Handout_PopelColor_4Slideperpage.pdf

(1)

CSE

409:

Computer

Graphics

Color

Department of CSE, BUET Department of CSE, BUET Acknowledgements: parts of information and pictures has been collected from MIT, Berkeley and UVa

Md. Tanvir Al Amin

Color

To

understand

how

to

make

realistic

images,

we

need

a

basic

understanding

of

the

physics

and

physiology

of

vision.

To

understand

how

to

make

realistic

images,

we

need

a

basic

understanding

of

the

physics

and

physiology

of

vision.

Elements

of

color

Elements

of

color

Elements

of

color

Elements

of

color

The

color

of

an

object

depends

on

• object

itself

The

color

of

an

object

depends

on

• object

itself

Department of CSE, BUET Department of CSE, BUET

• light

source

• color

of

the

surrounding

area

• Human

visual

system

• light

source

• color

of

the

surrounding

area

• Human

visual

system

Color

• Some

objects

reflect

light,

whereas

others

also

transmit

light

• Some

objects

reflect

light,

whereas

others

also

transmit

light

• When

a

surface

that

reflects

only

pure

blue

light

is

illuminated

with

pure

red

light,

it

appears

black.

• Similarly

a

pure

green

light

viewed

through

glass

that

transmits

only

pure

red

will

also

appear

black.

• When

a

surface

that

reflects

only

pure

blue

light

is

illuminated

with

pure

red

light,

it

appears

black.

• Similarly

a

pure

green

light

viewed

through

glass

that

transmits

only

y p

pure

red

will

also

appear

pp

black.

Basics

of

Color

Physics:

• Illumination

Physics:

• Illumination

Illumination

–

Electromagnetic

spectra

• Reflection

–

Material

properties

–

Surface

geometry

and

microgeometry

(i.e.,

polished

versus

matte

versus

brushed)

Perception

Illumination

–

Electromagnetic

spectra

• Reflection

–

Material

properties

–

Surface

geometry

and

microgeometry

(i.e.,

polished

versus

matte

versus

brushed)

Perception

(2)

Physiology

of

Vision

The

eye:

The

eye:

The

retina

• Rods

• Cones

C l !

The

retina

• Rods

• Cones

C l !

–

Color!

–

Color!

Physiology

of

Vision

The center of the retina is a densely packed region

ll d th

ff

The center of the retina is a densely packed region

ll d th

ff

called the

fovea

.

Cones much denser here than the

Cones much denser here than the periphery

periphery

called the

fovea

.

Cones much denser here than the

Cones much denser here than the periphery

periphery

Physiology

of

Vision:

Cones

Three

types

of

cones:

• L

or

R

,

most

sensitive

to

red

light

(610

nm)

• M

or

G

most sensitive to green light (560 nm)

Three

types

of

cones:

• L

or

R

,

most

sensitive

to

red

light

(610

nm)

• M

or

G

most sensitive to green light (560 nm)

• M

or

G

,

most

sensitive

to

green

light

(560

nm)

• S

or

B

,

most

sensitive

to

blue

light

(430

nm)

• M

or

G

,

most

sensitive

to

green

light

(560

nm)

• S

or

B

,

most

sensitive

to

blue

light

(430

nm)

• Color

blindness

results

from

missing

cone

type(s)

• Color

blindness

results

from

missing

cone

type(s)

Physiology

of

Vision:

The

Retina

Strangely,

rods

and

cones

are

at

the

back

of the retina behind a mostly

‐

Strangely,

rods

and

cones

are

at

the

back

of the retina behind a mostly

‐

back

of

the

retina,

behind

a

mostly

transparent

neural

structure

that

collects

their

response.

http://www.trueorigin.org/retina.asp

back

of

the

retina,

behind

a

mostly

transparent

neural

structure

that

collects

their

response.

http://www.trueorigin.org/retina.asp

(3)

Perception:

Metamers

A

given

perceptual

sensation

of

color

derives

from

the

stimulus

of

all

three

cone

types

A

given

perceptual

sensation

of

color

derives

from

the

stimulus

of

all

three

cone

types

Identical perceptions of color can thus be caused

by very different spectra

Identical perceptions of color can thus be caused

by very different spectra

What

is

Color?

