#16: Computer Cinematography:

The Toy Story Lighting Model

CSE167: Computer GraphicsInstructor: Ronen Barzel

UCSD, Winter 2006

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Film 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 eye

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Real-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 background

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Real-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…

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Computer 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 this

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Lighting Model

Several independent aspects selection shape shadowing texture dropoff direction properties

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Basic setup

only fill lights

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One key light

(will talk about the beam-of-light effect later)

(will talk about the beam of light effect later)

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Selection

torus unaffected by key light

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Shape

Region of space affected by light Real-world commonly use spotlights, barn door lights

CG model: Generalized cone/pyramid Soft edges Cuton and cutoff

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Generalized cone/pyramid

A “superellipse” shape swept into a pyramid superellipse varies between circle and rectangle pyramid may be truncated or sheared

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2-dimensional superellipse:

n=1 is ordinary ellipse or circle need to handle 4 quadrants carefully:

• avoid exponentiation of negative numbers• use

2D Superellipse

x

y

⎡

⎣⎢

⎤

⎦⎥=

acosn(θ)bsinn(θ)

⎡

⎣⎢

⎤

⎦⎥, where 0 ≤θ ≤ 2π and 0 ≤ n < ∞

x

a⎛⎝⎜

⎞⎠⎟

2 n

+y

b⎛⎝⎜

⎞⎠⎟

2 n

= 1

y = b 1−x

a⎛⎝⎜

⎞⎠⎟

2 n⎛

⎝⎜

⎞

⎠⎟

n 2

xn =−xnif x < 0

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Aside: 3D Superellipsoid

x

y

z

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥=

acosn1 (φ)cosn2 (θ)bcosn1 (φ)sinn2 (θ)

csinn1 (φ)

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥ 

where 0 ≤θ ≤ 2π

and - π2 ≤φ≤ π2

and 0 ≤ n < ∞

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Returning to lighting…

A 2D superellipse swept into a pyramid pyramid may be truncated or sheared

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Shape: Rounded Rectangle

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Shape: Sheared Barn Door

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Soft Edges

Full intensity in central pyramid No effect (zero intensity) outside outer pyramid Smooth falloff between them

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Shape: Hard Edge

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Shape: Soft Edges

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Step 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 )

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Linear Step

Vary linearly between a and b:

Simple, quick Not C1-continuous

• Abrupt• Can cause Mach banding

linearstep(s,a,b) =

0, s≤as−ab−a

, a≤s≤b

1 b≤s

⎧

⎨⎪⎪

⎩⎪⎪

s0

1

a b

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“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 library

smoothstep(s,a,b) =

0, s≤a

−2s−ab−a

⎛⎝⎜

⎞⎠⎟

3

+ 3s−ab−a

⎛⎝⎜

⎞⎠⎟

2

, a≤s≤b

1 b≤s

⎧

⎨

⎪⎪

⎩

⎪⎪

s0

1

a b

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Shape: Cutoff

Truncate bottom of pyramid smooth transition zone (fairly sharp in the example)

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Shape: Cuton

Truncate top of pyramid smooth transition zone (gradual in the example)

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Shadowing

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 shading

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Basic Shadows

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Shadow Selection

Cube casts no shadow

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Shadow Direction

move shadowmap camera away from light position to control where shadows fall surprisingly acceptable, especially for shadows cast on distant surface

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Shadow Sharing

use one light’s shadowmap for other lights prevent other lights from washing out important shadows

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Fake shadows

Place a virtual shadow-casting superellipse in space. known as a “blocker” (Real-world, place opaque cards in front of light)

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Shape Trimming

Make large blocker, place it to trim the shape of the light Can animate blocker for effects like door opening offscreen

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Shadow Softening

Effect of shadow on light doesn’t need to be binary on/off:

Only partially inhibit the light (scale down rather than remove)

Soften edges• shadowmap edge blurring techniques• smooth falloff in shadow blockers

Rather than inhibiting light, lerp to a different color• e.g. can be effective to lerp to dark blue• default: lerp to black gives normal shadowing

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Texture

We use images to create texture on surfaces Can also use images to create texture in lighting, via

projection Cookie: single-channel matte used as a “cookie cutter”

(a.k.a. cucaloris)• changes shape• breaks up light

Slide: color image used as a slide projector• can blur based on distance for getting unfocused projection (e.g.

from a TV) Noise: 2D procedural noise

• makes light “dirty”

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Texture files

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Cookie changing light shape

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Cookie breaking up light

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Projecting a color slide

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Intensity distribution

The intensity of the light can vary across the shape Beam distribution

• cosinen across beam Distance falloff

• inverse-linear, inverse-square, etc.

