Interactive Ray Tracing of Pointbased Models

1
Interactive Ray Tracingof Point-based Models

• Ingo Wald
• MPI Informatik, Saarbrücken, Germany
• wald_at_mpi-inf.mpg.de

2
Motivation

• Why point-based rendering ?
• Many advantages, but no details here at PBG
• But Why point-based ray tracing ?
• Actually, two different questions
• Q1 Why ray tracing for point-based models ?
• Q2 Why use point-based techniques in ray tracing
  ?

3
Q1 Why Ray Tracing ?

• Reason 1 Logarithmic scalability to large models
• Very handy for todays huge models
• But For PBR not that important
  (multiresolution)
• Reason 2 Image quality / shading quality
• Ray traced image quality not yet standard in PBG
• But would benice to have

4
Q2 Why Point-based techniques in Ray Tracing?

• Native point-based data (acquisition)
• Direct ray tracing avoids tesselation
• Much simpler
• And for native triangular models ?
• Triangular model ? Point-based model ? PBRT
• Does this make sense ?
• Highly controversial issue amongst ray tracing
  researchers
• RT is log. in tris, anyway. Dont need
  multiresolution
• Many new problems with points as primitives
• Consistency, efficiency, loss of information

5
Q2 Why Point-based techniques in Ray Tracing?

• Interactive ray tracing Three recent examples

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Q2 Why Point-based techniques in Ray Tracing?

• Interactive ray tracing Three recent examples
• Example 1 Boeing 777 (350 million triangles, 40
  GB data)

7
Q2 Why Point-based techniques in Ray Tracing?

• Interactive ray tracing Three recent examples
• Example 2 Lawrence Livermore isosurface (8
  billion voxels)

8
Q2 Why Point-based techniques in Ray Tracing?

• Interactive ray tracing Three recent examples
• Example 3 Forest landscape (1.5 billion
  triangles)

9
Q2 Why Point-based techniques in Ray Tracing?

• Interactive ray tracing Three recent examples
• Yes, can render all three already today without
  PB techniques
• But Doesnt make sense (gt1G prim. for lt1M pixels
  ?)
• Too much data Tiny camera move?totally different
  data
• Even though image doesnt change at all
• Sample only every 1000th pixel ? Severe aliasing
• Aliasing THE most important problem of RTRT
  today
• Need multiresolution approach also for RT
• Probably best via PB approach
• Current results only intermediate step
• towards long-term goal of multiresolution
  (PB)RT

10
How to ray trace PB models ?

• Problem Zero ray-point hit probability
• Rays infinitely thin
• Points infitnitely thin, too
• Two possible solutions
• Use thick rays that have volume (cones, beams,
  )
• Reconstruct from all points overlapping rays
  volume
• Grow points to have a surface
• can then be found by thin ray
• Long history of debate on which is better

11
How to ray trace PB models ?

• Alternative 1 Growing rays
• Experience Tracing beams very costly (much more
  than rays)

12
How to ray trace PB models ?

• Alternative 1 Growing rays
• Many un-answered questions
• Where exactly is the hit point ?
• E.g., where to start the shadow ray from ?

points gathered by beam
Position A
Hitpoint position ?
13
How to ray trace PB models ?

• Alternative 1 Growing rays
• Many un-answered questions
• Where exactly is the hit point ?
• How stable is the decision ?

points gathered by beam
Position A
Hitpoint position ?
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How to ray trace PB models ?

• Alternative 1 Growing rays
• Many un-answered questions
• Where exactly is the hit point ?
• How stable is the decision ?
• Is it consistent ?
• E.g., when viewed from different angle (shadow
  ray)

Position B (Light ?)
surface seen from A
Position A
Same surface, different viewpoint Same hitpoint
? Is it in shadow ?
Hitpoint position ?
15
How to ray trace PB models ?

• Alternative 1 Growing rays
• Many un-answered questions
• Where exactly is the hit point ?
• How stable is the decision?
• Is it consistent ?
• Too many open questions

16
How to ray trace PB models ?

• Alternative 2 Growing points
• Define unique surface through points
• Always consistent
• Most trivial solution Use disks/ellipses/
• Trivial

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How to ray trace PB models ?

• Alternative 2 Growing points
• Define unique surface through points
• Always consistent
• Most trivial solution Use disks/ellipses/
• Trivial, but shading discontinuities (worse w/
  shadows)

18
How to ray trace PB models ?

• Better Define smooth surface
• Use Adamson/Alexas model
• Each point xi has normal ni and radius ri of
  influence
• Influence wi falls off with distance to xi
• With wi(xi)1 and wi(x)0 for all x-xigtri
• Define weighted average position X(x) and N(x)
• Define distance function F(x) ((x-X(x)).N(x)
• Implicit fct F(x)0 is smooth surface
• Need
• Useful ri s (not given)
• Fast ray/surface intersection framework

