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An Optimized Diffusion Depth Of Field Solver (DDOF)

An Optimized Diffusion Depth Of Field Solver (DDOF). Holger Gruen – AMD. Agenda. Motivation Recap of a high-level explanation of DDOF Recap of earlier DDOF solvers A Vanilla Cyclic Reduction(CR) DDOF solver A DX11 optimized CR solver for DDOF Results. Motivation.

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An Optimized Diffusion Depth Of Field Solver (DDOF)

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  1. An Optimized Diffusion Depth Of Field Solver (DDOF) Holger Gruen – AMD AMD‘s Favorite Effects

  2. Agenda • Motivation • Recap of a high-level explanation of DDOF • Recap of earlier DDOF solvers • A Vanilla Cyclic Reduction(CR) DDOF solver • A DX11 optimized CR solver for DDOF • Results AMD‘s Favorite Effects

  3. Motivation • Solver presented at GDC 2010 [RS2010] has some weaknesses • Great implementation but memory reqs and runtime too high for many game developers • Looking for faster and memory efficient solver AMD‘s Favorite Effects

  4. Diffusion DOF recap 1 • DDOF is an enhanced way of blurring a picture taking an arbitrary CoC at a pixel into account • Interprets input image as a heat distribution • Uses the CoC at a pixel to derive a per pixel heat conductivity CoC=Circle of Confusion AMD‘s Favorite Effects

  5. Diffusion DOF recap 2 • Blurring is done by time stepping a differential equation that models the diffusion of heat • ADI method used to arrive at a separable solution for stepping • Need to solve tri-diagonal linear system for each row and then each colum of the input AMD‘s Favorite Effects

  6. DDOF Tri-diagonal system • row/colofinputimage • derivedfromCoCateachpixelof aninputrow/col • resultingblurredrow/col AMD‘s Favorite Effects

  7. Solver recap 1 • The GDC2010 solver [RS2010] is a ‚hybrid‘ solver • Performs three PCR steps upfront • Performs serial ‚Sweep‘ algorithm to solve small resulting systems • Check [ZCO2010]for details on other hybrid solvers AMD‘s Favorite Effects

  8. Solver recap 2 • The GDC2010 solver [RS2010] has drawbacks • It uses a large UAV as a RW scratch-pad to store the modified coefficients of the sweep algorithm • GPUs without RW cache will suffer • For high resolutions three PCR steps produce tri-diagonal system of substantial size • This means a serial (sweep) algorithm is run on a ‚big‘ system AMD‘s Favorite Effects

  9. Solver recap 3 • Cyclic Reduction (CR) solver • Used by [Kass2006] in the original DDOF paper • Runs in two phases • reduction phase • backward substitution phase AMD‘s Favorite Effects

  10. Solver recap 4 • According to [ZCO2010]: • CR solver has lowest computational complexity of all solvers  • It suffers from lack of parallelism though  • At the end of the reduction phase • At the start of the backwards substitution phase AMD‘s Favorite Effects

  11. Passes of a Vanilla CR Solver Input image … X reduce reduce Solve for the first y Stop at size 1 Pass 1: constructfromCoC … abc reduce reduce Blurredimage Y … substitute substitute AMD‘s Favorite Effects

  12. Vanilla Solver Results • Higher performancethanreported in [Bavoil2010]  (~6 ms vs. ~8ms at 1600x1200) • Memory footprintprohibitivelyhigh • >200 MB at 1600x1200 • Need an answertotacklingthe lack ofparallelismproblem – answergiven in [ZCO2010] AMD‘s Favorite Effects

  13. Vanilla CR Solver Input image … X reduce reduce Solve for the first y This is what kills parallelism Stop at size 1 Pass 1: constructfromCoC … abc reduce reduce Blurredimage Y … substitute substitute AMD‘s Favorite Effects

  14. Keeping the parallelism high Input image … X reduce reduce Stop at a reasonable size Solve for Y at that resolution to have a big enough parallel workload (e.g using PCR see [ZCO2010]) Pass 1: constructfromCoC … abc reduce reduce Blurred image Y … substitute substitute AMD‘s Favorite Effects

  15. Memory Optimizations 1 Input image … X reduce reduce Stop at a reasonable size Solve for Y at that resolution Pass 1: constructfromCoC … abc reduce reduce Blurred image Y … substitute substitute AMD‘s Favorite Effects

  16. Memory Optimizations 1 rgab32f rgab32f … X reduce reduce Stop at a reasonable size Solve for Y at that resolution rgab32f rgab32f … abc reduce reduce rgab32f rgba32f … Y substitute substitute substi-tute AMD‘s Favorite Effects

  17. Memory Optimizations 1 rgab16f rgab16f … X reduce reduce Stop at a reasonable size Solve for Y at that resolution Thissavessomesignificantamountofmemory - Wefoundnoartifactsforgoingfrom rgba32f to rgba16f rgab32f rgab32f … abc reduce reduce rgab16f rgba16f … Y substitute substitute substi-tute AMD‘s Favorite Effects

  18. Memory Optimizations 2 rgab16f rgab16f … X reduce reduce Stop at a reasonable size Solve for Y at that resolution Thisdoesagain save a significantamountofmemoryasthisisthebiggestsurfaceusedbythesolver rgab32f rgab32f … abc reduce reduce rgab16f rgba16f … Y substitute substitute substi-tute AMD‘s Favorite Effects

