Vertex Shader Tricks New Ways to Use the Vertex Shader to Improve Performance Bill Bilodeau...
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Transcript of Vertex Shader Tricks New Ways to Use the Vertex Shader to Improve Performance Bill Bilodeau...
Vertex Shader Tricks
New Ways to Use the Vertex Shader to Improve Performance
Bill BilodeauDeveloper Technology Engineer, AMD
Topics Covered
●Overview of the DX11 front-end pipeline●Common bottlenecks●Advanced Vertex Shader Features●Vertex Shader Techniques●Samples and Results
Graphics Hardware
DX11 Front-End Pipeline●VS –vertex data●HS – control points●Tessellator●DS – generated vertices●GS – primitives●Write to UAV at all stages
● Starting with DX11.1
Vector GPR’s(256 2048-bit registers)
Vector ALU(1 64-way single precision operation every 4 clocks)
Scalar ALU(1 operation every 4 clocks)
Scalar GPR’s(256 64-bit registers)
Vector/Scalar cross communication bus
Vector GPR’s(256 2048-bit registers)
Vector ALU(1 64-way single precision operation every 4 clocks)
Scalar ALU(1 operation every 4 clocks)
Scalar GPR’s(256 64-bit registers)
Vector/Scalar cross communication bus
Vector GPR’s(256 2048-bit registers)
Vector ALU(1 64-way single precision operation every 4 clocks)
Scalar ALU(1 operation every 4 clocks)
Scalar GPR’s(256 64-bit registers)
Vector/Scalar cross communication bus
.
.
.
Input Assembler
Hull Shader
Domain Shader
Tessellator
Geometry Shader
Stream Out
CB,SRV,or
UAV
Vertex Shader
Bottlenecks - VS
●VS Attributes● Limit outputs to 4 attributes (AMD)
●This applies to all shader stages (except PS)
●VS Texture Fetches● Too many texture fetches can add latency
●Especially dependent texture fetches●Group fetches together for better performance●Hide latency with ALU instructions
Bottlenecks - VS
●Use the caches wisely● Avoid large vertex formats
that waste pre-VS cache space
● DrawIndexed() allows for reuse of processed vertices saved in the post-VS cache
●Vertices with the same index only need to get processed once
Vertex Shader
Pre-VS Cache(Hides Latency)
Input Assembler
Post-VS Cache(Vertex Reuse)
Bottlenecks - GS
●GS ● Can add or remove primitives● Adding new primitives requires storing new
vertices●Going off chip to store data can be a bandwidth issue
● Using the GS means another shader stage●This means more competition for shader resources●Better if you can do everything in the VS
Advanced Vertex Shader Features
●SV_VertexID, SV_InstanceID●UAV output (DX11.1)●NULL vertex buffer
● VS can create its own vertex data
SV_VertexID
●Can use the vertex id to decide what vertex data to fetch●Fetch from SRV, or procedurally create a vertex
VSOut VertexShader(SV_VertexID id){
float3 vertex = g_VertexBuffer[id];…
}
UAV buffers
●Write to UAVs from a Vertex Shader● New feature in DX11.1 (UAV at any stage)
●Can be used instead of stream-out for writing vertex data
● Triangle output not limited to strips ●You can use whatever format you want
●Can output anything useful to a UAV
NULL Vertex Buffer
●DX11/DX10 allows this● Just set the number of vertices in Draw() ● VS will execute without a vertex buffer bound
●Can be used for instancing● Call Draw() with the total number of vertices● Bind mesh and instance data as SRVs
Vertex Shader Techniques
●Full Screen Triangle●Vertex Shader Instancing
● Merged Instancing●Vertex Shader UAVs
Full Screen Triangle
●For post-processing effects● Triangle has better performance
