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Perceptual Influence of Approximate Visibility in Indirect Illumination

Perceptual Influence of Approximate Visibility in Indirect Illumination. Insu Yu 27 May 2010. ACM Transactions on Applied Perception (Presented at APGV 2009). Introduction. Can you see difference ?

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Perceptual Influence of Approximate Visibility in Indirect Illumination

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  1. Perceptual Influence of Approximate Visibility in Indirect Illumination Insu Yu 27 May 2010 ACM Transactions on Applied Perception (Presented at APGV 2009)

  2. Introduction • Can you see difference ? Traditionally GI (Path tracing, photon mapping, ray-tracing) uses accurate visibility (ray casting)

  3. Motivation Direct Indirect Direct + Indirect • Indirectillum. is perceptually important in GI but high rendering cost • Low frequency nature in real world(Smooth gradation) • Visibility determination is most expensive • Intersecting rays with all polygons • Approximated Visibility for efficient GI?

  4. Previous works (Approx. Visibility) • Radiosity(Sillion 95): blur out small features • Lightcuts(Walter05) : grouping VLPs • Interactive GI for dynamic(Dachsbacher05) : indirect illum is neglected • GPU-based indirect illum(Bunnel05): a hierarchical link structure • Ambient occlusion (Zhukov98) • Hierarchical radiosity(Dachsbacher07) for IGI • Imperfect Shadow(Ritscehl08b) • Approx Visibility used in RT but perceptual impact not formally studied • No distinction of direct / indirect illumination

  5. Overview • Visibility Approximations (in our study) • Imperfect Visibility(Ritschel 2008) • Ambient Occlusion(Zhukov1998) • Direction Ambient Occlusion(Sloan 09. Ritschel 09) • Evaluate Perceptual influence of approximated visibility on scene with indirect illumination • Which visibility approx are perceptually acceptable • Scope • Only interested in indirect illumination • Direct illumination uses accurate visibility • Evaluate with Instant radiosity method

  6. Rendering Equation (Visibility ?) • Rendering Equation • Reflection operator ‘K’, Geometry operator ‘G’, visibility ‘V’ • Operator form • KG < 1

  7. Lambertian & Phong examples

  8. Path dependant notation • Neumann expansion • Path notation • G0 Direct lighting - use accurate visibility (Ga) • G1 first bounce • G2 light is reflected the second time • Superscript dots: path-length • Subscript: bounce number

  9. Path dependant notation IR (VPLs) shoot photons from light sources deposit on every bounce treat photons as point lights Direct illumination Indirect first bounce Shadow Ray

  10. Approximations - IMP • Imperfect Visibility (Gimp) • Ritschel08 used to speed up instant radiosity • Randomly setting N% of visibility to either 0 or 1 • Introduce noise • Last bounce is approximated X X Accurate Imperfect

  11. Approximations – AO • Ambient Occlusion(Gao) • Produce smooth visibility • The percentage of ‘visible sky’ -scalar value • visibility from ‘x’ in all directions • User defined radius ‘r’ Accurate Ambient Occlusion

  12. Approximations – DAO • Directional Ambient Occlusion(Gdao) • Add directional component to AO • Partial correct and errors • 5th order Spherical Harmonics to represent directional visibility Accurate Dir. Ambient Occlusion

  13. Approximations – Cont’ • No Visibility (Gno) • No visibility indirect illumination for Interactive GI • Validate whether no visibility is useful approximation • V(x,y) = 1

  14. Video (Approximations)

  15. Perceptual Influence Study • Goal • Evaluate the influence of visibility approximations • Carry out a series of psychophysical experiments • How perceptually similar to a reference • Paired comparison • Visibility approximations appear realistic (perceived realism) • Ranking Order

  16. Stimuli • Chosen parameter sets to speed up for real-time apps (IR, lightcuts, path-tracing) • l is path length • Accurate Visibility • IMP: (case 1~3) • 25% 50% 75% visibility corruptions • AO,DAO (case 4-6, 7-9) • r = 0.05, 0.1, 0.2 radius of scene diameter • No visibility (case 10) Accurate Visibility for Direct lighting

  17. Test Scenes Arches Tea house LivingroomSponza • Five seconds video instead of static images to take into account of temporal artifacts • A full range of scenes Arches : Fast light moving + strong direct lighting Tea house: Slow light moving + dominant Indirect Livingroom: Camera moving + dominant Direct Lighting Sponza: Fast Camera moving + strong indirect shadows

  18. Test Scenes (cont’) Arches Tea house LivingroomSponza • Rendered using Instant Radiosity(IR) • Four indirect illuminations • High number of VPLs to avoid artifaces • 640x480 resolutions and gamma corrected • Rendered in a PC cluster (1-4 hrs per image)

  19. Video (Reference and Approximations)

  20. Experiment- Paired comparison • Paired comparison plus category (Scheffe52) • Quantify perceptual similarity to the reference • How similar to the reference (pair of videos) • Five-point scoring scale • Assign 1(not similar), 2(slightly similar), 3(moderately similiar), 4(very much similar), 5 (extremely similar) • Category rating + pair comparison

  21. Experiment - Ordinal rank order • Ordinal rank order (Bartleson84) • Determine the perceived realism • Can be quickly performed than complete pair-wise comparison • Intuitive user interface (videos in a row is shown) • Rank the videos in order from highest to lowest by perceived realism

  22. Experiment Procedure • Procedure • Two sessions on different days • A training session was given • conducted in a controlled environment • Paired comparison • 40 estimates (4 scenes x 10 approx) took 15mins • 14 subjects for paired comparisons • Ranking order • 11 videos(reference + 10 approximations) • Rank 4 different sets, sorting 11 videos (25-35mins) • 18 subjects for ranking experiment • Pan/Zoom, Drag & Drop, Pause functions

  23. Video(Experiments)

  24. Results and Analysis • Perceptual Scales: • Five-point scores were scaled using Law of Categorical Judgment Torgerson(58) and Thurstonia(27) • Category boundaries • Estimates of the category boundaries • The scale values can be related to the original categories

  25. Results (Pair comparison) AO & DAO: slightly similar (radius >= 0.2) – Large Radius IMP: very much /moderately similar (accepted wide range of scenes) AO: very much /moderately similar (radius < 0.1) DAO: very much /moderately similar (radius < 0.1) Direct illumination dominant No Vis: moderately similar Arches Tea house Living room Sponza

  26. Results (Ranking) No Vis: Ranked higher than worst AO, DAO Ref, IMP: ranked equally very realistic AO & DAO(0.05, 0.1): ranked generally realistic AO & DAO(0.2): ranked less realistic Arches Tea house Living room Sponza

  27. Results (Overall) All IMP perceived very high realism. AO,DAO: Large radius perceived significantly less realistic AO,DAO(r=0.2) less realistic than ‘no visibility’ Correlation between similarity and perceived realism

  28. Discussion • Visibility approximations can be used in GI maintaining appearance is perceptually similar to ref • IMP ranked generally higher ‘perceived realism’ • Highly corrupted(random) is preferred to human eyes than inaccurate AO,DAO • Most visibility approx are ‘very much similar’ to the ref when direction illumination is dominant • Validates the use of visibility approximations

  29. Thanks you Questions ?

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