Participating Media & Vol. Scattering

1. Participating Media & Vol. Scattering • Applications • Clouds, smoke, water, … • Subsurface scattering: paint, skin, … • Scientific/medical visualization: CT, MRI, … • Topics • Absorption and emission • Scattering and phase functions • Volume rendering equation • Homogeneous media • Ray tracing volumes University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

2. Absorption Absorption cross-section: Probability of being absorbed per unit length University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

3. Optical distance or depth Homogenous media: constant Transmittance University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

4. Transmittance and Opacity Transmittance Opacity University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

5. Out-Scatter Scattering cross-section: Probability of being scattered per unit length University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

6. Albedo Attenuation due to both absorption and scattering Extinction Total cross-section University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

7. Black Clouds From Greenler, Rainbows, halos and glories University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

8. Phase function Reciprocity Energy conserving In-Scatter University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

9. Phase Functions • Phase angle • Phase functions (from the phase of the moon) 1. Isotropic -simple 2. Rayleigh -molecules 3. Mie scattering - small spheres • ... Huge literature ... University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

10. Blue Sky = Red Sunset From Greenler, Rainbows, halos and glories University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

11. Coronas and Halos Moon Corona Sun Halos From Greenler, Rainbows, halos and glories University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

12. g: average phase angle Henyey-Greenstein Phase Function Empirical phase function g = -0.3 g = 0.6 University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

13. The Volume Rendering Equation • Integro-differential equation • Integro-integral equation Attenuation: Absorption and scattering Source: Scatter (+ emission) University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

14. Simple Atmosphere Model • Assumptions • Homogenous media • Constant source term (airlight) • Fog • Haze University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

15. The Sky From Greenler, Rainbows, halos and glories University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

16. Atmospheric Perspective From Greenler, Rainbows, halos and glories University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

17. Atmospheric Perspective Aerial Perspective: loss of contrast and change in color From Musgrave University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

18. Semi-Infinite Homogenous Media • Reduced Intensity • Effective source term • Volume rendering equation • Integrating over depths University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

19. Semi-Infinite Homogenous Media • Integrating over depths University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

20. Semi-Infinite Homogenous Media • BRDF • Lommel-Seeliger Law for Diffuse Reflection University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

21. Subsurface Scattering • Skin University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

22. Volume Representations • 3D arrays (uniform rectangular) • CT data • 3D meshes • CFD, mechanical simulation • Simple shapes with solid texture • Ellipsoidal clouds with sum-of-sines densities • Hypertexture University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

23. Voxel Scalar Volumes • Interpolation • Map scalars to optical properties University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

24. Voxel Scalar Volumes • Scatter University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

25. Ray Marching Direct Lighting Primary ray University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

26. Ray Marching Direct Lighting Shadow ray University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

27. Beams of Light From Greenler, Rainbows, halos and glories From Minneart, Color and light in the open air University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

28. Color and Opacity Volumes M. Levoy, Ray tracing volume densities Voxel University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

29. Ray Marching Direct Lighting Primary ray University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

30. Volume Rendering Examples From Marc Levoy From Karl Heinz Hoehne University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell