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Light and Color in Nature (Today: Water)

Light and Color in Nature (Today: Water). by Benjamin Peters and Martin Schäf WS0304. Intorduction. Motivation Reflection and Refraction Illumination and Caustics Results Advanced Topics. Looking at a water surface. Light is reflected and refracted at the same time

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Light and Color in Nature (Today: Water)

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  1. Light and Color in Nature(Today: Water) by Benjamin Peters and Martin Schäf WS0304

  2. Intorduction • Motivation • Reflection and Refraction • Illumination and Caustics • Results • Advanced Topics

  3. Looking at a water surface • Light is reflected and refracted at the same time • There is a light pattern on the ground (Caustics)

  4. Reflection and Refraction • What we need to know: • Reflection angle • Refraction angle • (Reflected light)/(Refracted light) • What we know: • Incoming angle • Vertex Normal

  5. Reflection and Refraction Reflection Vertex Normal N α α Air Water β Refraction

  6. Snell’s Law • α Incoming = α Reflected • sin(α)/sin(β) = Refractive Index R (material depending) • e.g. RWater= 1.33; RGlass= 1.5 • Refractive Index can be found in any book about physics

  7. Fresnel Term • Fresnel Term is often approximated to recieve better results • Fresnel Term as found in books is F = ½ (sin²(α-β)/sin²(α+β)) + tan²(α-β)*tan²(α+β) • Returns good results for strong reflecting water surfaces • Not good to show the effect of caustics

  8. Example

  9. Approximated Fresnel Term • F0 = (R-1)2/(R+1)2 is a minimum of incoming light parallel to the normal of the surface. • Fα= F0+(1- cos(α))5 * (1- F0) is a value between 0 and 1 depending on the angle between the incoming ray and the surface normal. • e.g. F90 = 1, if the incoming ray is parallel to the surface, all light is reflected.

  10. Refraction and Reflection • Result: • β = arcsin(1/Rwater* sin(α)) = arcsin(1/1.33*sin(α)) • color of intersection point = • (Fα*colorreflected) + (1-Fα)*colorrefracted

  11. Result Scene without Fresnel Term and with one simple direcional light source Scene with Fresnel Term and with one simple direcional light source

  12. Illumination and Caustics • Light is refracted by the water surface • Some spots are stronger illuminated then others

  13. A simple introduction inPhoton Mapping • A sum of photons is send from each light source to the scene • Intersecting photons are stored in a kd-tree • Illumination of each point can be computed by counting the number of photons in a certain distance

  14. A simple introduction inPhoton Mapping Light source Camera

  15. Photon Mapping on a water surface dark bright

  16. Results • Left image: Fresnel Shading, no Photonmapping • Right image: Fresnel Shading, Photonmapping (only 100 000 Photons)

  17. Results • Left image: Fresnel Shading, Photonmapping (1 mil. Photons) • Left image: Fresnel Shading, Photonmapping (1 mil. Photons) and Phongshading

  18. Advanced Topics • Shafts of light and caustics: • http://nis-lab.is.s.u-tokyo.ac.jp/~nis/cdrom/pg/pg2001_iwa.pdf

  19. Advanced Topics • Modeling and animating realistic water surfaces • http://graphics.stanford.edu/~fedkiw/

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