CMSC427 Advanced shading – getting global illumination by local...

CMSC427Advancedshading–gettingglobalilluminationbylocalmethods

Credit:slidesProf.Zwicker

• Shadows• Environmentmaps• Reflectionmapping• Irradianceenvironmentmaps• Ambientocclusion• Reflectionandrefraction• Toonshading

Topics

Why are shadows important?

• Cuesonscenelighting

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Why are shadows important?

• Contactpoints• Depthcues

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Why are shadows important?

• Realism

Without self-shadowing Without self-shadowing

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Terminology

• Umbra:fullyshadowedregion• Penumbra:partiallyshadowedregion

(area) light source

receiver shadow

occluder

umbra

penumbra

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Hard and soft shadows

• Pointanddirectionallightsleadtohardshadows,nopenumbra

• Arealightsourcesleadtosoftshadows,withpenumbra

point directional area

umbra penumbra7

Hard and soft shadows

Hard shadow,point light source

Soft shadow,area light source

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Shadows for interactive rendering• Focusonhardshadows

• Softshadowsoftentoohardtocomputeininteractivegraphics

• Twomaintechniques• Shadowmapping• Shadowvolumes

• Manyvariations,subtleties• Stillactiveresearcharea

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Shadow mappinghttp://en.wikipedia.org/wiki/Shadow_mappinghttp://www.opengl-tutorial.org/intermediate-tutorials/tutorial-16-shadow-mapping/

Mainidea• Scenepointislitbylightsourceifitisvisible fromlightsource

• Determinevisibilityfromlightsourcebyplacingcameraatlightsourceposition andrenderingscene

Scene points are lit if visible from light source

Determine visibility from light source by placing camera

at light source position10

Two pass algorithm

Firstpass• Renderscenebyplacingcameraatlightsourceposition

• Storedepthimage(shadowmap)

Depth image seen from light source

depth valuein shadow map

Secondpass• Renderscenefromcamera(eye)position

• Ateachpixel,comparedistancetolightsource(yellow)withvalueinshadowmap(red)

• Ifyellowdistanceislargerthanred,weareinshadow

• Ifdistanceissmallerorequal,pixelislit

Two pass algorithm

Final image with shadows

vb is in shadow pixel seen

from eye vb

depth valuein shadow map

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Issues

• Limitedfieldofviewofshadowmap• Z-fighting• Samplingproblems

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Limited field of view

• Whatifascenepointisoutsidethefieldofviewoftheshadowmap?

field of view of shadow map

Limited field of view

• Whatifascenepointisoutsidethefieldofviewoftheshadowmap?

• Usesixshadowmaps,arrangedinacube

• Requiresrenderingpassforeachshadowmap!

shadowmaps

• Intheory,depthvaluesforpointsvisiblefromlightsourceareequalinbothrenderingpasses

• Becauseoflimitedresolution,depthofpixelvisiblefromcameracouldbelargerthanshadowmapvalue

• Needtoadd biasinfirstpasstomakesurepixelsarelit

z-fighting

Camera image

Shadow mapImagepixels

Shadow mappixels Pixel is

consideredin shadow!

Depth of pixel visiblefrom camera

Depth of shadow map

Solution

• Addbias whenrenderingshadowmap• Movegeometryawayfromlightbysmallamount

• Findingcorrectamountofbiasistricky

Correct bias Not enough bias Too much bias17

Bias

Correct

Not enough Too much

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Sampling problems

• Shadowmappixelmayprojecttomanyimagepixels

• Uglystair-steppingartifacts

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Solutions

• Increaseresolutionofshadowmap• Notalwayssufficient

• Splitshadowmapintoseveralslices• Tweakprojectionforshadowmaprendering

• Lightspaceperspectiveshadowmaps(LiSPSM)http://www.cg.tuwien.ac.at/research/vr/lispsm/

• WithGLSLsourcecode!