than quad●Fast and easy with VS generated coordinates
● No IB or VB is necessary●Something you should be using for full screen effects
Clip Space Coordinates
(-1, -1, 0)
(-1, 3, 0)
(3, -1, 0)
Full Screen Triangle: C++ code// Null VB, IBpd3dImmediateContext->IASetVertexBuffers( 0, 0, NULL, NULL, NULL );pd3dImmediateContext->IASetIndexBuffer( NULL, (DXGI_FORMAT)0, 0 );pd3dImmediateContext->IASetInputLayout( NULL );
// Set Shaders pd3dImmediateContext->VSSetShader( g_pFullScreenVS, NULL, 0 );pd3dImmediateContext->PSSetShader( … );pd3dImmediateContext->PSSetShaderResources( … );
pd3dImmediateContext->IASetPrimitiveTopology( D3D11_PRIMITIVE_TOPOLOGY_TRIANGLELIST );
// Render 3 vertices for the trianglepd3dImmediateContext->Draw(3, 0);
Full Screen Triangle: HLSL CodeVSOutput VSFullScreenTest(uint id:SV_VERTEXID){
VSOutput output;
// generate clip space positionoutput.pos.x = (float)(id / 2) * 4.0 - 1.0;output.pos.y = (float)(id % 2) * 4.0 - 1.0;output.pos.z = 0.0;output.pos.w = 1.0;
// texture coordinatesoutput.tex.x = (float)(id / 2) * 2.0;output.tex.y = 1.0 - (float)(id % 2) * 2.0;
// coloroutput.color = float4(1, 1, 1, 1);
return output;}
Clip Space Coordinates
(-1, -1, 0)
(-1, 3, 0)
(3, -1, 0)
VS Instancing: Point Sprites
●Often done on GS, but can be faster on VS● Create an SRV point buffer and bind to VS● Call Draw or DrawIndexed to render the full
triangle list. ● Read the location from the point buffer and
expand to vertex location in quad● Can be used for particles or Bokeh DOF sprites● Don’t use DrawInstanced for a small mesh
Point Sprites: C++ Code
pd3d->IASetIndexBuffer( g_pParticleIndexBuffer, DXGI_FORMAT_R32_UINT, 0 );
pd3d->IASetPrimitiveTopology( D3D11_PRIMITIVE_TOPOLOGY_TRIANGLELIST );
pd3dImmediateContext->DrawIndexed( g_particleCount * 6, 0, 0);
Point Sprites: HLSL CodeVSInstancedParticleDrawOut VSIndexBuffer(uint id:SV_VERTEXID){ VSInstancedParticleDrawOut output;
uint particleIndex = id / 4;uint vertexInQuad = id % 4; // calculate the position of the vertexfloat3 position;position.x = (vertexInQuad % 2) ? 1.0 : -1.0;position.y = (vertexInQuad & 2) ? -1.0 : 1.0;position.z = 0.0;position.xy *= PARTICLE_RADIUS;
position = mul( position, (float3x3)g_mInvView ) + g_bufPosColor[particleIndex].pos.xyz; output.pos = mul( float4(position,1.0), g_mWorldViewProj ); output.color = g_bufPosColor[particleIndex].color;
// texture coordinateoutput.tex.x = (vertexInQuad % 2) ? 1.0 : 0.0;output.tex.y = (vertexInQuad & 2) ? 1.0 : 0.0;
return output;}
Point Sprite Performance
Indexed, 500K Sprites
Non-Indexed, 500K Sprites
GS, 500K Sprites
DrawInstanced, 500K Sprites
Indexed, 1M Sprites
Non-Indexed, 1M Sprites
GS, 1M Sprites DrawInstanced, 1M Sprites
R9 290x (ms)
0.52 0.77 1.38 1.77 1.02 1.53 2.7 3.54
Titan (ms) 0.52 0.87 0.83 5.1 1.5 1.92 1.6 10.3
1
3
5
7
9
11
AMD Radeon R9 290x
Nvidia Titan
Point Sprite Performance
●DrawIndexed() is the fastest method●Draw() is slower but doesn’t need an IB●Don’t use DrawInstanced() for creating sprites on either AMD or NVidia hardware
● Not recommended for a small number of vertices
Merge Instancing
●Combine multiple meshes that can be instanced many times
● Better than normal instancing which renders only one mesh
● Instance nearby meshes for smaller bounding box
●Each mesh is a page in the vertex data● Fixed vertex count for each mesh
●Meshes smaller than page size use degenerate triangles
Merge Instancing
Mesh Vertex Data
Mesh Data 0
Mesh Data 1
Mesh Data 2
.