19
Choosing Splat Radii

• Step 1 Choosing the splat radii
• Too small ? Holes in the model
• Too large ? Many regions of influence will
  overlap (slow)
• Use Wu et al.s optimal subsampling technique
• Wu EG04 Determine ri such as to
• Optimally cover surface
• Achieve minimal overlap
• ?Optimal solution Use exactly that technique
• Note Can also use other radii
• e.g., user-specified
• Might just be slower or contain holes

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Intersecting the Surface

• Step 2 Determining the splats to intersect with
• Each point x always influenced by only few xi

C
B
D
A
21
Intersecting the Surface

• Step 2 Determining the splats to intersect with
• Enclose each splats ROI with tight fitting box

C
B
D
A
22
Intersecting the Surface

• Step 2 Determining the splats to intersect with
• Enclose each splats ROI with tight fitting box
• Build kd-tree over those AA boxes

C
B
D
A
23
Intersecting the Surface

• Step 2 Determining the splats to intersect with
• Enclose each splats ROI with tight fitting box
• Build kd-tree over those AA boxes

C
B
D
A
24
Intersecting the Surface

• Step 2 Determining the splats to intersect with
• Enclose each splats ROI with tight fitting box
• Build kd-tree over those AA boxes

C
B
D
A
25
Intersecting the Surface

• Step 2 Determining the splats to intersect with
• Enclose each splats ROI with tight fitting box
• Build kd-tree over those AA boxes

C
B
D
A
26
Intersecting the Surface

• Step 2 Determining the splats to intersect with
• Enclose each splats ROI with tight fitting box
• Build kd-tree over those AA boxes
• Leaves Store list of splats whose ROI overlaps
  cell

C
C
-
-
B
B
B,C
A,B
A
A,B,C
-
(right side incomplete)
D
A
A
-
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Intersecting the Surface

• Step 2 Determining the splats to intersect with
• Enclose each splats ROI with tight fitting box
• Build kd-tree over those AA boxes
• Leaves Store list of splats whose ROI overlaps
  cell
• KD-Tree Inherit advantages of fast triangular
  ray tracing
• Fast traversal, occlusion culling, good kd-trees
  (SAH)

C
C
-
-
B
B
B,C
A,B
A
A,B,C
-
D
A
A
-
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Intersecting the Surface

• Step 3 Ray/surface intersection in each voxel
• Find distance t F(t)0
• Equidistant sampling along t
• Have intersection if sgn(F(ti1)) ! sign(F(ti))
• But Too coarse sampling can miss the surface
• Optimizations Fast SSE implementation (no
  details)
• Quite fast, but Still main cost factor
• Further optimize kd-tree to minimize F(x) calls
• Need to take closer look at kd-tree

29
KD-Tree Post-optimizations

• Observation 1 Many full cells dont contain
  surf.
• Reason KD-tree was built over ROIs
• ROIs bound surface, but not closely

30
KD-Tree Post-optimizations

• Observation 1 Many full cells dont contain
  surf.
• Example from before

C
C
-
-
B
B
B,C
A,B
A
A,B,C
-
A,B,C
A
A
A,B,C
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KD-Tree Post-optimizations

• Observation 1 Many full cells dont contain
  surf.
• Actual surface only touches few voxels

C
C
-
-
B
B
B,C
A,B
A
A,B,C
-
A,B,C
A,B,C
A
A
A,B,C
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KD-Tree Post-optimizations

• Observation 1 Many full cells dont contain
  surf.
• Many non-empty cells that are not necessary

C
C
-
-
B
B
B,C
A,B
A
A,B,C
-
A,B,C
A,B,C
A
A
A,B,C
33
KD-Tree Post-optimizations

• Observation 1 Many full cells dont contain
  surf.
• Many non-empty cells that are not necessary
• Can make quite a difference

C
C
-
-
B
B
B,C
A,B
A
A,B,C
-
A,B,C
A,B,C
A
A
A,B,C
34
KD-Tree Post-optimizations

• Observation 1 Many full cells dont contain
  surf.
• Fix Determine min(F()) and max(F()) in each cell
• Cull cells with min gt 0 or max lt 0
• Currently Sample F with random x
• Can produce false culling (holes)
• Better Interval arithmetic (Not implemented yet)

35
KD-Tree Post-optimizations

• Observation 2 Most cells too large
• Reason Only split planes at ROI borders
• Example from earlier-on

C
C
-
-
B
B
B,C
A,B
A
A,B,C
-
A
A
36
KD-Tree Post-optimizations

• Observation 2 Most cells too large
• Reason Only split planes at ROI borders
• Example from earlier-on Green regions would
  suffice

C
C
-
-
B
B
B,C
A,B
A
A,B,C
-
A
A
A
37
KD-Tree Post-optimizations

• Observation 2 Most cells too large
• Reason Only split planes at ROI borders
• Example from earlier-on Green regions would
  suffice
• Fix Post-shrinking of cells
• Cut voxel into slices, cull slices
• Same problem as before False culling
• Important Not only fewer cell intersections
• Also Cells much thinner Finer sampling, fewer
  artifacts

38
KD-Tree Post-optimizations

• Observation 3 After shrink/cull, many cells
  empty
• Example from earlier-on Most cells empty after
  cull/shrink

