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UCSD CSE 167 - Computer Cinematography

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#16: Computer Cinematography: The Toy Story Lighting ModelFilm CinematographyReal-World Film LightingReal-World Film Lighting TrickeryComputer Graphics TrickeryLighting ModelBasic setupOne key lightSelectionShapeGeneralized cone/pyramid2D SuperellipseAside: 3D SuperellipsoidReturning to lighting…Shape: Rounded RectangleShape: Sheared Barn DoorSoft EdgesShape: Hard EdgeShape: Soft EdgesStep TransitionsLinear Step“SmoothStep”Shape: CutoffShape: CutonShadowingBasic ShadowsShadow SelectionShadow DirectionShadow SharingFake shadowsShape TrimmingShadow SofteningTextureTexture filesCookie changing light shapeCookie breaking up lightProjecting a color slideIntensity distributionUniform IntensityBeam DistributionDistance FalloffLight Ray DirectionLight PropertiesExample 1Example 1 notesExample 2Example 2 notesExample 3Example 3 notesExample 4Example 4 notesExample 5Example 5 notesExample 6Example 6 notesFog in computer graphicsAtmospheric effectsReal-time cloudsDone#16: Computer Cinematography:The Toy Story Lighting ModelCSE167: Computer GraphicsInstructor: Ronen BarzelUCSD, Winter 20062Film CinematographyWhat makes film look different from home movie?CameraLightingNot trying for “photorealism”Lighting in cinematographyContribute to storytellingSet moodCompose imageDirect viewer’s eye3Real-World Film LightingPractical lights:lights that are visible onscreene.g. desk lamps, ceiling lamps, flashlights, fires, …rarely major contributors to illuminationMain lights and spotlights placed off-cameraTo create desired illumination effectStandard three-point lighting setup:Key light -- provides major source of illumination & shadowFill Light -- fills in dark areas & shadows made by keyBack light -- creates subtle highlights around edges•helps give sense of 3D depth•separates subject from background4Real-World Film Lighting TrickeryCheat as necessary:Suspend sheet in front of light to soften shadowsPosition opaque cards or graded filters to shape lightFocus narrow “tickler” lights to get extra highlightsHide “kicker” light under a desk to fill dark areas under character’s chinetc…5Computer Graphics TrickeryWe’re not looking for realismreal-world lighting is constrained by reality, CG isn’tThings that can’t be done in real lifelights coming out of nowherelights that act on only certain objectsdecoupling lights and shadowsetc.Real life does some things easier than standard CG:soft shadowssoft area lightsglobal illumination techniques help this6Lighting ModelSeveral independent aspectsselectionshapeshadowingtexturedropoffdirectionproperties7Basic setuponly fill lights8One key light(will talk about the beam-of-light effect later)(will talk about the beam of light effect later)9Selectiontorus unaffected by key light10ShapeRegion of space affected by lightReal-world commonly use spotlights, barn door lightsCG model:Generalized cone/pyramidSoft edgesCuton and cutoff11Generalized cone/pyramidA “superellipse” shape swept into a pyramidsuperellipse varies between circle and rectanglepyramid may be truncated or sheared122-dimensional superellipse:n=1 is ordinary ellipse or circleneed to handle 4 quadrants carefully: •avoid exponentiation of negative numbers•use 2D Superellipsexy⎡⎣⎢⎤⎦⎥=acosn(θ)bsinn(θ)⎡⎣⎢⎤⎦⎥,where 0 ≤ θ ≤ 2π and 0 ≤ n < ∞xa⎛⎝⎜⎞⎠⎟2 n+yb⎛⎝⎜⎞⎠⎟2 n= 1y = b 1−xa⎛⎝⎜⎞⎠⎟2 n⎛⎝⎜⎞⎠⎟n 2xn=−xnif x < 013Aside: 3D Superellipsoidxyz⎡⎣⎢⎢⎢⎤⎦⎥⎥⎥=acosn1(φ)cosn2(θ)bcosn1(φ)sinn2(θ)csinn1(φ)⎡⎣⎢⎢⎢⎤⎦⎥⎥⎥where 0 ≤ θ ≤ 2π and -π2≤ φ≤π2 and 0 ≤ n < ∞14Returning to lighting…A 2D superellipse swept into a pyramidpyramid may be truncated or sheared15Shape: Rounded Rectangle16Shape: Sheared Barn Door17Soft EdgesFull intensity in central pyramidNo effect (zero intensity) outside outer pyramidSmooth falloff between them18Shape: Hard Edge19Shape: Soft Edges20Step TransitionsWant to transition between two valuesE.g. in a shader, transition between two colors on surfaceE.g. inside/outside light shapeWant function to provide control value between 0 to 1:• Function step(s,a,b) for a < b• Should return 0 for s ≤ a• Should return 1 for s ≥ b• Should vary smoothly between them• Can use, e.g., to trans ition between two colors:Lerp(step(s,a,b),Color1,Color2)21Linear StepVary linearly between a and b:Simple, quickNot C1-continuous•Abrupt•Can cause Mach bandinglinearstep(s,a,b) =0, s ≤as−ab−a, a ≤s ≤b1 b≤s⎧⎨⎪⎪⎩⎪⎪s01a b22“SmoothStep”Cubic curve:Tangent is horizontal at start and end•C1-continuousSpecial-case of a Hermite Spline:•Cubic spline defined by two interpolating points and two tangentsIn RenderMan shading language librarysmoothstep(s,a,b) =0, s ≤a−2s−ab−a⎛⎝⎜⎞⎠⎟3+3s−ab−a⎛⎝⎜⎞⎠⎟2, a ≤s≤b1 b≤s⎧⎨⎪⎪⎩⎪⎪s01a b23Shape: CutoffTruncate bottom of pyramidsmooth transition zone (fairly sharp in the example)24Shape: CutonTruncate top of pyramidsmooth transition zone (gradual in the example)25ShadowingShadows & shadow placement important for cinematographyCan control shadows for artistic effectThink of shadow as “volume of darkness”illumination inhibited inside the volumeControl:Shadow selectionShadow directionShadow sharingFake shadowsShape trimmingShadow softeningTechniques can be implemented using shadowmaps & procedural shading26Basic Shadows27Shadow SelectionCube casts no shadow28Shadow Directionmove shadowmap camera away from light position to control where shadows fallsurprisingly acceptable, especially for shadows cast on distant surface29Shadow Sharinguse one light’s shadowmap for other lightsprevent other lights from washing out important shadows30Fake shadowsPlace a virtual shadow-casting superellipse in space. known as a “blocker”(Real-world, place opaque cards in front of light)31Shape TrimmingMake large blocker, place it to trim the shape of the lightCan animate blocker for effects like door opening offscreen32Shadow SofteningEffect of shadow on light doesn’t need to


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UCSD CSE 167 - Computer Cinematography

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