GS-4147, TressFX 2.0, by Bill-Bilodeau

TressFX 2.0 AND BEYOND
BILL BILODEAU, AMD
DONGSOO HAN, AMD

TressFX 2.0 AND BEYOND
AGENDA

 TressFX Overview
 TressFX Rendering
‒ TressFX 2.0 improvements

 TressFX Physics
 Future Work

2 TressFX 2.0 and Beyond NOVEMBER 12, 2013 | AMD DEVELOPER SUMMIT

TressFX OVERVIEW
WHAT IS TressFX?

 Realistic hair rendering and simulation
‒ Used in Tomb Raider
 Goes beyond simple shells and fins representation used in games

 Hair is rendered as thousands of strands with self shadowing, antialiasing and transparency
 Physical simulation for each strand using GPU compute shaders
 Very flexible to allow for different hair styles and different conditions


TressFX RENDERING
WHAT MAKES IT LOOK GOOD

 What goes into good hair?
‒ Anti-aliasing
‒ Volumetric self shadowing
‒ Transparency

Basic Rendering

Antialiasing

Antialiasing

+ Self Shadowing

Antialiasing

+ Self Shadowing
+ Transparency

TressFX RENDERING
LIGHTING MODEL
Secondary Highlights

 Kajiya-Kay Hair Lighting Model
‒ Anisotropic hair strand lighting model
‒ Uses the tangent along the strand instead of the normal
for light reflections
‒ Instead of cos(N, H) , use sin(T,H)

 Marschner Model
‒ Two specular highlights
‒ Primary light colored highlight shifted towards the tip
‒ Secondary hair colored highlight shifted towards the root
‒ TressFX uses an approximation of the Marchner
technique when rendering two highlights


Primary Highlights

TressFX RENDERING
ANTI-ALIASING

 Every hair strand is anti-aliased manually
‒ Not using Hardware MSAA!

 Compute pixel coverage on edges of hair strands and convert it
to an alpha value


TressFX RENDERING
SELF SHADOWING

 Self Shadowing
‒ Uses a simplified Deep Shadow Map technique

No Self Shadows


With Self Shadows

TressFX RENDERING
TRANSPARENCY

 Order Independent Transparency (OIT) using a Per-Pixel Linked Lists (PPLL)

 Fragments are stored in link lists on the GPU
 Nearest K fragments are rendered in back to front order

No Transparency


With Transparency

TressFX 1.0 RENDERING
How rendering was done in version 1.0

 TressFX 1.0 Rendering
‒ Render hair strand geometry into A-buffer
‒ Do lighting, shadowing, and antialiasing
‒ Store fragment color with depth and coverage in per-pixel linked list (PPLL)

‒ Render the K nearest fragments (K-buffer) in back to front order
‒ Blend nearest K fragments in the correct order with transparency
‒ Blend the remaining fragments without sorting


A-BUFFER PASS

Coverage

Hair Geometry

Vertex Shader

Head UAV

Pixel Shader
Lighting
PPLL UAV

Shadows

depth
color
coverage


next

PER-PIXEL LINKED LIST

 GPU implementation of order independent transparency (OIT)

 Head UAV
‒ Each pixel location has a “head pointer” to a linked list in the PPLL
UAV

Head UAV

 PPLL UAV
‒ As new fragments are rendered, they are added to the next open
location in the PPLL (using UAV counter)
‒ A link is created to the fragment pointed to by the head pointer
‒ Head pointer then points to the new fragment
// Retrieve current pixel count and increase counter
uint uPixelCount = LinkedListUAV.IncrementCounter();
uint uOldStartOffset;
// Exchange indices in LinkedListHead texture corresponding to pixel location
InterlockedExchange(LinkedListHeadUAV[address], uPixelCount, uOldStartOffset);
// Append new element at the end of the Fragment and Link Buffer
Element.uNext = uOldStartOffset;
LinkedListUAV[uPixelCount] = Element;


PPLL UAV

K-BUFFER PASS

K-Buffer

Full Screen Quad

Vertex Shader


Pixel Shader

depth
depth
color depth
color depth
coverage color
coverage color
coverage
coverage

Transparency

HOW CAN WE MAKE IT FASTER?

 Observation
‒ All fragments are lit and shadowed equally
‒ Even the ones buried under dozens of hair fragments that you can’t see

 Solution
‒ Defer the lighting and shadowing until the k-buffer pass
‒ Render the nearest K fragments with high quality
‒ Render the remaining fragments with lower quality (but faster)


A-BUFFER PASS

Hair Geometry

Vertex Shader

Pixel Shader
Coverage

depth
coverage
tangent
next

K-BUFFER PASS

K-Buffer

Full Screen Quad

Vertex Shader

Pixel Shader

Lighting


depth
depth
coveragedepth
coverage depth
tangent
coverage
tangentcoverage
tangent
tangent

