not solve floating point precision issues Solution: Define patch ownership of the seams Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Watertight Displacement • Store 4 texture coordinates per vertex (0: interior, 1,2: edges, 3: corner) tx3 tx1 ty2 ty3 – 16 per quad patch, 12 per triangle tx2 tx0 ty3 ty0 ty1 patch – The corresponding texture ty1 coordinate can be selected using the value of the parametric tw1 tw0 tz0 tz2 tw2 coordinate UV – Compute Barycentric interpolation tw3 tw2 tz1 3 tz3 tw of texture coordinates Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Watertight Surface Evaluation Suggested Reading: • Tessellation of Subdivision Surfaces in DirectX 11 (Gamefest 08) http://developer.nvidia.com/object/gamefest-2008- subdiv.html • Next-Generation Rendering of Subdivision Surfaces (Siggraph08) http://developer.nvidia.com/object/siggraph-2008- Subdiv.html • Course Notes (Siggraph09) http://www.nitianyun.com/sigcourse09.html Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Live Demo Instanced Tessellation GPU Patch Construction Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Q&A Tianyun Ni Ignacio Castaňo tni@nvidia.com icastano@nvidia.com Efficient Substitutes for Subdivision Surfaces – SIGGRAPH Course 2009

Real-time Rendering of Efficient Substitutes for Subdivision Surfaces

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