Ad Advanced Computer Graphics d C G hi CS 563: Real Time Ocean - PowerPoint PPT Presentation

Ad Advanced Computer Graphics d C G hi CS 563: Real ‐ Time Ocean Rendering [Real ‐ Time Realistic Ocean Lighting using Seamless [R l Ti R li ti O Li hti i S l Transitions from Geometry to BRDF] Xin Wang March, 20, 2012 , , Computer Science Dept. Worcester Polytechnic Institute (WPI)

B Background k d  Photorealistic rendering image  Photorealistic rendering image  Cannot be used in games, simulators, etc…  Realistic animation and rendering

I t Introduction d ti  Hierarchical modeling of the ocean  Hierarchical modeling of the ocean  Illumination reflection using BRDF  Lighting effects h ff  BRDF model  Approximate formula for computing the surfaces f l f h f  Rendering

P Pervious Work i W k  Physical ocean models  Physical ocean models.  [CM54,PM64,RD07]  Computer graphics ocean models. C t hi d l  [Tes01,CC06,HVT*06]  Reflectance models. fl d l  [CT81,AS00,RDP05]  Multi ‐ resolution reflectance models.  [Kaj85,HSR07]

O Ocean Model – Phase I M d l Ph I  Dynamic scene no pre computations  Dynamic scene, no pre ‐ computations  physical facts about deep water waves  Trochoid Waves.  A gerstner wave is defined by p = [ x+hsin(wt − kx), hcos(wt − kx)]T , where w = gk. p [ ( ), ( )] , g

O Ocean Model – Phase II M d l Ph II  Ocean surface with sum of n trochoid wave trains  Ocean surface with sum of n trochoid wave trains  Three sub ‐ models.

O Ocean Model – Phase II M d l Ph II  Model hierarchy  Average positions  Compute inside a grid cell by filtering the trochoids  Average normals  Compute inside a pixel  BRDFs BRDFs  Subpixel surface details with statistical properties

O Ocean Model – Result M d l R lt

O Ocean BRDF BRDF  A very accurate BRDF model for anisotropic  A very accurate BRDF model for anisotropic rough surfaces.

O Ocean BRDF BRDF  BRDF model coordinates  BRDF model coordinates  v and l are unit vectors towards the viewer and the light f is the normal of a microfacet whose x and y light. f is the normal of a microfacet whose x and y

O Ocean Lighting – Sun Lighting Li hti S Li hti  Compute the light reflected from the Sun at P by  Compute the light reflected from the Sun at P by applying the BRDF  BRDF as constant over the Sum solid angle Ω sun BRDF t t th S lid l Ω  Self ‐ shadowing can be provided with a shadow map for close views

O Ocean Lighting – Sky Lighting Li hti Sk Li hti  Light reflected from the sky dome is difficult  Light reflected from the sky dome is difficult  Approximate method for specular to diffuse BRDFs assuming an isotropic or anisotropic BRDF i i t i i t i Gaussian slope distribution  Three steps: Th t  Approximate environment lighting  Average Fresnel reflectance A F l fl  Average sky radiance

Sky Lighting – Approximate environment lighting i t li hti  BRDF is proportional to the fraction of micro  BRDF is proportional to the fraction of micro ‐ facets  Approximation is exact when BRDF is purely specular l

Sky Lighting – Average Fresnel reflectance fl t  Plot of the reflectance of anisotropic rough  Plot of the reflectance of anisotropic rough surface (green), and filter function (red)

Sk Li hti Sky Lighting – Average sky radiance A k di  Environment map filtering  Environment map filtering  The reflected light L is an elliptical Gaussian filter  Environment map transformed filter

O Ocean Lighting – Refracted Lighting Li hti R f t d Li hti  Light coming from the Sun and Sky also refracted  Light coming from the Sun and Sky also refracted inside the water  Also refracted again to the viewer Al f t d i t th i  Radiance Lsea reaching the surface from below is diff diffuse

O Ocean Lighting – Result Li hti R lt  Reflected sun light reflected sky light light  Reflected sun light, reflected sky light, light refracted from the water to final result

S Summary of Lighting Algorithm f Li hti Al ith

E t Extensions i  Local waves  Local waves  Support other waves than trochoids  Local reflections L l fl ti  Use reflection map in screen space  Multiple reflections l l fl  Environment map approximate sky irradiance  Planet ‐ scale rendering  Render a sphere with Ross BRDF

I Implementation l t ti  Vertex shader projects the screen space regular  Vertex shader projects the screen space regular grid  Fragment shader computes the per pixel normals F t h d t th i l l and the Sun, Sky and refracted light  Use a geometric progression for the wavelengths U t i i f th l th

R Result lt

R Result lt

R Result lt

C Compare to real photo t l h t

References  An anisotropic phong BRDF model. Ashikhmin M., Shirley P. Journal of Graphics Tool 5(2000)  GPU ‐ based real ‐ time simulation and rendering of GPU b d l i i l i d d i f unbounded ocean surface. Yang X., Pi X., Zheng L., Li S In International Conference on Computer Aided S. In International Conference on Computer Aided Design and Computer Graphics (2005)  Simulating ocean water Tessendorf J ACM  Simulating ocean water. Tessendorf J. ACM SIGGRAPH course notes (2001)