Shadows
Transparency

TressFX 2.0 IMPROVEMENTS
CONTINUOUS LODs

 Distance to camera can be used for reducing the density of the hair
‒ Uniformly remove hair strands from the rendering
‒ To compensate for missing strands, thicken the hair
‒ Adjust the minimum pixel coverage with distance

Full Density Hair

Reduced Density Hair

Reduced Density with Thicker Strands

CODE RESTRUCTURING

 TressFX11 Sample Code is much more modular

 All of the necessary TressFX code in separate files
for
‒ Rendering
‒ Simulation
‒ Mesh management
‒ Asset loading

 Code for head rendering and sample framework
are completely separate

Main

TressFXSimulate

TressFXSimulate

TressFXRender

SceneRender

SceneRender

TressFXRender

TressFXMesh

Gaussian
Filter

‒ Take the “TressFX” files to get just what you need

 Better variable names
 Removal of dead code


DX11Mesh
TressFXAssetLoader

TressFX Code

ObjImport

MISCELLANEOUS IMPROVEMENTS

 Vertex shader optimizations for rendering
‒ Draw call for hair now uses an index buffer with a triangle list instead of looking up indices from a buffer

 PPLL head buffer uses a RWTexture2D for better caching (tiled)
 Hair shadow on model is softer and less blocky
 Various shader code optimizations
 Porting Guide
 Download the new TressFX 2.0 sample soon from our Radeon SDK :

http://developer.amd.com


MEMORY CONSIDERATIONS

 A-Buffer
‒ 2 UAVs
‒ Size determined by resolution
‒ Head of the Linked List UAV

250.00
200.00

‒ Screen resolution RWTexture2D, DXGI_FORMAT_R32_UINT

‒ Per-Pixel Linked List UAV
‒ Structured Buffer, size = (number of pixels) x (avg hair layers) x
(sizeof(LinkedListStructure))
‒ Default average number of hair layers is 8
‒ Linked list structure is currently 3 DWORDs: depth, coverage, tangent

 Limited memory, but unbounded linked list
‒ This means too many fragments for a given pixel can overflow the PPLL
‒ Can cause artifacts
‒ Typically this only happens if the camera gets too close


150.00
720p
100.00

1080p

50.00
0.00
Total A-Buffer
Memory (MB)

Linked List Head

Per-Pixel Linked List

PERFORMANCE RESULTS

3

2.5

2.5

2

2
1.5
TressFX 1.0
1

TressFX 2.0

1.5

TressFX 1.0
TressFX 2.0

1

0.5

0.5

0

0

Total Hair Render
Time (ms)

A-Buffer Pass

Total Hair Render
Time (ms)

K-Buffer Pass

R9 280x

R9 290x

> 2X performance increase!


A-Buffer Pass

K-Buffer Pass

TressFX Simulation
Topics

 TressFX 1.0 Simulation Overview
‒ Main Interest
‒ Simulation Overview
‒ Constraints
‒ Global shape constraints
‒ Local Shape Constraints
‒ Edge length constraints

‒ Problems

 TressFX Beyond
‒ General Constraint Formulation
‒ Tridiagonal Matrix-free Formulation
‒ Solving Linear System
‒ Benefits


Main Interest
 Main interest of TressFX simulation
‒ Performance, performance and performance! – DirectCompute
‒ Styled hair – bending and twisting forces are important
‒ Stability – position based dynamics
- Conditions – wet, dry or heavy
- Wind – helps express dynamics even the character in the idle mode


Simulation Overview

load hair data
precompute rest-state values – can be offline

CPU

while simulation running do
apply gravity

integrate
apply GSC (Global Shape Constraints)

GPU – DirectCompute

apply LSC (Local Shape Constraints)
apply wind
apply ELC (Edge Length Constraints)

collision handling

vertex buffer


GPU – Rendering
pipeline

GLOBAL SHAPE CONSTRAINTS
 GSC(Global Shape Constraints)
‒ The initial positions of particles serve as the global goal positions
‒ The goal positions are rigid w.r.t character head transform.
‒ You can think the initial positions are some cage and vertices are trapped in that cage during simulation.
‒ Easy and cheap. Help maintain the global shape but lose the detailed simulation

initial goal position
current position
final position


LOCAL SHAPE CONSTRAINTS
 LSC(Local Shape Constraints)
‒ The goal positions are determined in the local frames.
‒ Still the goal positions are transformed in world frames and applied to vertex positions.

initial goal position
current position
final position


LOCAL SHAPE CONSTRAINTS – CONT’
 Local Transforms
‒ As in robotic arm, an open-chain structure has joints and each joint has parent-child relationships to its connected
joints.
‒ 𝑖−1 𝑇𝑖 is to transform (translate and rotate) child space(i) to its parent space(i-1)
‒ With local transforms in chain structure, we can get a global transforms.
𝑤

𝑇𝑖 =

𝑤

𝑇0 ∙ 0 𝑇1 … ∙

𝑖−2

𝑇𝑖−1 ∙

‒ Local frames should be updated at each particles


𝑖−1

𝑇𝑖

LOCAL SHAPE CONSTRAINTS – CONT’
 Initialize and update local and global transforms
‒ Initialization is performed in CPU or offline only once.
‒ Update is performed at each frame in GPU.
‒ Update is serial process but independent to other strands. We update multiple strands in massive parallel processes
in GPU.
‒ With local and global transforms, we can calculate target vertex positions for local shape constraints.
‒ Finally, update two neighboring vertices to get stable convergence.