  19. Memory Optimizations 2 rgab16f rgab16f … X reduce reduce Stop at a reasonable size Solve for Y at that resolution Skip abc construction pass and compute abc on-the-fly during 1. reduction pass rgab32f … abc reduce rgab16f rgba16f … Y substitute substitute substi-tute AMD‘s Favorite Effects

  20. Intermediate Results 1600x1200 AMD‘s Favorite Effects

  21. Memory Optimizations 3 rgab16f rgab16f … X reduce reduce Stop at a reasonable size Solve for Y at that resolution Yetagainthissaves a significantamountofmemory ! Skip abc construction pass compute abc during 1. reduction pass rgab32f … abc reduce rgab16f rgba16f … Y substitute substitute substi-tute AMD‘s Favorite Effects

  22. Memory Optimizations 3 rgab16f … X reduce4 Stop at a reasonable size Solve for Y at that resolution Reduce 4-to-1 in a special first reduction pass Skip abc construction pass compute abc during 1. reduction pass … abc Substitute 1-to-4 in a special substitution pass rgba16f … Y substitute substitute substitute4 AMD‘s Favorite Effects

  23. Intermediate Results 1600x1200 AMD‘s Favorite Effects

  24. DX11 Memory Optimizations 1 rgab16f … X reduce4 Stop at a reasonable size Solve for Y at that resolution Reduce 4-to-1 in a special first reduction pass Skip abc construction pass compute abc during 1. reduction pass … abc Substitute 1-to-4 in a special substitution pass rgba16f … Y substitute substitute substitute4 AMD‘s Favorite Effects

  25. DX11 Memory Optimizations 1 Pack abc and X into one rgba_uint surface rgab16f … X reduce4 Stop at a reasonable size Solve for Y at that resolution Reduce 4-to-1 in a special first reduction pass Skip abc construction pass compute abc during 1. reduction pass … abc Substitute 1-to-4 in a special substitution pass rgba16f … Y substitute substitute substitute4 AMD‘s Favorite Effects

  26. Using SM5 for data packing uint rgab16f pack x,y channel X uint (f32tof16(X.x) + (f32tof16(X.y) << 16)) rgab32f uint abc uint AMD‘s Favorite Effects

  27. Using SM5 for data packing uint rgab16f X uint lower 5 bits of z channel pack rgab32f uint higher 27 bits of x channel abc uint (asuint(abc.x) &0xFFFFFFC0) | (f32tof16(X.z) & 0x3F)) Steal 6 lowest mantissa bits of abc.x to store some bits of X.z AMD‘s Favorite Effects

  28. Using SM5 for data packing uint rgab16f X uint central 5 bits of z channel pack rgab32f uint higher 27 bits of y channel abc uint (asuint(abc.y) &0xFFFFFFC0) | ((f32tof16(X.z) >>6 )& 0x3F)) Steal 6 lowest mantissa bits of abc.y to store some bits of X.z AMD‘s Favorite Effects

  29. SM5 Memory Optimizations 1 uint rgab16f X uint higher 5 bits of z channel rgab32f uint higher 27 bits of z channel pack abc uint (asuint(abc.z) &0xFFFFFFC0) | ((f32tof16(X.z) >>12 )& 0x3F)) Steal 6 lowest mantissa bits of abc.z to store some bits of X.z AMD‘s Favorite Effects

  30. Sample Screenshot AMD‘s Favorite Effects

  31. Abs(Packed-Unpacked) x 255.0f AMD‘s Favorite Effects

  32. DX11 Memory Optimizations 2 • Solver does a horizonal and vertical pass • Chain of lower res RTs needs to be there twice • Horizontal reduction/substitution chain • Vertical reduction/substitution chain • How can DX11 help? AMD‘s Favorite Effects

  33. DX11 Memory Optimizations 2 • UAVs allow us to reuse data of the horizontal chain for the vertical chain • A proof of concept implementation shows that this works nicely but impacts the runtime significantly • ~40% lower fps • Stayed with RTs as memory was already quite low • Use only if you are really concerned about memory AMD‘s Favorite Effects

  34. Final Results 1600x1200 AMD‘s Favorite Effects

  35. Future Work • Look into CS acceleration of the solver • 4-to-1 reduction pass • 1-to-4 substitution pass • Look into using heat diffusion for other effects • e.g. Motion blur AMD‘s Favorite Effects

  36. Conclusion • Optimized CR solver is fast and mem-efficient • Used in Dragon Age 2 • 4aGames considering its use for new projects • Detailed description in ‚Game Engine Gems 2‘ • Mail me (holger.gruen@amd.com) if you want access to the sources AMD‘s Favorite Effects

  37. References • [Kass2006] “Interactive depth of field using simulated diffusion on a GPU” Michael Kass, Pixar Animation studios, Pixar technical memo #06-01 • [ZCO2010] “Fast Tridiagonal Solvers on the GPU” Y. Zhang, J. Cohen, J. D. Owens, PPoPP 2010 • [RS2010] “DX11 Effects in Metro 2033: The Last Refuge” A. Rege, O. Shishkovtsov, GDC 2010 • [Bavoil2010] „Modern Real-Time Rendering Techniques“, L. Bavoil, FGO2010 AMD‘s Favorite Effects

  38. Backup AMD‘s Favorite Effects

  39. Results 1920x1200 AMD‘s Favorite Effects

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