High-Quality Volume Graphics on Consumer PC Hardware

KlausEngel

JoeKniss

MarkusHadwiger

ChristofRezk-Salama

Course 42 Joe M. Kniss

Human Tooth CT

(f)RGB(f)

Shading,Compositing…

Map data value f to color and opacity

Optical PropertiesOptical PropertiesColor (RGB)

Emissive term

Optical PropertiesOptical PropertiesAlpha (a.k.a. opacity or extinction)

Attenuates light based on density

)(expr

dxxA ~~

i xxxA0

)(exp=

=)( ixA

Optical PropertiesOptical PropertiesTraditional Volume Rendering Equation

Emission & absorption

)(exp*)(r

dxxAdssC

iic0 1

Optical PropertiesOptical PropertiesScale alpha values based on sample rate (sr)

oldnew )1(1

1.3 .7

Transfer FunctionTransfer FunctionTransform scalar data value into optical propertiesUse optical properties to solve integralVery easy to implement

Data is a textureTransfer function is a lookupCompute Riemann sum using “over operator”

},,,{)),,(( BGRzyxfT

Transfer FunctionTransfer FunctionPre-classification Post-classification

Transfer function evaluated before interpolation, i.e.

interpolation of colors not data

Transfer function evaluated after interpolation, i.e. data is

interpolated first

ImplementationImplementation

Two choices:Texture Lookup Table

glColorTable*Varies depending on hardware, may require a special texture data formatFast

Dependent Texture ReadVery hardware dependentBecoming more general, see OpenGl 2.0Slower but more flexible…

Transfer FunctionTransfer FunctionProblem:

No shape or depth queuesNo shading.

f(x,y,z)

alphacolor

Transfer FunctionTransfer FunctionSolution: Faux Shading

Ramp color to black with alphaSilhouette edges

f(x,y,z)

alphacolor

Transfer FunctionTransfer FunctionBetter: Surface Shading

Slower, requires normalMore flexible

Transfer FunctionTransfer FunctionBetter: Surface Shading

Slower, requires normalMore flexibleFaux shading enhances

Transfer FunctionTransfer FunctionProblem: Can’t surface shade homogeneous regions

Need good gradientsSensitive to noise

Transfer FunctionTransfer FunctionSolution: Surface scalar (s)

Only shade high gradient magnitudes:

Or, add s to TFInterpolate…

2||)||1(1 fs

cscshadingsCout )1()(

Transfer FunctionTransfer FunctionProblem: How did the light get there?

No attenuation through volumeNot realistic!

Transfer FunctionTransfer FunctionSolution: Shadows

Better depth queuesDramatic effects

ShadowsShadows

Image planer0

)(exp*)(exp*)(r

L dxxAdttAIdssC

ShadowsShadows

Image planer0

Sample ri (s)

kLi Ic0 1

ShadowsShadowsImplementation:

2 passesAttenuate from the light sourceRender from the eye

Store light attenuation in second volumeMultiply color by attenuation from shadow volume

ShadowsShadowsDisadvantages:

Difficult to build shadow volume on the cardSlow to build off the cardAdditional volume requiredAttenuation leakageBlurry shadow boundariesLow resolution shadows!

ShadowsShadowsAlternative: Incremental shadows

Generate shadows one slice at a timeOnly use a 2D bufferImage space shadow computationAll on cardHalf angle slicing

ShadowsShadows

Slicing from light’s point of view

ShadowsShadows

Slicing from eye’s point of view

Incremental ShadowsIncremental Shadows

Half angle slicing: good from either

point of view

Similar aspect ratio from both points of

Slice pass 1

Slice pass 2

Incremental ShadowsIncremental ShadowsAdvantages:

Screen space shadowsNo leakageUse render to texture to optimizeShades perturbed volumesSimple implementation

Disadvantages:Aliasing at sharp opacity changesFix with slightly larger light buffer

ShadowsShadowsProblem: Shadows still too dark

Direct attenuation is inadequateNeed to handle higher order light transport effects

Shadows

ShadowsShadowsSolution: Translucency

One consequence of light scatteringSmoke, clouds, skin, wax….

Shadows Translucent

TranslucencyTranslucencyWax:

ShadowsReal

TranslucencyTranslucencyWax:

ShadowsReal Translucent

TranslucencyTranslucencyAdd indirect attenuation

dsdxxAsIsIsCr

)(exp*)()(*)(

)(exp)(l

Ld dttAIsI

)()(exp)(0

blurdttAIsIl

Direct attenuation, same as shadows

Blurred indirect attenuation, includes an indirect alpha

TranslucencyTranslucency

Direct (Id) and indirect (Ii) attenuation

TranslucencyTranslucencyHow?