• CombinationofsplittingandLiSPSM• Basisformostseriousimplementations• Listofadvancedtechniquessee

http://en.wikipedia.org/wiki/Shadow_mapping

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LiSPSM

Basic shadow map Light space perspectiveshadow map

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Percentage closer filtering

• Goal:avoidstair-steppingartifacts• Similartotexturefiltering,butwithatwist

http://http.developer.nvidia.com/GPUGems/gpugems_ch11.html

Simpleshadow mapping Percentage closer filtering

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Percentage closer filtering

• Insteadoflookinguponeshadowmappixel,lookupseveral

• Performdepthtestforeachshadowmappixel• Computepercentageoflitshadowmappixels

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Percentage closer filtering

• Supportedinhardwareforsmallfilters(2x2shadowmappixels)

• Canuselargerfilters(lookupmoreshadowmappixels)atcostofperformancepenalty

• Fakesoftshadows• Largerfilter,softershadowboundary

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Shadow volumes

Shadowingobject

Partiallyshadowed object

Lightsource

Eye position(note that shadows are independent of the eye position)

Surface insideshadow volume(shadowed)

Surface outsideshadow volume(illuminated)

Shadowvolume(infinite extent)

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In shadow or not

• Testifsurfacevisibleingivenpixelisinsideoroutsideshadowvolume1. Allocateacounterperpixel2. Castarayintothescene,startingfromeye,goingthroughgivenpixel

3. Incrementthecounterwhentherayenterstheshadowvolume

4.Decrementthecounterwhentherayleavestheshadowvolume

5.Whenwehittheobject,checkthecounter.• Ifcounter>0,inshadow• Otherwise,notinshadow

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In shadow or not

OccluderLightsource

Eyeposition

+1 +2 +2+3

In shadow

+1

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Implementation in rendering pipeline• Raytracingnotpossibletoimplementdirectly• Useafewtricks...

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Shadow volume construction

• Needtogenerateshadowpolygonstoboundshadowvolume

• Extrudesilhouetteedgesfromlightsource

Extruded shadow volumes29

Shadow volume construction

• NeedstobedoneontheCPU• Silhouetteedgedetection

• Anedgeisasilhouetteifoneadjacenttriangleisfrontfacing,theotherbackfacingwithrespecttothelight

• Extrudepolygonsfromsilhouetteedges

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Shadow test without ray tracing

Usingthestencilbuffer• Aframebuffer channel(likeRGBcolors,depth)thatcontainsaper-pixelcounter(integervalue)

• AvailableinOpenGL• Stenciltest

• Similartodepthtest(z-buffering)• Controlwhetherafragmentisdiscardedornot• Stencilfunction:isevaluatedtodecidewhethertodiscardafragment

• Stenciloperation:isperformedtoupdatethestencilbufferdependingontheresultofthetest

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Shadow volume algorithms

Z-passapproach• Countleaving/enteringshadowvolumeeventsasdescribed

• Usestencilbuffertocountnumberofvisible(i.e.notoccludedfromcamera)front-facingandbackfacingshadowvolumepolygonsforeachpixel

• Ifequal,pixelisnotinshadowZ-failapproach• Countnumberofinvisible(i.e.occludedfromcamera)front-facingandback-facingshadowvolumepolygons

• Ifequal,pixelisnotinshadow

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Z-pass approach: details

• Renderscenewithonlyambientlight• Updatedepthbuffer

• Turnoffdepthandcolorwrite,turnonstencil,keepthedepthteston• Initstencilbufferto0• Drawshadowvolumetwiceusingfaceculling

• 1stpass:renderfront facesandincrement stencilbufferwhendepthtestpasses

• 2ndpass:renderback facesanddecrement whendepthtestpasses• Ateachpixel

• Stencil!=0,inshadow• Stencil=0,lit

• Renderthesceneagainwithdiffuseandspecularlighting• Writetoframebufferonlypixelswithstencil=0

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Issues

• Z-passfailsif• Eyeisinshadow• Shadowpolygonclippedbynearclipplane

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Shadow volumes

• Pros• Doesnotrequirehardwaresupportforshadowmapping• Pixelaccurateshadows,nosamplingissues

• Cons• MoreCPUintensive(constructionofshadowvolumepolygons)

• Fill-rateintensive(needtodrawmanyshadowvolumepolygons)

• Expensiveforcomplexgeometry• Trickytohandleallcasescorrectly• Hardtoextendtosoftshadows

