interactive ray tracing of point-based models

11Interactive Point Based Ray TracingInteractive Point Based Ray TracingJune 21, 2005June 21, 2005

Interactive Ray Tracingof Point-based ModelsInteractive Ray Tracingof Point-based Models

Ingo Wald SCI Institute, University of Utah

wald@sci.utah.edu

MotivationMotivation

• Why point-based rendering ?– Many advantages, but no details here …

• 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 ?

Q1: Why Ray Tracing ?Q1: Why Ray Tracing ?

• Reason 1: Logarithmic scalability to large models– Very handy for today‘s 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

Q2: Why Point-based techniques in Ray Tracing? 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. Don‘t need multiresolution…

• Many new problems with points as primitives

– Consistency, efficiency, loss of information…

Q2: Why Point-based techniques in Ray Tracing? Q2: Why Point-based techniques in Ray Tracing? Interactive ray tracing: Three recent examples

– Example 1: Boeing 777 (350 million triangles, 40 GB data)

– Example 2: Lawrence Livermore isosurface (8 billion voxels)

– Example 3: Forest landscape (1.5 billion triangles)

– Yes, can render all three already today without PB techniques

– But: Doesn‘t make sense (>1G prim. for <1M pixels ?)• Too much data: Tiny camera movetotally different data

– Even though image doesn‘t 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

How to ray trace PB models ?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 ray‘s volume

– „Grow“ points to have a surface • can then be found by „thin“ ray

– Long history of debate on which is better…

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

• Alternative 1: Growing rays– Many un-answered questions

• Where exactly is the hit point ?

– E.g., where to start the shadow ray from ?

Hitpoint Hitpoint position ?position ?

Position APosition Apoints gathered points gathered

by beamby beam

• How stable is the decision ?

by beamby beam

• How stable is the decision ?

• Is it consistent ?

– E.g., when viewed from different angle (shadow ray)

Same surface, different Same surface, different viewpoint: viewpoint:

Same hitpoint ?Same hitpoint ?

Is it in shadow ?Is it in shadow ?

surface seen from Asurface seen from APosition APosition APosition B Position B (Light ?)(Light ?)

• How stable is the decision?

• Is it consistent ?

Too many open questions

• Alternative 2: Growing points– Define unique surface through points

• Always consistent

– Most trivial solution: Use disks/ellipses/…• Trivial

• Alternative 2: Growing points– Define unique surface through points

• Always consistent

– Most trivial solution: Use disks/ellipses/…• Trivial, but shading discontinuities (worse w/ shadows)

• Better: Define smooth surface– Use Adamson/Alexa‘s 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-xi|>ri

• 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

Choosing Splat RadiiChoosing 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

Intersecting the SurfaceIntersecting the Surface

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

• Step 2: Determining the splats to intersect with• Enclose each splat‘s ROI with tight fitting box

• Build kd-tree over those AA boxes

• Leaves: Store list of splats whose ROI overlaps cell

DDA,BA,B

AA A,B,CA,B,C

B,CB,C

(right side (right side incomplete)incomplete)

• 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)…

DDA,BA,B

AA A,B,CA,B,C

B,CB,C

• Step 3: Ray/surface intersection in each voxel– Find distance t : F(t)=0

– Equidistant sampling along t

• Have intersection if sgn(F(ti+1)) != sign(F(ti))

• But: Too coarse sampling can miss the surface

– Optimizations: Fast SSE implementation (no details)

• Quite fast, but: Still main cost factorFurther optimize kd-tree to minimize F(x) calls

– Need to take closer look at kd-tree

KD-Tree Post-optimizationsKD-Tree Post-optimizations

• Observation 1: Many full cells don‘t contain surf.– Reason: KD-tree was built over ROI‘s

• ROI‘s bound surface, but not closely

• Observation 1: Many full cells don‘t contain surf.– Example from before

A,BA,B

AA A,B,CA,B,C

B,CB,C

A,B,CA,B,CA,B,CA,B,C

• Observation 1: Many full cells don‘t contain surf.– Actual surface only touches few voxels