Spectral Power Electromagnetic Wave

Reflectance Spectrum

Spectral

Distribution

Illuminant D65

(nm)

Power Distribution

What

is

Color?

Spectral Power Distribution

Neon Lamp

Reflectance Spectrum

Distribution

Illuminant F1

Spectral Power Distribution Under D65

Spectral Power Distribution

What

is

Color?

Observer

Stimulus

(4)

What

is

Color?

Spectral Sensibility of the L, M and S

S

M

L

Ganglion Cells

Horizontal Cells Bipolar

Cells RodCone

Cones

Rods

Cones

Cones and Rods

Distribution of

Light

Retina Optic Nerve Amacrine

Cells

What

is

Color?

Right LGN

Visual

Left LGN

Visual

Cortex

LGN = Lateral Geniculate Nucleus

Perception:

Other

Gotchas

Color

perception

is

also

difficult

because:

• It

varies

from

person

to

person

Color

perception

is

also

difficult

because:

• It

varies

from

person

p

to

person

p

• It

is

affected

by

adaptation

(stare

at

a

light

bulb…

don’t)

• It

is

affected

by

surrounding

color:

p

• It

is

affected

by

adaptation

(stare

at

a

light

bulb…

don’t)

• It

is

affected

by

surrounding

color:

Perception:

Relative

Intensity

We

are

not

good

at

judging

absolute

intensity

Let’s

illuminate

pixels

with

white

light

on

scale

of

0 ‐

1.0 We

are

not

good

at

judging

absolute

intensity

Let’s

illuminate

pixels

with

white

light

on

scale

of

0 ‐

1.0 Intensity

difference

of

neighboring

colored

rectangles

with

intensities:



0.10 →

0.11 (10%

change)



0.50 →

0.55 (10%

change)

will look the same

Intensity

difference

of

neighboring

colored

rectangles

with

intensities:



0.10 →

0.11 (10%

change)



0.50 →

0.55 (10%

change)

will look the same

will

look

the

same

We

perceive

relative

intensities,

not

absolute

will

look

the

same

(5)

Representing

Intensities

Remaining

in

the

world

of

black

and

white…

Use photometer to obtain min and max brightness of monitor

Remaining

in

the

world

of

black

and

white…

Use photometer to obtain min and max brightness of monitor

_II

Use

photometer

to

obtain

min

and

max

brightness

of

monitor

This

is

the

dynamic

range

Intensity

ranges

from

min,

I

0

,

to

max,

1.0 How

do

we

represent

256 shades

of

gray?

Use

photometer

to

obtain

min

and

max

brightness

of

monitor

This

is

the

dynamic

range

Intensity

ranges

from

min,

I

0

,

to

max,

1.0 How

do

we

represent

256 shades

of

gray?

Equal distribution between min and max fails

II

₀

=I

₀

II

₁

= rI

₀

II

₂

= rI

₁

= r

2

2 _II

0

0 relative change near max is much smaller than near I

relative change near max is much smaller than near I

₀₀

Preserve % change

II

nn

= r

nn

II

00

, n > 0 r = (1/I

00

))

1/n1/n

So, I

jj

= I

00(n(n--j)/nj)/n

0

0 ≤

≤

j

≤≤

n

For n = 3, r =2, we got four intensities

⅛

, ¼, ½, 1

For n = 3, r =2, we got four intensities

⅛

, ¼, ½, 1

relative change near max is much smaller than near I

₀₀

Preserve % change

II

nn

= r

nn

II

00

, n > 0 r = (1/I

00

))

1/n1/n

So, I

jj

= I

00(n(n--j)/nj)/n

0

0 ≤

≤

j

≤≤

n

For n = 3, r =2, we got four intensities

⅛

, ¼, ½, 1

For n = 3, r =2, we got four intensities

⅛

, ¼, ½, 1

…

II

₂₅₅

=rI

₂₅₄

=r

255

255 _II

0

Dynamic

Ranges

Dynamic

Range

Max

#

of

Display

(max

/

min

illum)

Perceived

Dynamic

Range

Max

#

of

Display

(max

/

min

illum)

Perceived

Intensities

(r

=

1.01)

CRT:

50 ‐

200

400 ‐

530 Photo

(print)

100

465 Photo

(slide)

1000

700 B/W printout

100

465 Intensities

(r

=

1.01)