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Shadow maps

• Pros:• LittleCPUoverhead• Noneedtoconstructextrageometrytorepresentshadows

• Hardwaresupport• Canfakesoftshadowseasily

• Cons:• Samplingissues• Depthbiasisnotcompletelyfoolproof

• Shadowmappinghasbecomemorepopularwithbetterhardwaresupport

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Resources

• Overview,lotsoflinkshttp://www.realtimerendering.com/

• Basicshadowmapshttp://en.wikipedia.org/wiki/Shadow_mapping

• Avoidingsamplingproblemsinshadowmapshttp://www.comp.nus.edu.sg/~tants/tsm/tsm.pdfhttp://www.cg.tuwien.ac.at/research/vr/lispsm/

• Fakingsoftshadowswithshadowmapshttp://people.csail.mit.edu/ericchan/papers/smoothie/

• Alternative:shadowvolumeshttp://en.wikipedia.org/wiki/Shadow_volume

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More realistic illumination

• Inrealworld,ateachpointinscenelightarrivesfromalldirections

• Notjustfrompointlightsources

• Environmentmaps• Store“omni-directional”illuminationasimages• Eachpixelcorrespondstolightfromacertaindirection

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Capturing environment maps

• “360degrees”panoramicimage• Insteadof360degreespanoramicimage,takepictureofmirrorball(lightprobe)

Light probes [Paul Debevec, http://www.debevec.org/Probes/] 39

Environment maps as light sourcesSimplifyingassumption• Assumelightcapturedbyenvironmentmapisemittedinfinitelyfaraway

• Environmentmapconsistsofdirectionallightsources

• Valueofenvironmentmapisdefinedforeachdirection,independentofpositioninscene

• Usesingleenvironmentmapaslightsourceatalllocations inthescene

• Approximation!

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Environment maps as light sources• Howdoyoucomputeshadingofadiffusesurfaceusinganenvironmentmap?

• Whatismoreexpensivetocompute,shadingadiffuseoraspecularsurface?

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Environment maps applications

• Useenvironmentmapas“lightsource”

Global illumination[Sloan et al.]

Reflection mapping

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Sphere & cube maps

• Storeincidentlightonsphereoronsixfacesofacube

Spherical map Cube map

Elevation, azimuthalangle

Elevationq const.

Northpoleq=90

Southpoleq=-90

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Cube maps in OpenGL

Applicationsetup• Load,bindacubeenvironmentmap

glBindTexture(GL_TEXTURE_CUBE_MAP, …);// the six cube facesglTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_X,…);glTexImage2D(GL_TEXTURE_CUBE_MAP_NEGATIVE_X,…);glTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_Y,…);…glEnable(GL_TEXTURE_CUBE_MAP);

• Moredetails• “OpenGLShadingLanguage”,RandiRost• “OpenGLSuperbible”,Sellersetal.• Onlinetutorials

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Cube maps in OpenGL

Look-up• Givendirection(x,y,z)• Largestcoordinatecomponentdeterminescubemapface

• Dividingbymagnitudeoflargestcomponentyieldscoordinateswithinface

• Look-upfunctionbuiltintoGLSL• Use(x,y,z) directionastexturecoordinatestosamplerCube

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Environment map data

• Alsocalled„lightprobes“http://www.debevec.org/Probes/

• Toolforhighdynamicrangedata(HDR)http://projects.ict.usc.edu/graphics/HDRShop/

• Pre-renderedlightprobesforgameshttp://docs.unity3d.com/Manual/LightProbes.html

Lightprobes(http://www.debevec.org/Probes/) 46

Reflection mapping

• Simulatemirrorreflection• Computereflectionvectorateachpixelusingviewdirectionandsurfacenormal

• Usereflectionvectortolookupcubemap• Renderingcubemapitselfisoptional

Reflection mapping 47

Reflection mapping in GLSL

Vertexshader• Computeviewingdirectionforeachvertex• Reflectiondirection

• UseGLSLbuilt-inreflect function

• PassreflectiondirectiontofragmentshaderFragmentshader• Look-upcubemapusinginterpolatedreflectiondirectionin float3 refl;uniform samplerCube envMap;texture(envMap, refl);

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Reflection mapping examples

• Approximation,reflectionsarenotaccurate

[NVidia]

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Shading using environment map

• Assumption:distantlighting• Incidentlightisafunctionofdirection,butnotposition

• Realisticshadingrequires• Takeintoaccountlightfromalldirections• Includeocclusion

Illumination from environment

Same environment map for both points“Illumination is a function of direction,but not position”

