© Copyright Khronos Group, 2011 - Page 1

OpenGL Shading LanguageHints/Kinks

Bill Licea-KaneAMD

© Copyright Khronos Group, 2011 - Page 2

OpenGL Shading Language Hints/KinksMade in the Shade• A brief WABAC Machine moment

• Our world is no longer side effect free

- (GL_ARB_conservative_depth)

- GL_ARB_shader_image_load_store

- GL_ARB_shader_atomic_counters

• Some examples

- Don't try this at home

- Something old school

- Something new school

© Copyright Khronos Group, 2011 - Page 3

Brief WABAC Machine momentA decade in the shade

OpenGL Birds of a Feather

Wednesday, 15 August, 6:30 pm - 8:30 pm

Wilshire Grand, Wilshire Room

© Copyright Khronos Group, 2011 - Page 4

Shader-centric viewThe way things were – side effect free

Shader

Uniforms Textures

Temporaries

in out

© Copyright Khronos Group, 2011 - Page 5

Shader-centric viewThe way things are – side effects!

Shader

Uniforms Textures Images

Temporaries

in out

© Copyright Khronos Group, 2011 - Page 6

Shader-centric view…but still embarrassingly parallel

© Copyright Khronos Group, 2011 - Page 7

memory qualifiersmemory functions// memory qualifiers

coherent

volatile

restrict

writeonly

readonly

// memory functions

void memoryBarrier();

© Copyright Khronos Group, 2011 - Page 8

built-in functions

gvec4 imageLoad(readonly IMAGE_PARAMS)

void imageStore(writeonly IMAGE_PARAMS, gvec4 data)

© Copyright Khronos Group, 2011 - Page 9

built-in functionscontinuedint imageAtomicAdd(IMAGE_PARAMS, int data)

int imageAtomicMin(IMAGE_PARAMS, int data)

int imageAtomicMax(IMAGE_PARAMS, int data)

int imageAtomicAnd(IMAGE_PARAMS, int data)

int imageAtomicOr(IMAGE_PARAMS, int data)

int imageAtomicXor(IMAGE_PARAMS, int data)

int imageAtomicExchange(IMAGE_PARAMS, int data)

int imageAtomicCompSwap(IMAGE_PARAMS, int compare, int data)

© Copyright Khronos Group, 2011 - Page 10

built-in functionscontinueduint imageAtomicAdd(IMAGE_PARAMS, uint data)

uint imageAtomicMin(IMAGE_PARAMS, uint data)

uint imageAtomicMax(IMAGE_PARAMS, uint data)

uint imageAtomicAnd(IMAGE_PARAMS, uint data)

uint imageAtomicOr(IMAGE_PARAMS, uint data)

uint imageAtomicXor(IMAGE_PARAMS, uint data)

uint imageAtomicExchange(IMAGE_PARAMS, uint data)

uint imageAtomicCompSwap(IMAGE_PARAMS, uint compare, uint data)

© Copyright Khronos Group, 2011 - Page 11

built-in functionscontinueduint atomicCounterIncrement (atomic_uint c)

uint atomicCounterDecrement (atomic_uint c)

uint atomicCounter (atomic_uint c)

© Copyright Khronos Group, 2011 - Page 12

We would rather not shade if possibleSome controls to helplayout (depth_any) out float gl_FragDepth; // [unhappy face]

// hints

// may allow depth test early, gl_FragDepth will be written

layout (depth_unchanged) out float gl_FragDepth; // [happy face]

layout (depth_greater) out float gl_FragDepth; // [half-happy face]

layout (depth_less) out float gl_FragDepth; // [half-happy face]

// early depth test, period. gl_FragDepth ignored

layout (early_fragment_tests) in; // [woot-woot face]

© Copyright Khronos Group, 2011 - Page 13

Examples - Don’t try this at homeWe are what you call "experts"// "Experts" wouldn’t do this either.

// For thought-experimentation ONLY!!!