Same as shadows (two light buffers)Sample previous light buffer multiple times for blurPing-pong blendingStore indirect in a color componentSum direct and indirect in fragment shader for the eye passOnly use direct attn. for eye pass

TranslucencyTranslucencyProblem: Still doesn’t look right

ShadowsReal Translucent

TranslucencyTranslucencySolution: add spectral attenuation

Real Translucent w/spectral attn.

TranslucencyTranslucencySpectral attenuation:

Attenuate some colors more than othersSpectral indirect attn. is simplestNeed separate alpha for RGBStore in RGB components of light buffer

TranslucencyTranslucencyIndirect alpha vs. transport color

Alpha:

Transport: =1-alpha

Transport color is easier to specify

Optical PropertiesOptical PropertiesRecap:

Reflective color/Emission (RGB)Direct attenuation/alpha (A)Surface scalar (s)Indirect attenuation (Ar,Ag,Ab)

Others?Scattering, absorption, phase function,

density, emission, index of refraction…

Transfer functionTransfer functionSpecification

Simple, easyExpressiveGuided

Transfer functionTransfer functionTypical: 1D linear ramps

Independent R,G,B,A controlDifficultTrial and error

f(x,y,z)

Transfer functionTransfer functionProblem:

RGB = bad color space for humansNo concept of features

f(x,y,z)

Transfer functionTransfer functionBetter: set color at control points

Use HSV or HLS color spacesSimplified interfaceStill no guidancealpha

f(x,y,z)

Transfer functionTransfer functionGuided techniques:

Design GalleriesThumbnailsSemi-AutomaticDual-domain interaction

Design GalleriesDesign GalleriesTreat TF and rendering as a high dimensional parameter spaceStochastically sample spaceCluster images based on fitnessSelect best looking image

Design GalleriesDesign Galleries

Design GalleriesDesign GalleriesComputationally expensiveDifficult to implementNot guided by dataset specificsOnly handles 1D transfer functions

ThumbnailsThumbnailsVisual history of changesShow important regions of TFShow effects of potential changes

ThumbnailsThumbnailsspreadsheets

ThumbnailsThumbnails

Parameterization

Semi-automaticSemi-automaticVolumetric edge detectionUse first and second derivativesBuild histogram volumesUse simplified interfaceGenerate 1D or 2D TFs

Semi-automaticSemi-automatic

Position of boundary center

Semi-automaticSemi-automatic

Semi-automaticSemi-automaticIdentifies data ranges of boundariesStill requires hand-editingNot really an interactive processDemonstrates the value of histograms

Transfer functionTransfer functionWhat is a 2D transfer function

Multiple values per sample point2D lookup tableEach value is an axis of TFBetter specificityMore complicated to use

Transfer functionTransfer function

( )RGB( )

Generalize…

( , )RGB( , )

Modify…

( , )RGB( , )

Modify…

Seconddirectionalderivative

measured withHessian

RGB ( , ,

Transfer functionTransfer functionMulti-dimensional TFs

Data value, gradient magnitude, second derivativeMultivariate, i.e. multiple values Implement as dependent texture read, 1D, 2D, 3D texture for TF

Multiple scans Color data Multiple variables

MRI-T1,T2,PD Cryosection Simulation

Transfer functionTransfer functionProblem: How do we interact with a higher dimensional TF?

Larger parameter spaceUnintuitive feature identificationGreater demands on user interface

Transfer functionTransfer functionSolution: It is a process!!

Data set guidanceImproved classificationIntuitive feature identification

“Default” transfer functionInitial

ProbingInitial

Explore

Transfer function widgetManualDual-domain interaction

Initial

Explore

Specify

Use widgets to tune the transfer functionInitial

Explore

Specify

Refine

Understanding from immediate feedbackInitial

Explore

Specify

Refine

Interactive

ClassificationClassificationDiscrete featuresBasic functionsDirect manipulation widgets

V3 slider(s)

GuidanceGuidanceHistograms

GuidanceGuidanceProbing

Identify featuresin transfer function

Chapel Hill CT

GuidanceGuidanceDual-domain interaction

Classify features by pointing at them

RefinementRefinementManipulate well defined control points

Transfer functionTransfer functionAdvantages:

Better feature discrimination

TechniqueTechniqueAnimated transfer function

High-Quality Volume Graphics on Consumer PC Hardware

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