Volume Stylizer:Tomography-based Volume PaintingOliver Klehm1, Ivo Ihrke1,2, Hans-Peter Seidel1, Elmar Eisemann3

1 Max-Planck-Institut für Informatik2 Saarland University3 TU Delft

I3D 201323/03/2012

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Want to influence the appearance of volumes (voxels)• Editing on a fine-grained level is non-intuitive• No suitable tools available

o A Programmable System For Artistic Volumetric Lighting [Nowrouzezahrai2011]

Approach:• Inverse rendering: editing by painting

Motivation

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What do we modify• Volume parameters (per voxel)

o Emissiono Albedoo Extinction

Our Goal

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What do we modify• Volume parameters (per voxel)

From what do we edit the parameters• Arbitrary input images (target views)• In practice: rendered images that were painted

Our Goal

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Our Goal: Volume Painting

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Our Goal: Volume Painting

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What do we modify• Volume parameters (per voxel)

From what do we edit the parameters• Arbitrary input images (target views)• In practice: rendered images that were painted

Additional required information• Per image: camera parameters (viewing position, direction)• Lighting, scene

How do we optimize• Per-pixel difference to input image• Linear problem• Inconsistent input -> least-squares solution

Our Goal

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Given: each pixel k of target view as a constraint pixel Data associated:

ray origin, direction, target radiance

Try to reconstruct volume properties, such thatcomputed radiance matches target radiance(per pixel k)

Specific Goal

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A single pixel may influence many voxels And vice versa…

Problem overview

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Volume Rendering

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Represent the target field (e.g. ) as linear combination of basis functions• Common for stored volumes (== voxels: box/triangle basis fct.)

Reconstruct coefficient vector

, such that

Volume Reconstruction

Includes target Includes volume rendering

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Emission Reconstruction

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W is huge, ill-conditioned, …!=> iterative conjugate gradient method

We need to compute:• Objective Function:• Derivative:

The trick:

=> this is rendering / back-projection!

Optimization

=

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Pre-computation• Cache incoming light in volume per voxel• Sub-sample environment map, deep shadow maps

Optimization• 1. objective function evaluation

o Voxel traversal, gathering pre-cached incoming light

Implementation

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Pre-computation• Cache incoming light in volume per voxel• Sub-sample environment map, deep shadow maps

Optimization• 1. objective function evaluation• 2. gradient of objective function

Implementation

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Pre-computation• Cache incoming light in volume per voxel• Sub-sample environment map, deep shadow maps

Optimization• 1. objective function evaluation• 2. gradient of objective function• Additional vector arithmetic (addition, norm2, …)

Implementation

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Results - Video

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How to handle inconsistent inputs? How to define intermediate views? How to generalize? – requires different optimization

Optimize for volume and lighting

Limitations / Future Work

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3D volume properties by 2D painting Standard Rendering Model Fast GPU Optimization for interactive sessions

Conclusion

Thank you

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All OpenGL Vectors are actually 3D Textures Operations as shaders• Matrix multiplication is volume rendering• Arithmetic operations are “simple” shaders

=> Reconstruction in order of seconds

Fast Implementation

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Results

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Results

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Results - Extinction

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Light Editing• Lighting with paint [Pellacini2007]

o Non-linear light optimization• BendyLights [Kerr2010]

o Editing of spot lights• Optimizing Environment Maps for Material Depiction [Bousseau2011]

o Optimization with adhoc metrics Volume editing

• A Programmable System For Artistic Volumetric Lighting [Nowrouzezahrai2011]o Derivation of volume properties for single light beam onlyo Replacing physically-motivated parts of rendering with shaderso Homogeneous media onlyo Treat light beam as string of light (e.g. flow of particle)

• Automatically Adjusting Parameters for Rendering Clouds using Photographs [Dobashi2012]

Related Work

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Tomography• Ihrke• Wetzstein• Lanmann

Fabrication• Shadow Art [Pauly2009]• Manufactoring Layered Attenuators for Multiple Prescribed Shadow Images

[Baran2012]

Related Work

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Properties to reconstruct:• Extinction (overall density of volume)• Albedo (amount of scattering)• Emission (self glow)

Assumption: Single Scattering!• Light scatters ONCE in volume

Volume Properties

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Emission reconstruction

Background / first surface, attenuated by volume

Attenuation by volume

Integration along view ray

In-scattered light

Number of particles (extinction)