Ray Casting on Programmable Graphics Hardware

Martin KrausPURPL group, Purdue University

Martin KrausPURPL group, Purdue University

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Overview• Parallel volume rendering with a single GPU

• Implementing ray casting for a GPU

– Basics

– Optimizations

• Published systems

• Open questions

• Parallel volume rendering with a single GPU

• Implementing ray casting for a GPU

– Basics

– Optimizations

• Published systems

• Open questions

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Parallel Volume Rendering with a single Graphics Processing Unit (GPU) (1)

Even a single GPU is a parallel computer:• Multiple vertex and pixel pipelines

(e.g. “8 pixel pipelines” of QuadroFX and FireGL)

• APIs support this parallelism (independent processing of vertices and fragments)

• When implementing algorithms for a GPU, it’s crucial to exploit this parallelism

Even a single GPU is a parallel computer:• Multiple vertex and pixel pipelines

(e.g. “8 pixel pipelines” of QuadroFX and FireGL)

• APIs support this parallelism (independent processing of vertices and fragments)

• When implementing algorithms for a GPU, it’s crucial to exploit this parallelism

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Parallel Volume Rendering with a single Graphics Processing Unit (GPU) (2)

Many volume rendering algorithms cannot fully exploit these parallel pixel pipelines:• Cell projection: requires visibility sorting of cells

• Textured slices: rasterize all fragments of all slices

Many volume rendering algorithms cannot fully exploit these parallel pixel pipelines:• Cell projection: requires visibility sorting of cells

• Textured slices: rasterize all fragments of all slices

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Parallel Volume Rendering with a single Graphics Processing Unit (GPU) (3)

Ray casting can exploit parallel pixel pipes:• Like ray tracing, ray casting is embarrassingly parallel

Ray casting can exploit parallel pixel pipes:• Like ray tracing, ray casting is embarrassingly parallel

Volume DataEye Point

View RayPixel Sampling

Points

View Plane

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Parallel Volume Rendering with a single Graphics Processing Unit (GPU) (4)

More benefits of ray casting on a GPU:• Applicable to uniform and tetrahedral meshes

• Important optimizations can be implemented:

– early ray termination

– empty space leaping

– adaptive sampling distance

More benefits of ray casting on a GPU:• Applicable to uniform and tetrahedral meshes

• Important optimizations can be implemented:

– early ray termination

– empty space leaping

– adaptive sampling distance

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Implementing Ray Casting for a GPU: Basics (1)

Multi-pass approach: Render screen-filling rectangles to call a program for each pixelMulti-pass approach: Render screen-filling rectangles to call a program for each pixel

View Plane

Volume DataEye Point

Pixels

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Implementing Ray Casting for a GPU: Basics (2)

Initialize pixels with first intersection of the rays with the volume dataInitialize pixels with first intersection of the rays with the volume data

View Plane

Volume DataEye Point

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Implementing Ray Casting for a GPU: Basics (3a)

In each pass, propagate view rays and store accumulated color and sampling positionIn each pass, propagate view rays and store accumulated color and sampling position

View Plane

Volume DataEye Point

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Implementing Ray Casting for a GPU: Basics (3b)

In each pass, propagate view rays and store accumulated color and sampling positionIn each pass, propagate view rays and store accumulated color and sampling position

View Plane

Volume DataEye Point

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Implementing Ray Casting for a GPU: Basics (3c)

In each pass, propagate view rays and store accumulated color and sampling positionIn each pass, propagate view rays and store accumulated color and sampling position

View Plane

Volume DataEye Point

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Implementing Ray Casting for a GPU: Basics (4)

Stop when all rays have left the volumeStop when all rays have left the volume

View Plane

Volume DataEye Point

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Implementing Ray Casting for a GPU: Basics (5a)

Sampling of volume data:• Just a 3D-texture lookup for uniform data

• For tetrahedral cells: sample at cell boundaries

Sampling of volume data:• Just a 3D-texture lookup for uniform data

• For tetrahedral cells: sample at cell boundaries

View PlaneTetrahedral

Mesh

Eye Point

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Implementing Ray Casting for a GPU: Basics (5b)