Usually only used for practical light effects Choosing dropoff exponent parameters not intuitive Easier to adjust lighting using soft shape, cutoff, blockers

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Uniform Intensity

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Beam Distribution

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Distance Falloff

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Light Ray Direction

Two common CG options: radial from apex of pyramid parallel

Subtle effect on shading and highlights effect more noticeable as light source is close to object

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Light Properties

Properties of the “photons” that are incident on the surfaces

Intensity: nominal intensity at a target point within the shape• Attenuated by: shape boundary, shadows, cookies, noise, dropoff

Color: nominal RGB color of the light• Filtered by: slide, noise, colored shadow

Effect: how much light contributes to CG illumination model• Separate scale factor for ambient, diffuse, specular• Fill lights typically ambient-only, to avoid unwanted specular

hilights Other… depending on what the surface shading model can handle

• “ultraviolet” 4th color channel. “fluorescent” surface responds by self-illuminating

Lights that don’t actually light… Surface can react specially to specific “light” types:

• lights whose effect is to attenuate other lights• lights that affect surface properties

• e.g. use a slide to project a bump map?

• etc…

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Example 1

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Example 1 notes

Light shape/placement for image composition Barn door light frames character Nominally shines from offscreen window Placed for visual effect, not necessarily consistent with window

Softened (partial opacity) shadows of bed on wall

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Example 2

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Example 2 notes

Strips of light on the characters Not actually cast by the venetian blind slats Generated using a cookie, adjusted for best effect

Slats are lit from below to get desired glow

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Example 3

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Example 3 notes

Cookie (the one from slide 33) used for dappled leaves Separate cone-shaped spotlight w/cookie on soldier in

rear Characters in foreground lit by blue light for blue

highlights Practical light source: red LED

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Example 4

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Example 4 notes

Practical light source: Flashlight Cookie for lens ring effect Two light sources:

• one illuminates character• one generates fog effect, with cutoff to avoid obscuring character

“Ultraviolet” light white shirt responds with blue glow

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Example 5

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Example 5 notes

Shadow placement. Milk crate shadows adjusted so that “X” lands on Woody’s face Doesn’t obscure eyes or mouth

Blockers darken background Darkens mood Focuses attention on face in the foreground

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Example 6

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Example 6 notes

Soft edges of key light Buzz’s shadow direction adjusted for compositional effect Grazing-angle light on desk only, accentuates its texture Separate lights on Buzz only for extra highlighting Cookie animates from one frame to next for falling-rain

effect

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Fog in computer graphics

Simple version: depth cueing After lighting computation Blend between lit surface color and constant fog color, based

on depth Typically have a “near fog plane” then linear or exponential

fogging with distance Compute in pixel shaders Supported by some hardware No beam of light effects

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Lerp(e−dz,Clit,C fog)

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Atmospheric effects

Participating Medium Light bounces off air particles Some light gets absorbed by air particles Full simulation of multiple scattering is possible, using

monte carlo techniques• Gets best results for clouds, etc.

Simple approximation: ray marching, single scattering (Used for examples today) Start at surface, with lit color Take incremental steps along ray towards eye At each step:

• attenuate existing color / merge towards fog color• look up lighting (with shadows)• compute amount of light scattered towards eye• add to current color

Can be implemented in pixel shader Some systems support procedural volume shaders to run after

surface shading

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Real-time clouds

Various approximation techniques. Billboards, transparency, frustum culling, … Video shows some of the acceleration techniques Steve talked

about last class [play video]

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Done

Next class: Guest Lecture: Daniel Maskit, Digital Domain “A View From

The Trenches”