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Mathematical model

• AssumeLambertian(diffuse)material,BRDFkd• Ignoreocclusionfornow

• Illuminationfrompointlightsources

• Illuminationfromenvironmentmapusinghemisphericalintegral

• Directionsw• HemisphereofdirectionsW• Environmentmap,radiancefromeachdirectionc(w)

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Irradiance environment maps

• Precomputeirradiance asafunctionofnormal

• Storeasirradianceenvironmentmap

• Shadingcomputationatrendertime• Dependsonlyonnormal,notposition

Environment map Irradiance map52

Irradiance environment maps

Directional light Environment illumination

Images from http://www.cs.berkeley.edu/~ravir/papers/envmap/ 53

Implementation

• Precomputeirradiancemapfromenvironment• HDRShoptool,“diffuseconvolution”

http://projects.ict.usc.edu/graphics/HDRShop/

• Atrendertime,lookupirradiancemapusingsurfacenormal

• Whenobjectrotates,rotatenormalaccordingly

• Canalsoapproximateglossyreflection• Blurenvironmentmaplessheavily• Lookupblurredenvironmentmapusingreflectionvector

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Today

Moreshading• Environmentmaps• Reflectionmapping• Irradianceenvironmentmaps• Ambientocclusion• Reflectionandrefraction• Toonshading

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Including occlusion

• Ateachpoint,environmentispartiallyoccludedbygeometry

• Addlightonlyfromun-occludeddirections

Visualization of un-occluded directions56

Including occlusion

VisibilityfunctionVx(w)• Binaryfunctionofdirectionw• Indicatesifenvironmentisoccluded• Dependsonpositionx

Environment map Visibility functions

Vx0=0

Vx1=0Vx0=1

Vx1=1

x0

x1

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Mathematical model

• Diffuseilluminationwithvisibility

• Ambientocclusion• “Fraction”ofenvironmentthatisnotoccludedfromapointx

• Scalarvalue

• Approximation:diffuseshadinggivenbyirradianceweightedbyambientocclusion

Vx=0

Vx=1

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Ambient occlusion

Ambientocclusion Diffuseshading Ambientocclusioncombined(usingmultiplication)withdiffuseshading

http://en.wikipedia.org/wiki/Ambient_occlusion59

Implementation

• Precomputation (off-line,beforerendering)• Computeambientocclusiononaper-vertexbasis• Usingraytracing• Freetoolthatsavesmesheswithper-vertexambientocclusion

http://www.xnormal.net/

• Caution• Basicpre-computedambientocclusiondoesnotworkforanimatedobjects

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Shading integral

• Ambient occlusion with irradiance environmentmaps is crude approximation to general shadingintegral

• BRDFfor (non-diffuse)material

Reflected radiance c(wo)

Outgoing direction wo

Incident directions wi

HemisphereW61

Shading integral

• Accurate evaluation is expensiveto compute• Requires numerical integration

• Many tricksfor more accurate and generalapproximation than ambient occlusion and irradianceenvironment maps exist

• Spherical harmonics shadinghttp://www.research.scea.com/gdc2003/spherical-harmonic-lighting.pdf

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Note

• Visuallyinterestingresultsusingcombination(sum)ofdiffuseshadingwithambientocclusionandreflectionmapping

http://www.research.scea.com/gdc2003/spherical-harmonic-lighting.pdf

Diffuseshading withambient occlusion Reflection mapping Combination (sum)

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Today

Moreshading• Environmentmaps• Reflectionmapping• Irradianceenvironmentmaps• Ambientocclusion• Reflectionandrefraction• Toonshading

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Toon shading

• Simplecartoonstyleshader• Emphasizesilhouettes• Discretestepsfordiffuseshading,highlights• SometimescalledCELshading

http://en.wikipedia.org/wiki/Cel-shaded_animation

Off-line toon shader GLSL toon shader 65

Toon shading

• Silhouetteedgedetection• Computedotproductofviewingdirectionv andnormaln

• Use1Dtexturetodefineedgerampuniform sample1D edgeramp; e=texture1D(edgeramp,edge);

0

1edgeramp

edge66

Toon shading

• Computediffuseandspecularshading

• Use1Dtexturesdiffuseramp, specularramp tomapdiffuseandspecularshadingtocolors

• Finalcoloruniform sampler1D diffuseramp;uniform sampler1D specularramp;c = e * (texture(diffuse,diffuseramp)+

texture(specular,specularramp));

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