Sampling of volume data:• Just a 3D-texture lookup for uniform data

• For tetrahedral cells: sample at cell boundaries

Sampling of volume data:• Just a 3D-texture lookup for uniform data

• For tetrahedral cells: sample at cell boundaries

View PlaneTetrahedral

Mesh

Eye Point

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Implementing Ray Casting for a GPU: Basics (5c)

Sampling of volume data:• Just a 3D-texture lookup for uniform data

• For tetrahedral cells: sample at cell boundaries

Sampling of volume data:• Just a 3D-texture lookup for uniform data

• For tetrahedral cells: sample at cell boundaries

View PlaneTetrahedral

Mesh

Eye Point

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Implementing Ray Casting for a GPU: Basics (5d)

Sampling of volume data:• Just a 3D-texture lookup for uniform data

• For tetrahedral cells: sample at cell boundaries

Sampling of volume data:• Just a 3D-texture lookup for uniform data

• For tetrahedral cells: sample at cell boundaries

View PlaneTetrahedral

Mesh

Eye Point

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Implementing Ray Casting for a GPU: Basics (5e)

Sampling of volume data:• Just a 3D-texture lookup for uniform data

• For tetrahedral cells: sample at cell boundaries

Sampling of volume data:• Just a 3D-texture lookup for uniform data

• For tetrahedral cells: sample at cell boundaries

View PlaneTetrahedral

Mesh

Eye Point

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Implementing Ray Casting for a GPU: Basics (5f)

Sampling of volume data:• Just a 3D-texture lookup for uniform meshes

• For tetrahedral cells: sample at cell boundaries

Sampling of volume data:• Just a 3D-texture lookup for uniform meshes

• For tetrahedral cells: sample at cell boundaries

View PlaneTetrahedral

Mesh

Eye Point

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Implementing Ray Casting for a GPU: Optimizations (1)

Overview:• Avoid work for rays that don’t intersect the volume

• Avoid work for rays that have accumulated full opacity

• Quickly test whether all rays have left the volume

• Quickly go through empty regions

• Adapt sampling distance to data

Overview:• Avoid work for rays that don’t intersect the volume

• Avoid work for rays that have accumulated full opacity

• Quickly test whether all rays have left the volume

• Quickly go through empty regions

• Adapt sampling distance to data

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Implementing Ray Casting for a GPU: Optimizations (2)

Avoid work for rays that don’t intersect or have left the volume:• Idea: avoid rasterization of corresponding pixels by

depth tests

• Advantage: exploits early-depth tests to avoid execution of fragment program

• Requires updates of the depth-buffer (not after each pass but from time to time)

Avoid work for rays that don’t intersect or have left the volume:• Idea: avoid rasterization of corresponding pixels by

depth tests

• Advantage: exploits early-depth tests to avoid execution of fragment program

• Requires updates of the depth-buffer (not after each pass but from time to time)

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Implementing Ray Casting for a GPU: Optimizations (3)

Avoid work for rays that have accumulated full opacity (early ray termination):• Idea: treat them as if they have left the volume

• Advantage: also exploits early-depth tests for these pixels

Avoid work for rays that have accumulated full opacity (early ray termination):• Idea: treat them as if they have left the volume

• Advantage: also exploits early-depth tests for these pixels

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Implementing Ray Casting for a GPU: Optimizations (4)

Quickly test whether all rays have left the volume:• Observation: once all rays have left the volume, all

fragments fail the depth test

• Just use an “occlusion query” to determine whether all fragments have failed the depth test!

Quickly test whether all rays have left the volume:• Observation: once all rays have left the volume, all

fragments fail the depth test

• Just use an “occlusion query” to determine whether all fragments have failed the depth test!

