b44 lecture 16: light and shadow - harvard...

B44 Lecture 16: Light and Shadow

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Lecture 6Light

Science B44

Ted Adelson

Dan

Ker

sten

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1. Contrast and cell response2. Estimate the light intensity3. Discovering light and paint together4. Is it a shadow or a dark stain5. Is it a reflection or a light spot

Light

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We see surfaces as defined by their reflectanceindependently of the amount of light falling on it.

They are white (reflect a lot), grey (medium) orblack (hardly at all) whether we see them insunlight or dim room light.

But photometers, photographs, and photoreceptorsregister the light reflected from the surface.This can be million times more in sunlight thanroom light.

How do we recover the constant “paint” value??

First pass: Contrast

1. Contrast and cell response

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Bright light

1000 1000

600 200

600 - 200600 + 200

= ……%Contrast=Contrast = 60 - 2060 + 20

= 50%

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Medium light

100 100

60

for two adjacent areasContrast (difference/sum) is independent of light level

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Cells do respond to edge contrastAnd not much to the mean light level (they adapt)Nice, but isn’t there a problemCells only respond to contrast at edgesIt is not a point by point record of image intensityThis distorts the representation of light and darkHere are some examples

Hermann GridSimultaneous contrastCraik Cornsweet Illusion

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Hermann Grid


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Simultaneous contrast

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Craik Cornsweet Illusion

Centers of each square are equiluminant

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physical properties of stimulus

luminance (a) = luminance (b)

luminanceprofilea b

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just to be sure that the centers are really equiluminant

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Centers are equal, why do they look different?Hint from response to real disc of uniform brightness

ON center cells

luminance

cell responses12

Contrast at border is the only available signalMust “fill in” brightness from edge to central regionFilling in process fooled by an edge that looks like the

cell response profile to a real edge--> fills in inappropriately


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First Pass: Cells estimate contrast

Properties of receptive fields correctly suggest thatperception of light and dark depends strongly oncontrast from edges

Responses independent of light level, to some degreeBut importance of edges leads to some errorsSuppressed image data must be reconstructed or “filled

in”But this “filling in” cannot reconstruct the exact

original: the edge-based description is not invertibleIt fails on corners, intersections, shallow gradients

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Reflected light is the product of the lightfalling on the surface

And the reflectance (paint) of the surfaceHow do we find the reflectance or “paint”when we only have the light arriving at our

eyes?--> Estimate the light

2. Second Pass: Estimate the light

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Judging the intensity of the illuminationCorrect for that intensity to recover the

reflectance of each surface1. Take the average illumination from all surfaces

as half of the light intensity. The averagesurface will be gray, reflect half of the light

2. Take the maximum intensity in the scene as anestimate of the light, probably reflected by awhite surface

Simple if there are many surfaces, all at the sameangle to one light

Real scenes much more complex

Lightness constancy

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Gilchrist and apparent illuminantLight inferred based on apparent location in space

Hidden lights

Viewingpeep-hole

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By removing a corner of the target square, the targetis made to appear to be in either the brightly lit frontroom or the dimly lit far room

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By removing a corner of the target square, the targetis made to appear to be in either the brightly lit frontroom or the dimly lit far room


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Although the target and the light coming from it neverchange, observers see it as white when it is apparentlyin the front room but dark gray when it appears to bein the back room

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A visible shadow also changesthe apparent light falling ona surface

The light in the illuminatedarea is taken as muchstronger

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The light in the shadowed areais taken as dimmer

Correcting for the illuminantcan make two areas thatsend equal amount of lightlook very different

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SecondExample

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We "see" a light source and what it covers based oncues in the image (illumination edges, shadowedges, spatial layout of scene and lights)

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But the model of the light is simpleHas no memoryAnd is independent of our conscious knowledge


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Sometimes it is not possible to determine whatthe light is without at the same time makinga choice about the reflectance (paint)

Sometimes the choice of what is light andwhat is paint follows a rule of simplicity

Chose the solution with the fewest elements

3. Choosing light and paint together

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ProblemHow many ways can this pattern be explained?

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Paint solutionCould be 9 differentpatches of paint

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Light solutionCould be 9 lights

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Bent object9 different surfacesbent at odd angles andone light. But you canonly look at it fromone special position

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Simplest solution1 light,2 bends,2 paints


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Highlight: a reflectionon a shiny surface

Shading: the variation inreflected light due tothe change in theorientation of thesurface

Shadow: an area wherelight is blocked by anintervening object

Highlight

Shadow

Shading

4. Is it a shadow or a dark spot of paint?

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In addition to estimating the light (illuminant), italso helps to identify shadows and highlights(illumination effects, not paint)

Shadows are extremely complex shapes in thescene determined by the direction of thelight, the shape of the object casting theshadow and the surface on which it falls

Computer graphics can control and solve all ofthese variables

Vision does not have the time"Simple" rules to infer shadows

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Is this a shadow here?What are the cues?Darker but same colorNo thickness, substance of its

ownSimilar texture inside and

outsideX junctions of shadow and

surface contoursMany possible cues could be

usedBut few are

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Shadows must:1. Be darker than the

surface they are seen on2. Make X junctions with

surface contours3. Lie flat on the surface

(no volume)

Shadow Rules

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Just like a shadow, areflection does notbelong to the surface onwhich it is seen

It is not paintIt changes if the observer

moves, or the scenechanges, or the objectmoves

5. Is it a reflection or light spot of paint

Sphere examples from Roland W. Fleming36

Reflections are not likepaint

When the object moves,the reflections are notstuck on like paint

They change in complexways

5. Is it a reflection or light spot of paint

Teapot example from Dan Kersten


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A small highlight at one spot on a surface makes the wholesurface look glossy.

Remove the highlight or cover it with your finger and thewhole surface now looks matte (not glossy)

38What is wrong here?Verticality rule: an object’s reflections in a horizontalsurface are always vertically aligned with the object

39What is wrong here?Verticality rule: an object’s reflections in a horizontalsurface are always vertically aligned with the object 40

Verticality rule: an object’s reflections in a horizontalsurface are always vertically aligned with the object

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Reflections must be:1. Lighter than the surface they are seen on2. Vertically aligned with reflected objects if

reflection is on a horizontal surface (water)

Reflection Rules

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How do we see surface reflectance(lightness constancy)?

Contrast response in cells is a startAlso estimate the lightSometimes, solve for light and paint

together, choose simplest pairingShadows need to be darker, lie on surfaceReflections need to be lighter

1 Minute QuizNo lecture MondayReadings for Wednesday BV 12: 365-373Electronic Article: EA 7 on web site.

Summary


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Surface reflectance: the proportion ofthe incident light that is reflected. Asurface with high reflectance is white,low reflectance is black

Lightness constancy: the ability to see thesurface reflectance independently ofthe intensity of the incident light

X-junction: two contours that cross overto form an X and serve as a cue totransparency or shadow

Glossary

b44 lecture 16: light and shadow - harvard...

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