Direct3D 11 Tessellation

Speaker: Kevin Gee

Research: Charles Loop / Scott SchaferSlides: Shanon Drone, Matt Lee, Michael Oneppo

Design BackgroundProgrammable pipeline can target any surface approach.One primary scenario facilitates subdivision surfaces as a primitive type.Charles Loop and Scott Schaefer provided a reference approximation to Catmull-Clark.Converts Sub-D surface into Bezier patches.Other approaches are possible too.

Why Tessellate?Many reasons including

Reduced asset memory sizeMore morph targetsCheap / free LODs

Reduced asset creation time

Improved pixel shader utilizationReduced GPU skinning costsRun faster simulationsMove Sub-D costs to GPU

Pre-Tesselated Mesh: ~5500 kb

Asset Size: Comparison

Sub-D Mesh: ~130 kb

Morph Targets

Huge potential memory / size winsMorph targets in Sub-D take up less space than fully-tessellated \ sparse morph targetsEnable richer animations for the same memory cost

Level of Detail

Continuous LOD becomes possible.Reduces content creation time

Cheaper than building & testing explicit LODs

Improves pixel shader quad utilization

Faster SimulationSkin at the control mesh level

Saves skinning costs

Cloth in Sub-DReduces the resolution of the simulationKeeps a smooth surface for renderingThe more complex the simulation, the bigger the savings

Compute surface constraints at a lower frequency

Limit high-frequency positions to avoid penetrations

DIRECT3D 11 PIPELINE OVERVIEW

InputAssembler

VertexShader

Vertex Buffer

Index Buffer

Texture

GeometryShader

Texture

Stream Output

Rasterizer/Interpolator

PixelShader

OutputMerger

Depth/Stencil

Texture

Render Target

HullShader

Texture

DomainShader

Texture

Tessellator

New Primitives

Hull ShaderOperates per input primitive

E.g. patch

Computes control point transformsE.g. Basis Change

Computes tessellation factors per edge of generated patches

Hull Shader Syntax[patchsize(12)][patchconstantfunc(MyPatchConstantFunc)]MyOutPoint main(uint Id : SV_ControlPointID, InputPatch<MyInPoint, 12> InPts){ MyOutPoint result; …

result = TransformControlPoint( InPts[Id] );

return result;}

TessellatorInputs

Takes in “Tessellation Factors” provided by the Hull shader Tess factors per-side in the range of [2.0..64.0]

OutputsUV or UVW domain pointsConnectivity of those points (tris, lines)No adjacency information

Many possible partitioning schemes

Tessellation Scheme

demo

Domain Shader

Operates on each point generated by the tessellatorGets ALL control points as input

Control points and patch constant data are passed directly to the domain shader

Evaluate primitive surface to compute position of points

Convert from U,V space into positions, tangents

Domain Shader Syntaxvoid main( out MyDSOutput result,

float2 myInputUV : SV_DomainPoint, MyDSInput DSInputs,

OutputPatch<MyOutPoint, 12> ControlPts, MyTessFactors tessFactors ){

…

result.Position = EvaluateSurfaceUV( ControlPoints, myInputUV );

}

APPLYING SUBDIVISION SURFACES TO THE PIPE

What Are Subdivision Surfaces?

Surfaces defined by iterative refinementMany different techniques

Catmull-Clark (1978)Doo-Sabin (1978)Loop (1987)

Techniques differ primarily in edge cases and fixing trouble spots in previous techniques

Catmull-Clark Subdivision

Start with a quad meshFaces and edges are split in the centerVertices are averaged with their surrounding neighborsInfinite iteration results in the “limit surface”

Why Catmull-Clark?

Broad support from industry and modeling packagesParametric evaluation introduced in 1998 (Stam) at Alias|wavefrontFurther refinements added edges and creasesPixar adopted Catmull-Clark early

Facilitates rich character animation

Sub-D’s In Current SystemsBuild the model in

Sub-D’sModeling, texturing, rigging

Configure & preview displacement mapsAt export time

Tessellate into a poly meshApply displacement mapsWrite to disk

Game engineApply skinning transformRasterize

Proposed Future SystemBuild the model in Sub-D’sConfigure & preview displacement mapsExport Sub-D meshGame engine

Convert Sub-D mesh into parametric surfaceTessellate to desired LOD levelApply displacement maps and skinningRasterize

Catmull-Clark Terminology

Vertex, edge, quadValence is number of incident edges to a vertexRegular vertex has a valence of 4, otherwise it is an extraordinary vertex

Regular

VertexExtraordina

ryVertex

Loop/Schaefer ResearchRepresent each quad’s limit surface as a bicubic patch (16 knots, 4x4)Add two biquadratic patches that create a U and V tangent field

12 knots, 3x4 eachCross-product is the normal vector

Adjust the U and V patch edges to account for surface discontinuities around extraordinary vertices

Implementation OverviewInitialization time

Load Sub-D mesh (quad mesh)Build adjacency-based patches

Use 1-ring of vertices around each quad

Compute texture tangent space for each vertex

Run timeSkin the quad mesh in the vertex shader (VS)Convert Sub-D mesh into patches in the Hull shader (HS)Evaluate patches using the domain shader (DS)

Quad Mesh

Input Quads

Each patch consists of 4 inner quad vertices and a 1-ring neighborhood

Sub-D Patch

1-Ring Neighborhood

D3D11 Sub-D Pipeline Overview

Hull Shader

VS

Tessellator

Sub-D Patch Buffer

PS

Domain Shader

Draw

GS

o Single passo No additional memoryo Avoids 16 fetches per vertexo Variable tessellation within a draw

o Subsets of HS can operate in parallel

Skin

Regular and ExtraordinaryRegular patches

All vertices have 4 edge-adjacent neighbors

Valence 4

Predictable amount of data and work

Extraordinary patchesAny irregular patchNot quite as predictableRequire a little more workDraw call per valence supported

Direct3D 10 SDK SampleSubdivision Surfaces

demo

Handling CreasesAdd redundant geometry

Defined crease

Redundant geometry

More Loop/Schaefer ResearchLatest version:

Modified Approximate Catmull-Clark Patches (ACC2)Outputs a Bezier patch consisting of 16 control vertices for regular patchesOutputs a Gregory patch consisting of 20 control vertices for extraordinary patches

New Research (ACC2)Collapse position and tangent into a single bicubic patch

Fewer control points, less memory

Modification of a Gregory patch

Bicubic patch with 2 sets of interior knots (20 knots total)

b10b00 b20

b30

b01

b11v

b11u

b21v

b21u

b02

b03

b13 b23b33

b32

b31

b12v

b12u

b22v

b22u

ACC2 Patch

ACC2 Patch - Position

Average the inner point pairs and evaluate the resulting 4x4 bicubic patch for position

ACC2 Patch - Tangents

ACC2 Math

vpi

pi+1

pi-1

qi

b00 b10,i

b10,i+1

b20,i

b20,i+1

b11v,ib11u,i

qi-1

This is a lot…There’s a lot of complexity here, but it’s worth itD3D11 can target almost any surface algorithm you want

BezierNURBsDynamic and static tessellationDisplacementSubdivision (using Loop transforms)

and more…

Call to Action!

Experiment with the D3D10 Subdivision Surface Sample from the DirectX SDK NOW!Build support for Sub-D meshes into your pipelines, tools, and engines.

Look for a future Community Tech Preview (CTP) of Direct3D 11.

www.xnagamefest.com

© 2008 Microsoft Corporation. All rights reserved.This presentation is for informational purposes only.

Microsoft makes no warranties, express or implied, in this summary.