Advanced D3D10 Rendering Emil Persson May 24, 2007

Overview Introduction to D3D10 Rendering techniques in D3D10 Optimizations May 24, 2007 Advanced D3D10 Rendering

Introduction Best D3D revision yet!  Clean and powerful API Lots of new features SM 4.0 New geometry shader Stream Out Texture arrays Render to volume texture MSAA individual sample access Constant buffers Sampler state decoupled from texture unit Dual-source blending Etc… May 24, 2007 Advanced D3D10 Rendering

Clean API Vista only Everything is mandatory (almost) No legacy hardware support Clean starting point for future evolution of the API Limited market short-term Some old features deprecated Fixed function Assembly shaders Alpha test Triangle fans Point sprites Clip planes May 24, 2007 Advanced D3D10 Rendering

Dealing with deprecated features Fixed function Write a few über-shaders Assembly shaders Convert to HLSL Alpha test Use discard or clip() in pixel shader Use alpha-to-coverage Triangle fans Seldom used anyway, usually just for a quad Convert to triangle list or strip Point sprites Expand point to 2 triangles in GS Clip planes Use clip distance and/or cull distance May 24, 2007 Advanced D3D10 Rendering

SM 4.0 Geometry shader Processes a full primitive (point, line, triangle) Has access to adjacency information (optional) Useful for silhouette detection, shadow volume extrusion etc. May output multiple primitives Output limitation is 1024 floats May output nothing (to kill primitive) May 24, 2007 Advanced D3D10 Rendering

SM 4.0 Infinite instruction count Integer and bitwise instruction Very long shaders may have lower throughput though Integer and bitwise instruction Indexable temporaries Allows for local arrays May be used to emulate a stack Useful system generated values SV_VertexID SV_PrimitiveID SV_InstanceID SV_Position (Like VPOS, but now .zw are defined too) SV_IsFrontFace (Like VFACE) SV_RenderTargetArrayIndex SV_ViewportArrayIndex SV_ClipDistance SV_CullDistance May 24, 2007 Advanced D3D10 Rendering

SM 4.0 Integer & bitwise instructions Signed and unsigned No idiv though, just udiv Same registers as floats Can alias without conversion with asint(), asuint(), asfloat() etc. Integer texture sample values Syntax: Texture2D <uint4> myTex; Access to individual samples in MSAA surface Allows for custom AA resolve Syntax: Texture2DMS <float4, 4> myTex; May 24, 2007 Advanced D3D10 Rendering

Pixel center Half pixel offset is gone!  DX10 DX9 Affects SV_Position as well Now matches OpenGL DX10 DX9 May 24, 2007 Advanced D3D10 Rendering

Pixel center Pixels and texels align Texel center Screenspace TexCoord = SV_Position.xy / float2(width, height) Texel center Screenspace May 24, 2007 Advanced D3D10 Rendering

The small batch problem D3D10 designed to minimize batch overhead Pulls work from draw time to creation time Validation Shader input/output configuration Immutable State Objects Input layout Rasterizer state Sampler state Depth stencil state Blend state May 24, 2007 Advanced D3D10 Rendering

The small batch problem D3D10 also provides tools to reduce draw calls Improved instancing interface Geometry shader More shader resources Constant indexing in PS Render target arrays Texture arrays May 24, 2007 Advanced D3D10 Rendering

Rendering techniques in D3D10 May 24, 2007 Advanced D3D10 Rendering

Global Illumination May 24, 2007 Advanced D3D10 Rendering

Global Illumination Probes on a volume grid across the scene Each probe captures light environment into a tiny “cubemap” Probes are converted to Spherical Harmonics coefficients Indirect lighting is computed using interpolated SH coefficients Do the same in probe passes to get multiple light bounces May 24, 2007 Advanced D3D10 Rendering

Global Illumination Awful lot of work Solution Each probe is 6 slices. We need loads of probes. Sample scene has over 300 probes Solution Use geometry shader to reduce work Distribute work across multiple frames Sample updates 40 cubes per frame Scatter updates to hide artifacts Skip over “empty” space probes May 24, 2007 Advanced D3D10 Rendering

Global Illumination The Geometry Shader advantage 40 cubes x 6 faces x n draw calls = Pain DX9 style unrealistic even for simple scenes Update multiple slices per pass with GS GS output limit is 1024 floats Keep number of interpolators down to maximize primitive count Managed to update 5 probes (30 slices) per pass 8 passes is more manageable than 240 ... May 24, 2007 Advanced D3D10 Rendering