C
-
-
B
B,C
A,B
A
A,B,C
-
A
39
KD-Tree Post-optimizations

• Observation 3 After shrink/cull, many cells
  empty
• Example from earlier-on Most cells empty after
  cull/shrink

C
B,C
A,B
A
40
KD-Tree Post-optimizations

• Observation 3 After shrink/cull, many cells
  empty
• Example from earlier-on Most cells empty after
  cull/shrink
• Optimization Post-collapse
• Undo splits that dont make sense any more

C
B,C
A,B
A
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Results

• Overall Framerate1Dual-Opteron 2.4GHz, 512x512

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Results

• Overall Framerate 5-15 fps _at_ 1PC
• Nice scalability to very complex models
• ManyIffis 24M points w/ shadows ? 2fps_at_1PC

43
Results

• Overall Framerate 5-15 fps _at_ 1PC
• Nice scalability to very complex models
• High-quality shading
• Global Illumination

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Results

• Overall Framerate 5-15 fps _at_ 1PC
• Nice scalability to very complex models
• High-quality shading
• Problems and limitations
• Sometimes small holes (false culling,
  undersampling, )
• All is static Cant move points, cant change
  radii
• Assumes continuous surface
• Not applicable to random point cloud (forest
  leaves)

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Summary Future Work

• Summary
• Motivated/Argued for Point-based Ray Tracing
• Discussed thin rays vs thick rays issue
• Outlined framework for Interactive PBRT
• Future work
• Performance Stability
• Bounding the surface, interval arithmetic,
• Multiresolution PBRT
• In particular, for massively complex models

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Questions ?
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Questions ?
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Q1 Why Ray Tracing ?

• Impact of Shadows and Reflections Example

49
Q2 Why Point-based techniques in Ray Tracing?

• Need multiresolution approach also for RT
• Trivial for nice (manifold) meshes (Progressive
  meshes,)
• But problematic for real-world models (Forest,
  Boeing, )
• Should be much easier for point-based
  representation
• Eventual goal of (our) PBRT activities
• Not ray tracing (nice) point-based models
• Rather Multiresolution ray tracing using a PB
  approach
• Reduce (in-core) data
• Fight aliasing
• Current activities only first step in that
  direction

50
AdamsonAlexas model

• Observations
• Need position and normal for each point ? assume
  as given
• Need radius of influence for each point
• For each x, most wi(x) will be zero
• Fast computation Need to know which...
• Evaluating F(x) will be costly
• Need to minimize F(x) calls
• Need to quickly determine where surface can NOT
  be
• Need to minimize number of points contributing to
  x
• ? Need to minimize regions of influence (minimize
  ri !)

51
So Why Interactive Point-based Ray Tracing ?

• Advantages of PBG in particular for ray tracers
• Native point-based data (acquisition)
• Avoid tesselation sure, makes sense.
• And for native triangular models ?
• Triangular model ? Point-based model ? PBRT
• Does this make sense ?

52
So Why Interactive Point-based Ray Tracing ?

• Advantages of PBG in particular for ray tracers
• ? Highly controversial issue amongst ray tracers
• Ray tracing is logarithmic in scene size, anyway
• Dont need point-based multiresolution
  capabilities
• Triangles very well suited for fast ray tracing
• How to (efficiently) intersect points ?
• Triangles well-defined surface
• How to consistently intersect points ?
• In particular for secondary rays
• shadow ray has to see exactly the same surface
• Sampling Loss of information ?
• Many new problems, PBG advantages over rast.
  dont apply
• Where are the advantages ?

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Q1 Why Ray Tracing

• Impact of Shadows and Reflections Example

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MotivationWhy Point-based Rendering ?

• Point-based models become commonplace
• Acquisition Almost all models are point clouds
• Direct PBR simplifies 3D processing pipeline
• Today Acquisition?triangulation?rendering
• Direct PBR Get rid of (non-trivial)
  triangulation
• Plus Increasing use in rendering/modelling
• Multiresolution, no connectivity/topology, much
  simpler,

55
MotivationWhy Point-based Rendering ?

• Point-based models become commonplace
• Acquisition Almost all models are point clouds
• Direct PBR simplifies 3D processing pipeline
• Today Acquisition?triangulation?rendering
• Direct PBR Get rid of (non-trivial)
  triangulation
• Plus Increasing use in rendering/modelling
• Multiresolution, no connectivity/topology, much
  simpler,
• But Why point-based ray tracing ?
• Actually, two different questions
• Q1 Why ray tracing for point-based models ?
• Q2 Why point-based approach to ray tracing ?

56
Outline

• Motivation
• Why point based ray tracing
• Our approach
• Thin rays vs thick rays
• Fast traversal and intersection
• Results
• Summary and Future Work

57
How to ray trace PB models ?

• Alternative 1 Growing rays
• Many un-answered questions
• Where exactly is the hit point ?
• How sensitive is the procedure to exact form of
  beam ?
• E.g., what if the beam were traced a little bit
  further ?

points gathered by beam
Position A
Hitpoint position ?
Same surface ?