CRT:

50 ‐

200

400 ‐

530 Photo

(print)

100

465 Photo

(slide)

1000

700 B/W printout

100

465 B/W

printout

100

465 Color

printout

50

400 Newspaper

10

234 B/W

printout

100

465 Color

printout

50

400 Newspaper

10

234

Gamma

Correction

But

most

display

devices

are

inherently

nonlinear:

Intensity =

k(voltage)



is between 2 2 and 2 5 on most monitors

But

most

display

devices

are

inherently

nonlinear:

Intensity =

k(voltage)



is between 2 2 and 2 5 on most monitors

Intensity

=

k(voltage)



,



is

between

2.2 and

2.5 on

most

monitors

For

intensity

I,

we

need

to

find

j

=

ROUND(log

r

(I/I

0

))

from

lookup

table

After

j

is

found,

we

calculate

I

j

=

r

j

I

0

and

V

j

=

ROUND((I

j

/K)

1/

Common

solution:

gamma

correction

• Post

‐

transformation

on

intensities

to

map

them

to

linear

range

on

display

device:

Intensity

=

k(voltage)



,



is

between

2.2 and

2.5 on

most

monitors

For

intensity

I,

we

need

to

find

j

=

ROUND(log

r

(I/I

0

))

from

lookup

table

After

j

is

found,

we

calculate

I

j

=

r

j

I

0

and

V

j

=

ROUND((I

j

/K)

1/

Common

solution:

gamma

correction

• Post

‐

transformation

on

intensities

to

map

them

to

linear

range

on

display

device:

• Can

have

separate



for

R,

G,

B

How

many

intensities

are

enough?

r

=

(1/I

0

)

1/n

So,

n

=

log

1.01

(1/I

0

)

• Can

have

separate



for

R,

G,

B

How

many

intensities

are

enough?

r

=

(1/I

0

)

1/n

So,

n

=

log

1.01

(1/I

0

)



1

x

y



Gamma

Correction

Some

monitors

perform

the

gamma

correction

in

hardware

(SGIs)

Some

monitors

perform

the

gamma

correction

in

hardware

(SGIs)

Others

do

not

(most

PCs)

Tough

to

generate

images

that

look

good

on

both

platforms

(i.e.

images

from

web

pages)

Others

do

not

(most

PCs)

Tough

to

generate

images

that

look

good

on

both

(6)

Specifying

Color

perception

usually

involves

three

quantities:

• Hue

: Distinguishes between colors like red, green, blue, etc

Color

perception

usually

involves

three

quantities:

• Hue

: Distinguishes between colors like red, green, blue, etc

• Saturation

: How far the color is from a gray of equal intensity

• Lightness

: The perceived intensity of a reflecting object

Sometimes

lightness

is

called

brightness

if

the

object

is

emitting

light

instead

of

reflecting

it.

• Saturation

: How far the color is from a gray of equal intensity

• Lightness

: The perceived intensity of a reflecting object

Sometimes

lightness

is

called

brightness

if

the

object

is

emitting

light

instead

of

reflecting

it.

In

order

to

use

color

precisely

in

computer

graphics,

we

need

to

be

able

to

specify

and

measure

colors.

In

order

to

use

color

precisely

in

computer

graphics,

we

need

to

be

able

to

specify

and

measure

colors.

Combining

Colors

Additive (RGB)

_{Subtractive (CMYK)}

Additive (RGB)

Shining colored lights

on a white ball

Subtractive (CMYK)

Mixing paint colors and

illuminating with white light

How

Do

Artists

Do

It?

Artists

often

specify

color

as

tints,

shades,

and

tones

of

saturated

(pure)

pigments

Artists

often

specify

color

as

tints,

shades,

and

tones

of

saturated

(pure)

pigments

Tint

:

Gotten

by

adding

white

to

a

pure

pigment,

decreasing

saturation

Shade

:

Gotten

by

adding

black

to

a

pure

pigment,

decreasing

lightness

pigments

Tint

:

Gotten

by

adding

white

to

a

pure

pigment,

decreasing

saturation

Shade

:

Gotten

by

adding

black

to

a

pure

pigment,

decreasing

lightness

White

Tone

:

Gotten

by

adding

white

and

black

to

a

pure

pigment

Tone

:

Gotten

by

adding

white

and

black

to

a

pure

pigment

Pure Color

Black

Grays

Tones

HSV

Color

Space

Computer

scientists

frequently

use

an

intuitive

color

space

that

corresponds

to

tint,

shade,

and

tone:

Computer

scientists

frequently

use

an

intuitive

color

space

that

corresponds

to

tint,

shade,

and

tone:

• Hue

‐

The

color

we

see

(red,

green,

purple)

• Saturation

‐

How

far

is

the

color

from

gray

(pink

is

less

saturated

than

red,

sky

blue

is

less

saturated

than

royal

blue)

• Brightness

(Luminance)

‐

How

bright

is

the

color

(how

bright

are

• Hue

‐

The

color

we

see

(red,

green,

purple)

• Saturation

‐

How

far

is

the

color

from

gray

(pink

is

less

saturated

than

red,

sky

blue

is

less

saturated

than

royal

blue)

• Brightness

(Luminance)

‐

How

bright

is

the

color

(how

bright

are

(7)

HSV

Color

Model

Hue (H) is the angle

around the vertical axis

Saturation (S) is a value

from 0 to 1 indicating

how far from the vertical

axis the color lies

Value (V) is the height of

the hexcone”

HSV

Color

Model

H S V Color

0

1.0

1.0 Red

120

1.0

1.0 Green

240

1.0

1.0 Blue

*

0.0

1.0 White

*

0.0

0.5 Gray

*

0.0 Black

60

1.0

1.0 ?

270

0.5

1.0 ?

270

0.0

0.7 ?

Intuitive

Color

Spaces

A top

A top--down view of hexcone

down view of hexcone

A top

A top--down view of hexcone

down view of hexcone

HSV

Color

Space

A

more

intuitive

color

space

A

more

intuitive

color

space

Value

Saturation

• H

=

Hue

• S

=

Saturation

• V

=

Value

(or

brightness)

• H

=

Hue

• S

=

Saturation

• V

=

Value

(or

brightness)

Value

(8)

Precise

Color

Specifications

• Pigment

‐

mixing

is

subjective

‐‐‐

depends

on

human

observer,

surrounding

colors,

lighting

of

the

environment,

etc

• Pigment

‐

mixing

is

subjective

‐‐‐

depends

on

human

observer,

surrounding

colors,

lighting

of

the

environment,

etc

• We

need

an

objective

color

specification

• We

seek

help

from

a

branch

of

Physics

“Colorimetry”

• Light

is

electromagnetic

energy

in

the

400 to

700 nm

wavelength

range

• Dominant wavelength

is

the

wavelength

of

the

color

we

“see”

E it ti

it

i th

ti

f

l

d li ht t

hit li ht

• We

need

an

objective

color

specification

• We

seek

help

from

a

branch

of

Physics

“Colorimetry”

• Light

is

electromagnetic

energy

in

the

400 to

700 nm

wavelength

range

• Dominant wavelength

is

the

wavelength

of

the

color

we

“see”

E it ti

it

i th

ti

f

l

d li ht t

hit li ht

• Excitation purity

is

the

proportion

of

pure

colored

light

to

white

light

• Luminance

is

the

amount

(or

intensity)

of

the

light

• Excitation purity

is

the

proportion

of

pure

colored

light

to

white

light

• Luminance

is

the

amount

(or

intensity)

of

the

light

Electromagnetic

Spectrum

Visible

light

frequencies

range

between

...

• Red

=

4.3 x

1014

hertz

(700nm)

Visible

light

frequencies

range

between

...

• Red

=

4.3 x

1014

hertz

(700nm)

(

)

• Violet

=

7.5 x

1014

hertz

(400nm)

(

)

• Violet

=

7.5 x

1014

hertz

(400nm)

Visible

Light

Hue

=

dominant

frequency

(highest

peak)

Saturation

=

excitation

purity

(ratio

of

highest

to

rest)

Hue

=

dominant

frequency

(highest

peak)

Saturation

=

excitation

p

purity

y (

(ratio

of

f g

highest

to

rest)

)

Lightness

=

luminance

(area

under

curve)

p

y (

f g

)

Lightness

=

luminance

(area

under

curve)

White Light

Orange Light

How

well

do

we

see

color?

