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Vertex Shader Tricks New Ways to Use the Vertex Shader to Improve Performance Bill Bilodeau Developer Technology Engineer, AMD.

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Presentation on theme: "Vertex Shader Tricks New Ways to Use the Vertex Shader to Improve Performance Bill Bilodeau Developer Technology Engineer, AMD."— Presentation transcript:

1 Vertex Shader Tricks New Ways to Use the Vertex Shader to Improve Performance Bill Bilodeau Developer Technology Engineer, AMD

2 Topics Covered ● Overview of the DX11 front-end pipeline ● Common bottlenecks ● Advanced Vertex Shader Features ● Vertex Shader Techniques ● Samples and Results

3 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 DX Input Assembler Hull Shader Domain Shader Tessellator Geometry Shader Stream Out CB, SRV, or UAV Vertex Shader

4 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

5 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)

6 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

7 Advanced Vertex Shader Features ● SV_VertexID, SV_InstanceID ● UAV output (DX11.1) ● NULL vertex buffer ● VS can create its own vertex data

8 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]; … }

9 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

10 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

11 Vertex Shader Techniques ● Full Screen Triangle ● Vertex Shader Instancing ● Merged Instancing ● Vertex Shader UAVs

12 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)

13 Full Screen Triangle: C++ code // Null VB, IB pd3dImmediateContext->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 triangle pd3dImmediateContext->Draw(3, 0);

14 Full Screen Triangle: HLSL Code VSOutput VSFullScreenTest(uint id:SV_VERTEXID) { VSOutput output; // generate clip space position output.pos.x = (float)(id / 2) * ; output.pos.y = (float)(id % 2) * ; output.pos.z = 0.0; output.pos.w = 1.0; // texture coordinates output.tex.x = (float)(id / 2) * 2.0; output.tex.y = (float)(id % 2) * 2.0; // color output.color = float4(1, 1, 1, 1); return output; } Clip Space Coordinates (-1, -1, 0) (-1, 3, 0) (3, -1, 0)

15 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

16 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);

17 Point Sprites: HLSL Code VSInstancedParticleDrawOut VSIndexBuffer(uint id:SV_VERTEXID) { VSInstancedParticleDrawOut output; uint particleIndex = id / 4; uint vertexInQuad = id % 4; // calculate the position of the vertex float3 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 coordinate output.tex.x = (vertexInQuad % 2) ? 1.0 : 0.0; output.tex.y = (vertexInQuad & 2) ? 1.0 : 0.0; return output; }

18 Point Sprite Performance AMD Radeon R9 290x Nvidia Titan

19 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

20 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

21 Merge Instancing Mesh Vertex Data Mesh Data 0 Mesh Data 1 Mesh Data Mesh Instance Data Instance 0 Mesh Index 2 Instance 1 Mesh Index Degenerate Triangle Vertex 0 Vertex 1 Vertex 2 Vertex 3. 0 Fixed Length Page

22 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()

23 Merge Instancing Performance ● 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

24 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)

25 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 …

26 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

27 Bounding Box: HLSL Code void UAVBBoxSkinVS(VSSkinnedIn input, uint id:SV_VERTEXID ) { // skin the vertex... // output the max and min for the bounding box int x = (int) (vSkinned.Pos.x * FLOAT_SCALE); // convert to integer int 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);...

28 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

29 Particle System: HLSL Code uint particleIndex = id / 4; uint vertexInQuad = id % 4; // calculate the new position of the vertex float3 oldPosition = g_bufPosColor[particleIndex].pos.xyz; float3 oldVelocity = g_bufPosColor[particleIndex].velocity.xyz; // Euler integration to find new position and velocity float3 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...

30 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

31 Demos ● Particle System ● UAV Skinning ● Bbox

32 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

33 Questions ●


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