REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

Real-Time Volume Graphics

[05] Transfer Functions

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

Classification

During Classification the user defines the „Look“ of the data.

Which parts are transparent? Which parts have which color?

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

Classification

During Classification the user defines the „Look“ of the data.

Which parts are transparent? Which parts have which color?

The user defines a Transfer function.

Emission RGB

Absorption Ascalar S Transfe

r Functio

n

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

Classification

Real-Time update of the transfer function necessary!!!

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

Classification

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

Pre- vs Post-Interpolative Classificationo

pti

cal p

rop

erti

es

data value

inte

rpo

lati

on

PRE-INTERPOLATIVE

op

tica

l pro

per

ties

data value

interpolation

POST-INTERPOLATIVE

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

Pre-Classification

GeometryProcessing

Rasterization

FragmentOperations

TransferFunction

A color value is fetched from a tablefor each Voxel

A RGBA Value is determined for each Voxel

Pre-Classification: Pre-Classification:Color table is applied before interpolation.

(pre-interpolative Transfer function)

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

Possible Implementations

The best approach: Paletted TexturesStore the scalar volume together with the color table directly in graphics memory.Hardware-Support necessary!Low memory consumption

Main Memory (scalar volume can be deleted!)Graphics Memory (scalar volume + TF)

Low load on memory busScalar volume must be transferred only once!

Only the color table must be re-uploaded on TF change

Main Memory Graphics Memory

Scalar Textureskalar value S

Texture paletteA

GP /

PC

IeA

GP /

PC

Ie

Transfer function

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

Summary Pre-ClassificationApplication of the Transfer function before RasterizationOne RGBA Lookup for each VoxelDifferent Implementations:

Texture Transfer Texture Color Tables (paletted textures)

Simple and EfficientGood for coloring segmented data

Pre-Classification Summary

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

Post-Classification

GeometryProcessing

Rasterization

FragmentOperations

TransferFunction

A color is fetched from the color table for each Fragment

Post-Classification: Post-Classification:The color table is applied after Interpolation

(post-interpolative Transfer function).

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

Post-ClassificationTexture 0 = Scalar

field

Texture 1 = Transferfunction [Emission RGB, Absorption A]

R=G=B=A=Scalar field S

R

RGBA

= T(S)Polygon

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

CG Implementation//fragment program for post-classification //using 3D texturesfloat4 main (float3 texUV : TEXCOORD0, uniform sampler3D volume_texture, uniform sampler1D transfer_function) :

COLOR{ float index = tex3D(volume_texture, texUV); float4 result = tex1D(transfer_function, index); return result;}

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

Comparison of image quality

Post-ClassificationPre-Classification

Same TF, same Resolution, same Sampling Rate

Quality: Pre- vs. Post-Classification

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

Pre-Classification Post-Classification

Quality

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

Post-interpolative TF

Classified data

SupersamplingTransfer Function

Supersampling

Transfer Function

Analytical Solution Pre-interpolative TF

Transfer Function

Continuous data Discrete data

Scalar value

alph

a va

lue

Pre- vs Post-Classification

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

Pre-Integrated Classification

REAL-TIME VOLUME GRAPHICSKlaus EngelSiemens AG, Erlangen, Germany Eurographics 2006

When to use which Classification

Pre-Interpolative ClassificationIf the graphics hardware does not support fragment shadersFor simple segmented volume data visualization

Post-Interpolative ClassificationIf the transfer function is “smooth”For good quality and good performance (especially when slicing)