Real-Time Volume Graphics [07] Global Volume Illumination

1. Real-Time Volume Graphics[07] Global Volume Illumination

2. 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

3. 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 atevery point • Scattering from phase function

4. Surface Illumination The incoming radiance , at a point from direction will partially be reflectedinto direction To obtain the radiance at , we must account for all possibleincoming directions:

5. 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

6. Physical Model of Radiative Transfer Increase true emission in-scattering Decrease true absorption out-scattering

7. Scattering Phase function Incomingradiance Out-scattering coefficient • In-scattering • Volume Rendering Equation:

8. Volume Illumination • Up until now: External light is not attenuated • Now: Attenuation of light as it travels through the volume • Single scattering, no attenuation. • Light reaches every point unimpededly • Light is scattered once before it reaches the eye • Not physically plausible • Single scattering with attenuation. • Light is attenuated along ist way through the volume(Volumetric shadows) • Light is scattered once before it reaches the eye

9. Shadows Maps • For Surfaces:

10. Shadow Maps • For Volume Rendering: • Update image and shadow buffer slice-by-slice • Need proxy geometry, that can be rendered from twodifferent views. Half Angle Slicing

11. Light Source Attenuation • Draw slice into light buffer • Draw slice into image buffer • Proceed with next slice

12. Light Source Attenuation Example: Visible Human CT Head Direct Light +Attenuation

13. Volume Illumination • Up until now: External light is attenuated by volume • Now: Light is scattered inside the volume • Single scattering with attenuation. • Light is attenuated along ist way through the volume(Volumetric shadows) • Light is scattered once before it reaches the eye • Multiple scattering • Light is scattered multiple times before it reaches the eye(Global illumination)

15. Scattering • Approximation:

16. Volumetric Scattering 2. Draw slice into image buffer Sample light buffer multiple timesjittered around the original position

17. Volumetric Scattering • Scattering part: • Chromatic out-scattering term can be used to change color of light as it travels throught the volume

18. Volumetric Scattering Volumetric Scattering+ Chromatic Attenuation Photographof realwax block Bright bluereflective color Directattenuationonly

19. Examples Phase Function Henyey-Greenstein Lit from front Henyey-Greenstein Lit from behind Henyey-Greenstein Lit 45° degrees from above Henyey-Greenstein + Mie Phase function

20. Examples

21. Examples Direct light only Achromatic indirect light Chromatic indirect light

22. Examples • Approximation of opalescent materials (lit from above)

23. Examples Direct Light+ Shadows Surface Shading Direct Light+Indirect Light Direct Light+Indirect Light Surface shading on the leaves only