Direct3D 11 Tessellation

Speaker: Kevin Gee

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

Design Background

Programmable 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 size
More morph targets
Cheap / free LODs

Reduced asset creation time

Improved pixel shader utilization
Reduced GPU skinning costs
Run faster simulations
Move Sub-D costs to GPU

Pre-Tesselated Mesh: ~5500 kb

Asset Size: Comparison

Sub-D Mesh: ~130 kb

Morph Targets

Huge potential memory / size wins
Morph targets in Sub-D take up less

space than fully-tessellated \ sparse

morph targets
Enable 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 Simulation

Skin at the control mesh level

Saves skinning costs

Cloth in Sub-D

Reduces the resolution of the simulation
Keeps a smooth surface for rendering
The 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

New Primitives

Hull Shader

Operates per input primitive

E.g. patch

Computes control point transforms

E.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;
}

Tessellator

Inputs

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

Outputs

UV or UVW domain points
Connectivity of those points (tris, lines)
No adjacency information

Many possible partitioning schemes

Tessellation Scheme

demo

Domain Shader

Operates on each point generated by

the tessellator
Gets 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 Syntax

void 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

refinement
Many 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

mesh
Faces and edges

are split in the

center
Vertices are

averaged with

their surrounding

neighbors
Infinite iteration

results in the

“limit surface”

Why Catmull-Clark?

Broad support from industry and

modeling packages
Parametric evaluation introduced in

1998 (Stam) at Alias|wavefront
Further refinements added edges and

creases
Pixar adopted Catmull-Clark early

Facilitates rich character animation

Sub-D’s In Current
Systems

Build the model in
Sub-D’s

Modeling, texturing,
rigging

Configure & preview
displacement maps
At export time

Tessellate into a poly
mesh
Apply displacement maps
Write to disk

Game engine

Apply skinning transform
Rasterize

Proposed Future System

Build the model in Sub-D’s
Configure & preview displacement

maps
Export Sub-D mesh
Game engine

Convert Sub-D mesh into parametric
surface
Tessellate to desired LOD level
Apply displacement maps and skinning
Rasterize

Catmull-Clark

Terminology

Vertex, edge,

quad
Valence is

number of

incident edges to

a vertex

Regular vertex

has a valence of

4, otherwise it is

an

extraordinary

vertex

Regul

ar

Vertex

Extraordina

ry

Vertex

Loop/Schaefer Research

Represent 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 each
Cross-product is the normal vector

Adjust the U and V patch edges to

account for surface discontinuities

around extraordinary vertices

Implementation Overview

Initialization 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 time

Skin 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

Dra

w

GS

o Single pass

o No additional memory

o Avoids 16 fetches per vertex

o Variable tessellation within

a draw

o Subsets of HS can operate in

parallel

Skin

Regular and Extraordinary

Regular patches

All vertices have 4 edge-adjacent
neighbors

Valence 4

Predictable amount of data and work

Extraordinary patches

Any irregular patch
Not quite as predictable
Require a little more work
Draw call per valence supported

Direct3D 10 SDK
Sample
Subdivision Surfaces

demo

Handling Creases

Add redundant geometry

Defined
crease

Redundan
t
geometry

More Loop/Schaefer

Research

Latest version:
Modified Approximate Catmull-Clark

Patches (ACC2)
Outputs a Bezier patch consisting of

16 control vertices for regular

patches
Outputs 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)

b

10

b

00

b

20

b

30

b

01

b

11v

b

11u

b

21v

b

21u

b

02

b

03

b

13

b

23

b

33

b

32

b

31

b

12v

b

12u

b

22v

b

22u

ACC2 Patch

ACC2 Patch - Position

Average the inner point pairs and evaluate the

resulting 4x4 bicubic patch for position

ACC2 Patch - Tangents

ACC2 Math

v

p

i

p

i+1

p

i-1

q

i

b

00

b

10,i

b

10,i+1

b

20,i

b

20,i+1

b

11v,i

b

11u,i

q

i-1

This is a lot…

There’s a lot of complexity here, but

it’s worth it
D3D11 can target almost any surface

algorithm you want

Bezier
NURBs
Dynamic and static tessellation
Displacement
Subdivision (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.