A,BA,B

AA A,B,CA,B,C

B,CB,C

-- A,B,CA,B,C

• Observation 1: Many full cells don‘t contain surf. – Many non-empty cells that are not necessary

A,BA,B

AA A,B,CA,B,C

B,CB,C

-- A,B,CA,B,C

• Observation 1: Many full cells don‘t contain surf. – Many non-empty cells that are not necessary

• Can make quite a difference

A,BA,B

AA A,B,CA,B,C

B,CB,C

-- A,B,CA,B,C

• Observation 1: Many full cells don‘t contain surf.

• Fix: Determine min(F()) and max(F()) in each cellCull cells with min > 0 or max < 0

– Currently: Sample F with random xCan produce false culling (holes)

– Better: Interval arithmetic (Not implemented yet)

• Observation 2: Most cells too large– Reason: Only split planes at ROI borders

– Example from earlier-on:

A,BA,B

AA A,B,CA,B,C

B,CB,C

– Example from earlier-on: Green regions would suffice…

A,BA,B

AA A,B,CA,B,C

B,CB,C

– 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

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

A,BA,B

AA A,B,CA,B,C

B,CB,C

A,BA,B

B,CB,C

– Optimization: Post-collapse• Undo splits that don‘t make sense any more

A,BA,B

B,CB,C

ResultsResults

• Overall Framerate:1Dual-Opteron 2.4GHz, 512x512

Model Size Diffuse Phong w/Shadows

Iphigenia Head 1M 6.8 Not measured

Iphigenia Full 1M 15.9 Not measured

Octopus 465K 8.9 7.5

Dragon 1.3M 7.4 7.1

David 1.5M 11.2 7.9

ResultsResults

• Overall Framerate: ~5-15 fps @ 1PC

• Nice scalability to very complex models– „ManyIffis“: 24M points w/ shadows ~2fps@1PC

ResultsResults

• Nice scalability to very complex models

• High-quality shading– Global Illumination

ResultsResults

• Nice scalability to very complex models

• High-quality shading

• Problems and limitations:– Sometimes small holes (false culling, undersampling, …)

– All is static: Can‘t move points, can‘t change radii

– Assumes continuous surface: • Not applicable to random point cloud (forest leaves…)

Summary & Future WorkSummary & 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

Questions ?Questions ?

Q1: Why Ray Tracing ?Q1: Why Ray Tracing ?

• Impact of Shadows and Reflections: Example

Q2: Why Point-based techniques in Ray Tracing? 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…

Adamson&Alexa‘s modelAdamson&Alexa‘s 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 !)

So: Why Interactive Point-based Ray Tracing ? 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 ?

So: Why Interactive Point-based Ray Tracing ? 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• Don‘t 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. don‘t apply…Where are the advantages ?

Q1: Why Ray Tracing Q1: Why Ray Tracing

• Impact of Shadows and Reflections: Example

Motivation:Why Point-based Rendering ?Motivation:Why Point-based Rendering ?• Point-based models become commonplace

– Acquisition: Almost all models are point clouds• Direct PBR simplifies 3D processing pipeline

– Today: Acquisitiontriangulationrendering

– Direct PBR: Get rid of (non-trivial) triangulation

– Plus: Increasing use in rendering/modelling• Multiresolution, no connectivity/topology, much simpler,…

Motivation:Why Point-based Rendering ?Motivation:Why Point-based Rendering ?• Point-based models become commonplace

– Acquisition: Almost all models are point clouds• Direct PBR simplifies 3D processing pipeline

– Today: Acquisitiontriangulationrendering

– 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 ?

OutlineOutline

• Motivation– Why point based ray tracing

• Our approach– Thin rays vs thick rays

– Fast traversal and intersection

• Results

• Summary and Future Work

• How sensitive is the procedure to exact form of beam ?

– E.g., what if the beam were traced a little bit further ?

by beamby beam

Same surface ?Same surface ?

interactive ray tracing of point-based models

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