An Optimized Diffusion Depth of Field Solver Optimized Diffusion... · 2012-07-23 · References...
Transcript of An Optimized Diffusion Depth of Field Solver Optimized Diffusion... · 2012-07-23 · References...
An Optimized Diffusion Depth Of Field Solver (DDOF)
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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
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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
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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
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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
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DDOF Tri-diagonal system
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1 1 1 1
2 2 2 2 2
3 3 3 3 3
0
0 n n n n
b c y x
a b c y x
a b c y x
a b y x
• row/col of input image
• derived from CoC at each pixel of an input row/col
• resulting blurred row/col
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
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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
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Solver recap 3 • Cyclic Reduction (CR) solver
– Used by [Kass2006] in the original DDOF paper
– Runs in two phases
1. reduction phase
2. backward substitution phase
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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
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Passes of a Vanilla CR Solver
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Input image X
Pass 1: construct from CoC
abc
1 1 1 1
2 2 2 2 2
3 3 3 3 3
0
0 n n n n
b c y x
a b c y x
a b c y x
a b y x
reduce
reduce
reduce
reduce
…
…
Stop at size 1 Solve for the
first y
Y substitute substitute
…
Blurred image
Vanilla Solver Results
• Higher performance than reported in [Bavoil2010] (~6 ms vs. ~8ms at 1600x1200)
• Memory footprint prohibitively high
– >200 MB at 1600x1200
• Need an answer to tackling the lack of parallelism problem – answer given in [ZCO2010]
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Vanilla CR Solver
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Input image X
Pass 1: construct from CoC
abc
reduce
reduce
reduce
reduce
…
…
Stop at size 1 Solve for the
first y
Y substitute substitute
…
Blurred image
This is
what kills
parallelism
Keeping the parallelism high
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Input image X
Pass 1: construct from CoC
abc
reduce
reduce
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])
Y substitute substitute
…
Blurred image
Memory Optimizations 1
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Input image X
Pass 1: construct from CoC
abc
reduce
reduce
reduce
reduce
…
…
Stop at a
reasonable
size
Solve for Y at
that resolution
Y substitute substitute
…
Blurred image
Memory Optimizations 1
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rgab32f X
rgab32f abc
rgab32f
rgab32f
reduce
reduce
reduce
reduce
…
…
Stop at a
reasonable
size
Solve for Y at
that resolution
Y substitute substitute
…
rgba32f rgab32f
substi-
tute
Memory Optimizations 1
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rgab16f X
rgab32f abc
rgab16f
rgab32f
reduce
reduce
reduce
reduce
…
…
Stop at a
reasonable
size
Solve for Y at
that resolution
Y substitute substitute
…
rgba16f rgab16f
substi-
tute
This saves some significant
amount of memory - We found
no artifacts for going from
rgba32f to rgba16f
Memory Optimizations 2
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rgab16f X
rgab32f abc
rgab16f
rgab32f
reduce
reduce
reduce
reduce
…
…
Stop at a
reasonable
size
Solve for Y at
that resolution
Y substitute substitute
…
rgba16f rgab16f
substi-
tute
This does again save a
significant amount of
memory as this is the
biggest surface used
by the solver
Memory Optimizations 2
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rgab16f X
abc
rgab16f
rgab32f
reduce reduce
reduce
…
…
Stop at a
reasonable
size
Solve for Y at
that resolution
Y substitute substitute
…
rgba16f rgab16f
substi-
tute
Skip abc
construction pass
and compute abc
on-the-fly during 1.
reduction pass
Intermediate Results 1600x1200
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Solver
Time in ms
Memory in Megabytes
HD5870 GTX480
GDC2010 hybrid solver on GTX480
~8.5
8.00 [Bavoil2010]
~117 (guesstimate)
Standard Solver (already skips high res abc construction)
3.66
3.33
~132
Memory Optimizations 3
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rgab16f X
abc
rgab16f
rgab32f
reduce reduce
reduce
…
…
Stop at a
reasonable
size
Solve for Y at
that resolution
Y substitute substitute
…
rgba16f rgab16f
substi-
tute
Skip abc
construction
pass compute
abc during 1.
reduction pass
Yet again this saves a
significant amount of
memory !
Memory Optimizations 3
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rgab16f X
abc
reduce4
…
…
Stop at a
reasonable
size
Solve for Y at
that resolution
Y substitute substitute
…
rgba16f
substitute4
Skip abc
construction
pass compute
abc during 1.
reduction pass
Reduce 4-to-1
in a special first
reduction pass
Substitute
1-to-4 in a
special
substitution pass
Intermediate Results 1600x1200
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Solver
Time in ms
Memory in Megabytes
HD5870 GTX480
GDC2010 hybrid solver on GTX480
~8.5
8.00 [Bavoil2010]
~117 (guesstimate)
Standard Solver (already skips high res abc construction)
3.66
3.33
~132
4–to-1 Reduction
2.87
3.32
~73
DX11 Memory Optimizations 1
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rgab16f X
abc
reduce4
…
…
Stop at a
reasonable
size
Solve for Y at
that resolution
Y substitute substitute
…
rgba16f
substitute4
Skip abc
construction
pass compute
abc during 1.
reduction pass
Reduce 4-to-1
in a special first
reduction pass
Substitute
1-to-4 in a
special
substitution pass
DX11 Memory Optimizations 1
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rgab16f X
abc
reduce4
…
…
Stop at a
reasonable
size
Solve for Y at
that resolution
Y substitute substitute
…
rgba16f
substitute4
Skip abc
construction
pass compute
abc during 1.
reduction pass
Reduce 4-to-1
in a special first
reduction pass
Substitute
1-to-4 in a
special
substitution pass
Pack abc and X into
one rgba_uint surface
Using SM5 for data packing
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rgab16f X
rgab32f abc
uint
uint
uint
uint
pack x,y channel
(f32tof16(X.x) +
(f32tof16(X.y) << 16))
Using SM5 for data packing
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rgab16f X
rgab32f abc
uint
uint
uint
uint
higher 27 bits of x channel
(asuint(abc.x) &0xFFFFFFC0) |
(f32tof16(X.z) & 0x3F))
Steal 6 lowest mantissa bits of abc.x to store some bits of X.z
Using SM5 for data packing
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rgab16f X
rgab32f abc
uint
uint
uint
uint
higher 27 bits of y channel
(asuint(abc.y) &0xFFFFFFC0) |
((f32tof16(X.z) >>6 )& 0x3F))
Steal 6 lowest mantissa bits of abc.y to store some bits of X.z
SM5 Memory Optimizations 1
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rgab16f X
rgab32f abc
uint
uint
uint
uint
higher 27 bits of z channel
(asuint(abc.z) &0xFFFFFFC0) |
((f32tof16(X.z) >>12 )& 0x3F))
Steal 6 lowest mantissa bits of abc.z to store some bits of X.z
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?
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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
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Final Results 1600x1200
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Solver
Time in ms
Memory in Megabytes
HD5870 GTX480
GDC2010 hybrid solver on GTX480
~8.5
8.00 [Bavoil2010]
~117 (guesstimate,)
Standard Solver (already skips high res abc construction)
3.66
3.33
~132
4–to-1 Reduction
2.87
3.32
~73
4-to-1 Reduction + SM5 Packing
2.75
3.14
~58
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
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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 ([email protected]) if you want access to the sources
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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
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