Updating position
i

Computing on local transform

i-1

Zero


EDGE LENGTH CONSTRAINTS


how much
stretched or
compressed

0.5


unit edge vector

Problems
 Extreme acceleration
‒ When character makes a sudden move, it can generate extreme linear and
angular acceleration which stretch hair very long.
‒ Even with high iterations with Edge Length Constraints, hair doesn’t recover the
original length and as a result, hair can look too stretchy.
‒ Possible solution was to enforce Edge Length Constraints in the serial fashion
from the root to the end of hair with extra damping – used for Tomb Raider
‒ We need a better way! And we did research!


Problems
EXTREME ACCELERATION


General Constraint Formulation



Tridiagonal Matrix Formulation
 Special Formulation for Chain Structure such as Hair
‒ We don’t want to solve a big matrix equation, especially in GPU!
‒ Let’s take advantage of linear topology and serial indexing
 General
case. We
don’t
want this!
 Known. Easy to compute them.
 Special
case. Much
simpler!
 Unknown and what we are solving for

SOLVING LINEAR SYSTEM
 Solving Linear System
‒ The formulation doesn’t require explicit matrix – Good for GPU!
‒ Diagonal, super and sub diagonal elements are non-zero - Sparse!
‒ The equation is diagonally dominant – Good for choice of direct solver!
‒ We can use tridiagonal matrix algorithm (Thomas algorithm)
‒ So we can solve it in GPU!


FUR CASTLE


FUR MUSHROOM


GRASS


BENEFITS
 No more iterations for Edge Length Constraints
‒ Needn’t have to guess number of iterations
‒ Fixed computation cost
‒ Fast convergence


TressFX 2.0
CONCLUSIONS

 TressFX 2.0 performance now makes hair rendering faster than the previous version
‒ More than 2X faster in some cases
 TressFX is now fast enough to use on consoles
 More modular code structure means easier porting to your game
 Realistic physics for hair simulation can now be extended to other objects
 Stay tuned for more!
‒ Ongoing research to improve and expand the use of this technology


REFERENCE
 Real-time Hair Simulation with Efficient Hair Style Preservation – Han, et al. VRIPHYS 2012

 Tridiagonal Matrix Formulation for Inextensible Hair Strand Simulation – Han, et al. VRIPHYS 2013


DISCLAIMER & ATTRIBUTION

The information presented in this document is for informational purposes only and may contain technical inaccuracies, omissions and typographical errors.
The information contained herein is subject to change and may be rendered inaccurate for many reasons, including but not limited to product and roadmap
changes, component and motherboard version changes, new model and/or product releases, product differences between differing manufacturers, software
changes, BIOS flashes, firmware upgrades, or the like. AMD assumes no obligation to update or otherwise correct or revise this information. However, AMD
reserves the right to revise this information and to make changes from time to time to the content hereof without obligation of AMD to notify any person of
such revisions or changes.
AMD MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE CONTENTS HEREOF AND ASSUMES NO RESPONSIBILITY FOR ANY
INACCURACIES, ERRORS OR OMISSIONS THAT MAY APPEAR IN THIS INFORMATION.
AMD SPECIFICALLY DISCLAIMS ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE. IN NO EVENT WILL AMD BE
LIABLE TO ANY PERSON FOR ANY DIRECT, INDIRECT, SPECIAL OR OTHER CONSEQUENTIAL DAMAGES ARISING FROM THE USE OF ANY INFORMATION
CONTAINED HEREIN, EVEN IF AMD IS EXPRESSLY ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

ATTRIBUTION
© 2013 Advanced Micro Devices, Inc. All rights reserved. AMD, the AMD Arrow logo and combinations thereof are trademarks of Advanced Micro Devices,
Inc. in the United States and/or other jurisdictions. SPEC is a registered trademark of the Standard Performance Evaluation Corporation (SPEC). Other names
are for informational purposes only and may be trademarks of their respective owners.

GS-4147, TressFX 2.0, by Bill-Bilodeau

Recomendados

Recomendados

Mais conteúdo relacionado

Mais procurados

Mais procurados (20)

Semelhante a GS-4147, TressFX 2.0, by Bill-Bilodeau

Semelhante a GS-4147, TressFX 2.0, by Bill-Bilodeau (20)

Mais de AMD Developer Central

Mais de AMD Developer Central (20)

Último

Último (20)

GS-4147, TressFX 2.0, by Bill-Bilodeau