What color do we see the best?

• Yellow

‐

green

at

550 nm

What color do we see the worst?

What color do we see the best?

• Yellow

‐

green

at

550 nm

What color do we see the worst?

• Blue

at

440 nm

Flashback: Colortables (colormaps) for color storage

• Which

RGB

value

gets

the

most

bits?

Can perceive color differences of 10 nm at extremes (violet and red) and 2 nm between blue

and yellow

diff

di i

l

k lik

h

l

What color do we see the worst?

• Blue

at

440 nm

Flashback: Colortables (colormaps) for color storage

• Which

RGB

value

gets

the

most

bits?

Can perceive color differences of 10 nm at extremes (violet and red) and 2 nm between blue

and yellow

diff

di i

l

k lik

h

l

Metamers

– different energy radiations look like the same color

Color perception also affected by surrounding light and adaptation

(9)

Just

noticeable

difference

(JND)

128 fully

saturated

hues

can

be

distinguished

Cannot

perceive

hue

differences

with

less

saturated

light.

128 fully

saturated

hues

can

be

distinguished

Cannot

p

perceive

hue

differences

with

less

saturated

light.

g

Sensitivity

to

changes

in

saturation

for

a

fixed

hue

and

brightness

ranges

from

16 to

23 depending

on

hue.

p

g

Sensitivity

to

changes

in

saturation

for

a

fixed

hue

and

brightness

ranges

from

16 to

23 depending

on

hue.

Human

Color

Vision

Humans

have

3 light

sensitive

pigments

in

their

cones,

called

L,

M,

and S

Humans

have

3 light

sensitive

pigments

in

their

cones,

called

L,

M,

and S

and

S

Each

has

a

different

spectral

response

curve

:

and

S

Each

has

a

different

spectral

response

curve

:

L





L

(



)

E

(



)

d

M





M

(



)

E

(



)

d

This

leads

to

metamerism

“Tristimulus”

color

theory

This

leads

to

metamerism

“Tristimulus”

color

theory

M



M

(



)

E

(



)

d

S





S

(



)

E

(



)

d

Color

Spaces

Three

types

of

cones

suggests

color

is

a

3D

quantity.

How

to

define

3D

color

space?

Three

types

of

cones

suggests

color

is

a

3D

quantity.

How

to

define

3D

color

space?

Idea:

• Shine given wavelength (



) on a screen

• User must control three lasers producing three wavelengths (say R=700nm,

G=546nm, and B=436nm)

• Adjust intensity of RGB until colors are identical

Idea:

• Shine given wavelength (



) on a screen

• User must control three lasers producing three wavelengths (say R=700nm,

G=546nm, and B=436nm)

• Adjust intensity of RGB until colors are identical

• Note phosphors of TV are not perfect RGB

emitters as the results to right demonstrate

• Adjust intensity of RGB until colors are identical

• Note phosphors of TV are not perfect RGB

emitters as the results to right demonstrate

CIE

color

matching:

same

for

color

Primaries

(synthesis)

at

435.8,

546.1 and

700

• Chosen

for

robust

reproduction,

good

separation

in

red

‐

green

Primaries

(synthesis)

at

435.8,

546.1 and

700

• Chosen

for

robust

reproduction,

good

separation

in

red

‐

green

(10)

CIE

color

matching

Primaries

(synthesis)

at

435.8,

546.1 and

700

• For

robust

reproduction,

good

separation

in

red

‐

green

Primaries

(synthesis)

at

435.8,

546.1 and

700

• For

robust

reproduction,

good

separation

in

red

‐

green

Measure

matching

curves

as

function

of

wavelength

(analysis)

N t th t th

i

Measure

matching

curves

as

function

of

wavelength

(analysis)

N t th t th

i

Note

that

the

primaries

(monochromatic

435.8,

546.1 and

700nm)

are

not

the

same

as

the

matching

curve!!!)

Note

that

the

primaries

(monochromatic

435.8,

546.1 and

700nm)

are

not

the

same

as

the

matching

curve!!!)

Color Matching Problem

Some colors cannot be produced using only positively

weighted primaries

Some colors cannot be produced using only positively

weighted primaries

Solution: add light on the other side!

Color

matching

experiment

1 Color

matching

experiment

1