RealTime Volume Graphics 07 Global Volume Illumination REALTIME

Real-Time Volume Graphics [07] Global Volume Illumination REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Why Global Illumination Local illumination might sufficient for many application areas in scientific visualization (e. g medicine) Not sufficient for visual arts/photorealism! Appearance of many common objects is dominated by scattering effects Smoke, clouds Wax, skin, translucent materials REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Surface vs Volume Illumination Surface Lighting Light transport in vacuum Lighting calculation is performed at surface points Reflectivity from BRDF Volume Lighting Light transport in participating medium Lighing Calculation at every point Scattering from phase function REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Surface Illumination The incoming radiance at a point from direction , will partially be reflected into direction To obtain the radiance at , we must account for all possible incoming directions: REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Phase Functions For surfaces, the BRDF describes the probability of light being reflected from one direction on the hemisphere into another direction. For volumes, the phase function describes the probability of light being scattered from direction into direction REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Increase Physical Model of Radiative Transfer Decrease true emission true absorption REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany in-scattering out-scattering Eurographics 2006

Scattering In-scattering Outscattering coefficient Phase function Incoming radiance Volume Rendering Equation: REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Volume Illumination Up until now: External light is not attenuated Now: Attenuation of light as it travels through the volume Single scattering, with no attenuation. Light is reaches attenuated everyalong pointist way unimpededly through the volume (Volumetric shadows) Light is scattered once Light before is itscattered reaches once the eye before it reaches the eye Not physically plausible REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Shadows Maps For Surfaces: REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Shadow Maps For Volume Rendering: Update image and shadow buffer slice-by-slice Need proxy geometry, that can be rendered from two different views. Half Angle Slicing REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Light Source Attenuation 1. Draw slice into light buffer 2. Draw slice into image buffer 3. Proceed with next slice REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Light Source Attenuation Example: Visible Human CT Head Direct Light + Attenuation REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Volume Illumination Up until now: External light is attenuated by volume Now: Light is scattered inside the volume Single scattering Multiple scatteringwith attenuation. Light is scattered attenuatedmultiple along ist times way before through it reaches the volume the eye (Volumetric (Global illumination) shadows) Light is scattered once before it reaches the eye REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Scattering Approximation: REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Volumetric Scattering 2. Draw slice into image buffer Sample light buffer multiple times jittered around the original position REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Volumetric Scattering part: Chromatic out-scattering term can be used to change color of light as it travels throught the volume REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Volumetric Scattering Photograph of real wax block Volumetric Scattering + Chromatic Attenuation Bright blue reflective color Direct attenuation only REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Examples Phase Function Henyey. Greenstein Lit from behind Henyey. Greenstein Lit 45° degrees from above REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Henyey. Greenstein Lit from front Henyey. Greenstein + Mie Phase function Eurographics 2006

Examples REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Examples Direct light only Achromatic indirect light REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Chromatic indirect light Eurographics 2006

Examples Approximation of opalescent materials (lit from above) REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006

Examples Surface Shading Direct Light + Shadows Direct Light+ Indirect Light Surface shading on the leaves only REAL-TIME VOLUME GRAPHICS Christof Rezk Salama Computer Graphics and Multimedia Group, University of Siegen, Germany Eurographics 2006