Post tone-mapping resolve D3D10 allows for custom AA resolves Can drastically improve HDR AA quality Standard resolve occurs before tone-mapping Ideally resolve should be done after tone-mapping Standard resolve Custom resolve May 24, 2007 Advanced D3D10 Rendering

Post-tonemapping resolve Texture2DMS<float4, SAMPLES> tHDR; float4 main(float4 pos: SV_Position) : SV_Target { int3 coord; coord.xy = (int2) pos.xy; coord.z = 0; // Tone-map individual samples and sum it up float4 sum = 0; [unroll] for (int i = 0; i < SAMPLES; i++) float4 c = tHDR.Load(coord, i); sum.rgb += 1.0 – exp2(-exposure * c.rgb); } // Average sum *= (1.0 / SAMPLES); // sRGB sum.rgb = pow(sum.rgb, 1.0 / 2.2); return sum; May 24, 2007 Advanced D3D10 Rendering

Optimizations May 24, 2007 Advanced D3D10 Rendering

Geometry shader GS optimizations Input/output usually the bottleneck Reduce outputs with frustum and/or backface culling Keep input small by packing data TexCoord could be 2x16 bits in an uint Or use for instance asuint(normal.w) Merge to full float4 vectors Don’t do 2x float2 Keep output small Could be faster to trade for some work in PS Pass just position, don’t interpolate both lightVec and viewVec Or even back-project SV_Position.xyz to world space in PS Small output means more work fits within 1024 floats limit May 24, 2007 Advanced D3D10 Rendering

GS frustum and backface culling // Transform to clip space float4 pos[3]; pos[0] = mul(mvp, In[0].pos); pos[1] = mul(mvp, In[1].pos); pos[2] = mul(mvp, In[2].pos); // Use frustum culling to improve performance float4 t0 = saturate(pos[0].xyxy * float4(-1, -1, 1, 1) - pos[0].w); float4 t1 = saturate(pos[1].xyxy * float4(-1, -1, 1, 1) - pos[1].w); float4 t2 = saturate(pos[2].xyxy * float4(-1, -1, 1, 1) - pos[2].w); float4 t = t0 * t1 * t2; [branch] if (!any(t)) { // Use backface culling to improve performance float2 d0 = pos[1].xy * pos[0].w - pos[0].xy * pos[1].w; float2 d1 = pos[2].xy * pos[0].w - pos[0].xy * pos[2].w; if (d1.x * d0.y > d0.x * d1.y || min(min(pos[0].w, pos[1].w), pos[2].w) < 0.0) // Output primitive here ... } May 24, 2007 Advanced D3D10 Rendering

Miscellaneous optimizations Pre-baked constant buffers Don’t update per-material constants in DX9 style PS don’t need to return float4 anymore Use float3 if you only care about RGB May reduce instruction count Use GS to reduce draw calls Single pass render-to-cubemap Update multiple render targets per pass May 24, 2007 Advanced D3D10 Rendering

The new shader compiler SM4 shader compiler preserves semantics better This means more responsibility for you guys Be careful about your assumptions Periodically check the resulting assembly D3D10DisassembleShader() Use GPUShaderAnalyzer for performance critical shaders May 24, 2007 Advanced D3D10 Rendering

The new shader compiler Example: HLSL code: float4 main(float4 t: TEXCOORD0) : SV_Target { if (t.x > t.y) return t.xyzw; else return t.wzyx; } DX9 assembly: add r0.x, -v0.x, v0.y cmp oC0, r0.x, v0.wzyx, v0 DX10 assembly: lt r0.x, v0.y, v0.x if_nz r0.x // <--- Did you really want a branch here? mov o0.xyzw, v0.xyzw ret else mov o0.xyzw, v0.wzyx endif May 24, 2007 Advanced D3D10 Rendering

The new shader compiler Use [branch], [flatten], [unroll] & [loop] to control output code This is not for everyone Poor use could reduce performance Make sure you know what you’re doing Only use if you’re familiar with assembly code Verify that you get the code you expect Always benchmark both options New DX10 assembly (using [flatten]): lt r0.x, v0.y, v0.x movc o0.xyzw, r0.xxxx, v0.xyzw, v0.wzyx ret May 24, 2007 Advanced D3D10 Rendering

Questions? emil.persson@amd.com May 24, 2007 Advanced D3D10 Rendering