1. Advanced D3D10 Rendering
Emil Persson
May 24, 2007

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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

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

10. 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

11. 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

12. 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

13. Rendering techniques in D3D10
May 24, 2007
Advanced D3D10 Rendering

14. Global Illumination
May 24, 2007
Advanced D3D10 Rendering

15. 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

16. 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

17. 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
May 24, 2007
Advanced D3D10 Rendering

18. 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

19. 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

20. Optimizations
May 24, 2007
Advanced D3D10 Rendering

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. Questions?
May 24, 2007
Advanced D3D10 Rendering