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Implementing Ray Casting for a GPU: Optimizations (5)

Quickly go through empty regions (empty space leaping):• Increase sampling distance in empty regions by

lookup in a 3D-texture that encodes empty regions

• Works only for uniform meshes

• Generalization: Adapt sampling distance to data

Quickly go through empty regions (empty space leaping):• Increase sampling distance in empty regions by

lookup in a 3D-texture that encodes empty regions

• Works only for uniform meshes

• Generalization: Adapt sampling distance to data

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Published Systems (1)Purcell, Buck, Mark, and Hanrahan. Ray Tracing on Programmable Graphics Hardware. SIGGRAPH 2002.• “Ray tracing” (not ray casting)• Discusses concepts, not an actual implementation• Features several similar techniques, e.g., for

– invoking fragment programs– ray termination

Purcell, Buck, Mark, and Hanrahan. Ray Tracing on Programmable Graphics Hardware. SIGGRAPH 2002.• “Ray tracing” (not ray casting)• Discusses concepts, not an actual implementation• Features several similar techniques, e.g., for

– invoking fragment programs– ray termination

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Published Systems (2)Röttger, Guthe, Weiskopf, and Ertl. Smart Hardware-Accelerated Volume Rendering. VisSym 2003.• Ray casting on a GPU for uniform meshes• Features:

– early ray termination– pre-integrated classification– adaptive sampling distance

Röttger, Guthe, Weiskopf, and Ertl. Smart Hardware-Accelerated Volume Rendering. VisSym 2003.• Ray casting on a GPU for uniform meshes• Features:

– early ray termination– pre-integrated classification– adaptive sampling distance

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Published Systems (3)Krüger and Westermann. Acceleration Techniques for GPU-based Volume Rendering. Visualization 2003 (Session P10).• Ray casting on a GPU for uniform meshes

• Features:– early ray termination– empty-space leaping– multiple sampling points per pass

Krüger and Westermann. Acceleration Techniques for GPU-based Volume Rendering. Visualization 2003 (Session P10).• Ray casting on a GPU for uniform meshes

• Features:– early ray termination– empty-space leaping– multiple sampling points per pass

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Published Systems (4)Weiler, Kraus, Merz, and Ertl. Hardware-Based Ray Casting for Tetrahedral Meshes. Visualization 2003 (Session P11).• Ray casting on a GPU for tetrahedral meshes

• Features:– early ray termination– pre-integrated classification– imaginary tetrahedra for non-convex meshes

Weiler, Kraus, Merz, and Ertl. Hardware-Based Ray Casting for Tetrahedral Meshes. Visualization 2003 (Session P11).• Ray casting on a GPU for tetrahedral meshes

• Features:– early ray termination– pre-integrated classification– imaginary tetrahedra for non-convex meshes

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Open QuestionsRay casting on GPUs is a rapidly developing method, thus there are many open question:• What about worst-case performances?

• What about limited texture memory?

• What about hierarchical methods?

• What about multiple GPUs?

Ray casting on GPUs is a rapidly developing method, thus there are many open question:• What about worst-case performances?

• What about limited texture memory?

• What about hierarchical methods?

• What about multiple GPUs?

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More Open Question?

Thanks!Thanks!

Ray Casting on Programmable Graphics HardwareOverviewParallel Volume Rendering with a single Graphics Processing Unit (GPU) (1)Parallel Volume Rendering with a single Graphics Processing Unit (GPU) (2)Parallel Volume Rendering with a single Graphics Processing Unit (GPU) (3)Parallel Volume Rendering with a single Graphics Processing Unit (GPU) (4)Implementing Ray Casting for a GPU: Basics (1)Implementing Ray Casting for a GPU: Basics (2)Implementing Ray Casting for a GPU: Basics (3a)Implementing Ray Casting for a GPU: Basics (3b)Implementing Ray Casting for a GPU: Basics (3c)Implementing Ray Casting for a GPU: Basics (4)Implementing Ray Casting for a GPU: Basics (5a)Implementing Ray Casting for a GPU: Basics (5b)Implementing Ray Casting for a GPU: Basics (5c)Implementing Ray Casting for a GPU: Basics (5d)Implementing Ray Casting for a GPU: Basics (5e)Implementing Ray Casting for a GPU: Basics (5f)Implementing Ray Casting for a GPU: Optimizations (1)Implementing Ray Casting for a GPU: Optimizations (2)Implementing Ray Casting for a GPU: Optimizations (3)Implementing Ray Casting for a GPU: Optimizations (4)Implementing Ray Casting for a GPU: Optimizations (5)Published Systems (1)Published Systems (2)Published Systems (3)Published Systems (4)Open QuestionsMore Open Question?