Perceptual Influence of Approximate Visibility in Indirect Illumination

Insu Yu27 May 2010

ACM Transactions on Applied Perception (Presented at APGV 2009)

Introduction• Can you see difference ?

Traditionally GI (Path tracing, photon mapping, ray-tracing) uses accurate visibility (ray casting)

Motivation

• Indirect illum. 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?

Direct Indirect Direct + Indirect

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

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

Rendering Equation (Visibility ?)• Rendering Equation

• Reflection operator ‘K’, Geometry operator ‘G’, visibility ‘V’

• Operator form

• KG < 1

Lambertian & Phong examples

1 11 2

( ) cos( , ) ( ) cos ( , ) cos( , )

( ) cos( , ) ( ) cos ( , ) cos( , )1 1( ) ( )

x x

nd x s x

nN Ni d x i i s x

i ii i

Diffuse Specular

L x K N d L x K R N d

L x K N L x K R NN p N p

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

Path dependant notation

Direct illumination Indirect first bounce Shadow Ray

shoot photons from light sources

deposit on every bounce

treat photons as point lights

IR (VPLs)

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

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

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

Approximations – Cont’

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

Video (Approximations)

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

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

Test Scenes

• Five seconds video instead of static images to take into account of temporal artifacts

• A full range of scenesArches : Fast light moving + strong direct lightingTea house: Slow light moving + dominant IndirectLivingroom: Camera moving + dominant Direct LightingSponza: Fast Camera moving + strong indirect shadows

Arches Tea house Livingroom Sponza

Test Scenes (cont’)

• 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)

Arches Tea house Livingroom Sponza

Video (Reference and Approximations)

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

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

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 15 mins• 14 subjects for paired comparisons

• Ranking order• 11 videos(reference + 10 approximations)• Rank 4 different sets, sorting 11 videos (25-35 mins)• 18 subjects for ranking experiment• Pan/Zoom, Drag & Drop, Pause functions

Video(Experiments)

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

Results (Pair comparison)

Arches Tea house

Living room Sponza

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)AO & DAO: slightly similar (radius >= 0.2) – Large RadiusNo Vis: moderately similarDirect illumination dominant

Results (Ranking)

Arches Tea house

Living room Sponza

No Vis: Ranked higher than worst AO, DAORef, IMP: ranked equally very realisticAO & DAO(0.05, 0.1): ranked generally realisticAO & DAO(0.2): ranked less realistic

Results (Overall)

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

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

Thanks you

Questions ?