// Seriously.

layout (early_fragment_tests) in;

layout (binding=0, offset=0) uniform atomic_uint counter;

void main()

{

atomicCounterIncrement(counter);

}

© Copyright Khronos Group, 2011 - Page 14

Examples - Don’t try this at homeOcclusion querry-ish// "Experts" wouldn’t do this either.

// For thought-experimentation ONLY!!!

// Seriously.

layout (early_fragment_tests) in;

layout (binding=0, offset=0) uniform atomic_uint counter;

void main()

{

atomicCounterIncrement(counter);

}

© Copyright Khronos Group, 2011 - Page 15

Examples – Something old school

layout (triangles) in;

layout (points , max_vertices=1) out;

// ...

bool classifyTriangle();

void emitHit();

void main()

{

if classifyTriangle()

emitHit();

}

© Copyright Khronos Group, 2011 - Page 16

Examples – Something old schoolSelection Modelayout (triangles) in;

layout (points , max_vertices=1) out;

// ...

bool classifyTriangle();

void emitHit();

void main()

{

if classifyTriangle()

emitHit();

}

© Copyright Khronos Group, 2011 - Page 17

Examples – something new schoolPer fragment (even per sample) selection• Based on example from GDC 2010

- OIT and Indirect Illumination using DX11 Linked Lists

Holger Gruen, Nicolas Thibieroz, AMD ISV Relations

• Create a selection-sized offset image (4x4)

• Bind headOffset and dataStore images

• Bind counter buffer

• Initialize headOffset and counter

• Setup scissor

• DISABLE depth testing (we want EVERYTHING in the list)

• No framebuffer output

• Render

© Copyright Khronos Group, 2011 - Page 18

Examples – something new schoolImages – headOffset and dataStore

-1 -1 -1 -1

-1 -1 -1 -1

-1 -1 -1 -1

-1 -1 -1 -1

link id depthoffset

© Copyright Khronos Group, 2011 - Page 19

Examples – something new schoolImages – 1st triangle

-1 -1 -1 -1

-1 0 -1 -1

-1 -1 -1 -1

-1 -1 -1 -1

-1 id-red depth-red

link id depthoffset

© Copyright Khronos Group, 2011 - Page 20

Examples – something new schoolImages – 2nd triangle

-1 -1 -1 3

-1 0 4 2

-1 -1 -1 -1

-1 -1 1 -1

-1 id-red depth-red

-1 id-green depth-green

link id depthoffset

© Copyright Khronos Group, 2011 - Page 21

Examples – something new schoolImages – 3rd triangle

-1 -1 -1 3

-1 0 4 2

-1 -1 -1 -1

-1 -1 1 -1

-1 id-red depth-red

-1 id-green depth-green

-1 id-cyan depth-cyan

-1 id-cyan depth-cyan

-1 id-cyan depth-cyan

link id depthoffset

© Copyright Khronos Group, 2011 - Page 22

Examples – something new schoolImages – nth triangle

-1 id-red depth-red

-1 id-green depth-green

-1 id-cyan depth-cyan

-1 id-cyan depth-cyan

0 id-blue depth-blue

-1 id-cyan depth-cyan

link id depthoffset

-1 -1 -1 3

-1 4 2

-1 -1 -1 -1

-1 -1 1 -1

f-1

© Copyright Khronos Group, 2011 - Page 23

Examples – Something new school

layout (binding=0, offset=0) uniform atomic_uint fragmentCounter;

layout (r32ui) coherent uniform image2D restrict headOffset;

layout (rgba32ui) writeonly uimageBuffer restrict dataStore;

int2 coord(); // transform to coordinate system of headOffset buffer

uvec3 data(); // id, depth, whatever (color?)

void main()

{

uint NewOffset, PrevOffset;

NewOffset = atomicCounterIncrement(fragmentCounter);

PrevOffset = imageAtomicExchange(headOffset, coord(), NewOffset);

imageStore(dataStore, NewOffset, uvec4(PrevOffset, data()));

}

© Copyright Khronos Group, 2011 - Page 24

OpenGL Shading Language Hints/Kinks

Thank you!

http://blogs.amd.com/developer/