High-Quality Volume Graphics on Consumer PC Hardware Klaus Engel Joe Kniss Markus Hadwiger Christof...

High-Quality Volume Graphics on Consumer PC Hardware


KlausEngel

JoeKniss

MarkusHadwiger

ChristofRezk-Salama


Course 42 Joe M. Kniss

Human Tooth CT

(f)RGB(f)

f

RGB

Shading,Compositing…

Map data value f to color and opacity



Optical PropertiesOptical PropertiesColor (RGB)

Emissive term

1

0

)(r

r

dxxC

xxCn

i

i 0

)(

n

i

ic0

~~

=



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

Attenuates light based on density

0

1

)(expr

r

dxxA ~~

n

i

i xxxA0

)(exp

n

i

i xxxA0

)(exp=

=)( ixA

i e

n

i

i

0



Optical PropertiesOptical PropertiesTraditional Volume Rendering Equation

Emission & absorption

00

1

)(exp*)(r

s

r

r

dxxAdssC

N

i

N

ij

iic0 1

* ~~



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

new

old

sr

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

Eye

Image planer0

r1

Light



00

1

0

)(exp*)(exp*)(r

s

r

r

l

s

L dxxAdttAIdssC

ShadowsShadows

Eye

Image planer0

r1

Sample ri (s)

l0

N

i

N

ij

i

L

k

kLi Ic0 1

*

0

*

IL

~~



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

Eye

Slicing from light’s point of view



ShadowsShadows

Eye

Slicing from eye’s point of view



Incremental ShadowsIncremental Shadows

Eye

Half angle slicing: good from either

point of view




Similar aspect ratio from both points of

view




*

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

s

r

r

Id

00

1

)(exp*)()(*)(

0

)(exp)(l

s

Ld dttAIsI

)()(exp)(0

blurdttAIsIl

s

ILI

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

alpha

f(x,y,z)



Transfer functionTransfer functionProblem:

RGB = bad color space for humansNo concept of features

alpha

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…




Seconddirectionalderivative

measured withHessian

0

+

-

RGB ( , ,

)




0

+

-

RGB ( , ,

)

Done



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

1



ProbingInitial

Explore

12



Transfer function widgetManualDual-domain interaction

Initial

Explore

Specify

12

3



Use widgets to tune the transfer functionInitial

Explore

Specify

Refine

12

3

4



Understanding from immediate feedbackInitial

Explore

Specify

Refine

Interactive

12

3

4



ClassificationClassificationDiscrete featuresBasic functionsDirect manipulation widgets

V3 slider(s)

V2

V1



GuidanceGuidanceHistograms

Red

Green

Blu

e



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 Klaus Engel Joe Kniss Markus Hadwiger Christof...

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Transcript of High-Quality Volume Graphics on Consumer PC Hardware Klaus Engel Joe Kniss Markus Hadwiger Christof...