.
.
Mesh Instance Data
Instance 0
Mesh Index 2
Instance 1
Mesh Index 0
.
.
.Degenerate
Triangle
Vertex 0Vertex 1Vertex 2Vertex 3
.
.
.000
Fixed Length Page
Merged Instancing using VS
●Use the vertex ID to look up the mesh to instance
● All meshes are the same size, so (id / SIZE) can be used as an offset to the mesh
● Faster than using DrawInstanced()
Merge Instancing Performance
DrawInstanced Soft Instancing0
5
10
15
20
25
30
R9 290xGTX 780
●Instancing performance test by Cloud Imperium Games for Star Citizen●Renders 13.5M triangles (~40M verts)●DrawInstanced version calls DrawInstanced() and uses instance data in a vertex buffer●Soft Instancing version uses vertex instancing with Draw() calls and fetches instance data from SRV
AMD Radeon R9 290X
Nvidia GTX 780
ms
Vertex Shader UAVs
●Random access Read/Write in a VS●Can be used to store transformed vertex data for use in multi-pass algorithms●Can be used for passing constant attributes between any shader stage (not just from VS)
Skinning to UAV
●Skin vertex data then output to UAV● Instance the skinned UAV data multiple times
●Can also be used for non-instanced data● Multiple passes can reuse the transformed
vertex data – Shadow map rendering●Performance is about the same as stream-out, but you can do more …
Bounding Box to UAV
●Can calculate and store Bbox in the VS● Use a UAV to store the min/max values (6)● InterlockedMin/InterlockedMax determine min
and max of the bbox●Need to use integer values with atomics
●Use the stored bbox in later passes● GPU physics (collision)● Tile based processing
Bounding Box: HLSL Codevoid UAVBBoxSkinVS(VSSkinnedIn input, uint id:SV_VERTEXID ){
// skin the vertex. . .// output the max and min for the bounding boxint x = (int) (vSkinned.Pos.x * FLOAT_SCALE); // convert to integerint y = (int) (vSkinned.Pos.y * FLOAT_SCALE);int z = (int) (vSkinned.Pos.z * FLOAT_SCALE);
InterlockedMin(g_BBoxUAV[0], x);InterlockedMin(g_BBoxUAV[1], y);InterlockedMin(g_BBoxUAV[2], z);InterlockedMax(g_BBoxUAV[3], x);InterlockedMax(g_BBoxUAV[4], y);InterlockedMax(g_BBoxUAV[5], z);. . .
Particle System UAV
●Single pass GPU-only particle system●In the VS:
● Generate sprites for rendering● Do Euler integration and update the particle
system state to a UAV
Particle System: HLSL Codeuint particleIndex = id / 4;uint vertexInQuad = id % 4;
// calculate the new position of the vertexfloat3 oldPosition = g_bufPosColor[particleIndex].pos.xyz;float3 oldVelocity = g_bufPosColor[particleIndex].velocity.xyz;
// Euler integration to find new position and velocityfloat3 acceleration = normalize(oldVelocity) * ACCELLERATION;float3 newVelocity = acceleration * g_deltaT + oldVelocity;float3 newPosition = newVelocity * g_deltaT + oldPosition;g_particleUAV[particleIndex].pos = float4(newPosition, 1.0);g_particleUAV[particleIndex].velocity = float4(newVelocity, 0.0);
// Generate sprite vertices. . .
Conclusion
●Vertex shader “tricks” can be more efficient than more commonly used methods
● Use SV_Vertex ID for smarter instancing●Sprites●Merge Instancing
● UAVs add lots of freedom to vertex shaders●Bounding box calculation●Single pass VS particle system
Demos
●Particle System●UAV Skinning
● Bbox
Acknowledgements
●Merge Instancing● Emil Person, “Graphics Gems for Games”
SIGGRAPH 2011● Brendan Jackson, Cloud Imperium
●Thanks to● Nick Thibieroz, AMD● Raul Aguaviva (particle system UAV), AMD● Alex Kharlamov, AMD
Questions