Fitting Virtual Lights For Non-Diffuse Walkthroughs

1. Fitting Virtual Lights For Non-Diffuse Walkthroughs B.Walter, G.Alppay, E.Lafortune, S.Fernandez, D.P.Greenberg SIGGRAPH 1997 Presented By Aditi Majumder Paul Zimmons

2. Different Illumination Methods Global Illumination Methods Natural Appearance Phong Illumination Performance

3. Global Vs Phong Illumination • Global Illumination • Realistic physical model • Slow and not interactive • Large storage requirement • Phong Illumination • Fast and interactive • Too simple to generate non diffuse effects Is there a way to get the best of both worlds?

4. Previous Work • View Independent Non Diffuse Global Illumination • Immel, Cohen, Greenberg; Sillion, Arvo, Westin, Greenberg. • Image Based Rendering • Levoy, Hanrahan; McMillan, Bishop. • Environment and Reflection Maps • Blinn, Newall; Greene; Voorhies, Foran • Multipass Rendering Techniques • Diefenbach • Superposition Rendering • Bastos

5. Virtual Lights Another Solution Global Illumination Methods Natural Appearance Phong Illumination Performance

6. Advantages • Can approximate non-diffuse effects of global illumination, hence generates better pictures • Interactive display rate • Makes the model portable and suitable for existing highly optimized 3D graphics pipelines NON DIFFUSE WALKTHROUGHS !!!

7. Compute a View Independent Non-Diffuse Global Illumination Solution 1 PREPROCESS Approximate the effect with a set of virtual Phong lights per object 2 Render the scene with the virtual lights using the conventional graphics hardware 3 INTERACTIVE Algorithm Overview

8. VIDEO

9. Compute a View Independent Non-Diffuse Global Illumination Solution 1 PREPROCESS Approximate the effect with a set of virtual Phong lights per object 2 Render the scene with the virtual lights using the conventional graphics hardware 3 INTERACTIVE Algorithm Overview

10. Output of Global Illumination • Object discretized into vertices • Global Illumination Solution = Amount of light leaving each vertex in all directions

11. Phong Lighting Model Treated as appearance basis functions • Diffuse/Directionally Invariant • Directionally dependent Phong lobes caused by specific lights Light Source Light Emitted Diffuse

12. Light Source Light Emitted Diffuse Goal of the Algorithm Global Illumination Solution Fitted Virtual Light Solution

13. Steps in Fitting Lights • Design the shape and distribution of the Phong lobes by fitting virtual lights for each object • Find the appropriate diffuse and specular coefficients to weight the diffuse and specular components

14. Output of Step 1 • L2 • L4 • L3 • Shininess factor • Number of Phong lights • Direction and Intensity of each of the phong lights • L5 • L1 Fitting Virtual Lights

15. L2 L4 L3 • At each vertex choose • diffuse coefficient • specular coefficient L5 L1 Lighting Desired Lighting Obtained Fitting Virtual Lights

16. Find the kd and ks for each vertex by using a least square method and obtain the error E Error = MAXVAL shine = INIT_SHINE Yes Yes No E< Error? Find the maximum illuminated V and Û No Error = E ; Fkd = kd; Fks = ks; F_shine = shine Fit a virtual phong light L(maxI, -Û) shine += INCR Yes Number of lights > MAXNUM? No Subtract the effect of L at all the vertices shine > 128.0? END The Procedure

17. Limitations • Limit on the maximum number of lights in OpenGL • Sets a limit on the level of specularity • Fixed shininess per object • Gouraud interpolation of phong highlights leads to artifacts (needs high tesselation) • Performance penalty for varying more than one property per vertex

18. Superposition • Superposition - sum of scaled and shifted impulse responses • Phong lobe is response (kernel) • Taking 8 impulses per obj (8 lights) • Needs many lights to get a mirror • Non diffuse solution resolves visibility

19. Conclusion • Technique for using simple Phong lighting model to approximate the effects of a view-independent non-diffuse global illumination model • Compact Storage • Fast interactive display by using standard graphics API like OpenGL • Portable