lecture iii: color in image & video · most sources used for illumination emit white, or...

LECTURE III

COLOR IN IMAGE amp VIDEO

DR OUIEM BCHIR

1

COLOR SCIENCE

Light and Spectra

Light is a narrow range of electromagnetic energy

Electromagnetic waves have the properties of frequency and

wavelength

The distance from peak to peak of the wave is called the wavelength

Color is characterized by the wavelength content of the light

Laser light consists of a single wavelength eg a ruby laser

produces a bright scarlet-red beam

2

Most light sources produce

contributions over many

wavelengths

1048698 Human cannot detect all light just contributions that fall in the ldquovisible wavelengthsrdquo

Short wavelengths produce a blue sensation long wavelengths produce a red one

The human eye responds to the band of wavelengths between 300nm and 700nm

3

Most sources used for illumination emit white or nearly white

light

Using a prism Newton showed that white light is made up of

contributions from all of the visible wavelengths

4

Spectrophotometer device used to measure visible light by

reflecting light from a diffraction grating (a ruled surface)

that spreads out the different wavelengths

Visible light is electromagnetic wave in [300nm 700nm]

5

White light contains all

the colors of a rainbow

HOW MATERIALS MODIFY

LIGHT

Vision occurs when light from surrounding objects reaches the eye

Materials modify the light incident upon them in several ways

Light can be reflected from a surface

Light can be absorbed by or transmitted through a surface

In many cases light is both absorbed by and reflected from a surfaces

The amount of absorption and reflection depends on the wavelength resulting in some wavelengths being absorbed and others reflected to varying degrees

The wavelengths that are reflected from an object and perceived by the human eye is what gives an object its color

6

In this example

red light is reflected

Blue and green are absorbed

Object appears red

7

Light can be transmitted or passed through an object in varying

degrees The amount of transmission can also be dependent on

the wavelength

8

Light is passed through from a

colorless transparent object

unchanged

Object will appear clear

(eg window)

9

Light is partially transmitted

Certain wavelengths are

transmitted others are absorbed

This type of objects appear

translucent and colored (eg

colored glass)

This is the principle behind absorptive filters which are used to

remove or pass certain wavelengths of light

A surface can create the perception of color by absorbing

certain wavelengths and reflecting others

10

Inks are formulated to absorb certain

wavelengths while reflecting others

Only wavelengths that are present can be reflected

An object will appear its ldquousualrdquo color under white light because

all wavelengths are present to be reflected

11

A ldquoredrdquo apple appears red

under white light because red

is reflected and other

wavelengths are absorbed

12

Apple is illuminated with

light of a single color

(green)

bull No wavelength in the red region

to be reflected

bull All light is absorbed

bull Apple appears dark

The apparent color of an object is the

product of

1 Object characteristics

2 Light source used for illumination of that object

+

Human vision

13

HUMAN VISION

It seems likely that the brain makes use of differences R-G G-B

and B-R as well as combining all of R G and B into a high-light-

level achromatic channel

14

SPECTRAL SENSITIVITY

OF THE EYE

The eye is most sensitive to light in the middle of the visible

spectrum

The sensitivity of our receptors is also a function of

wavelength

The Blue receptor sensitivity is much smaller than the Red and

Green

15

Luminous-efficiency function= Overall sensitivity

1048698 Denoted V(λ)

1048698 Formed as the sum of the response curves for Red Green and Blue

16

R G and B cones and

Luminance Efficiency

curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS

1048698 Camera systems are made in a similar fashion a studio-quality

camera has three signals produced at each pixel location

(corresponding to a retinal position)

1048698 Analog signals are converted to digital truncated to integers

and stored

1048698 If the precision used is 8-bit then the maximum value for any of

R G B is 255 and the minimum is 0

1048698 However the light entering the eye of the computer user is that

which is emitted by the screen Therefore we need to know the

light E(λ) entering the eye

18

GAMMA CORRECTION

1048698 The light emitted is in fact roughly proportional to the voltage

raised to a power this power is called gamma (γ)

1048698 If the value in the red channel is R

1048698 The screen emits light proportional to R^γ

1048698 The value of gamma is around 22

19

EFFECT OF CRT ON LIGHT EMITTED

FROM SCREEN

Light output with no gamma-correction applied

darker values are displayed too dark

20

Effect of pre-correcting signals by applying the power law R1γ

21

COLOR SCIENCE

Light and Spectra

Light is a narrow range of electromagnetic energy

Electromagnetic waves have the properties of frequency and

wavelength

The distance from peak to peak of the wave is called the wavelength

Color is characterized by the wavelength content of the light

Laser light consists of a single wavelength eg a ruby laser

produces a bright scarlet-red beam

2



wavelengths




3


light



4





5




LIGHT








6

In this example



Object appears red

7



the wavelength

8



unchanged


(eg window)

9






colored glass)





10






11





12



(green)


to be reflected




product of



+

Human vision

13

HUMAN VISION




14


OF THE EYE


spectrum


wavelength


Green

15




16

R G and B cones and


curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21



wavelengths




3


light



4





5




LIGHT








6

In this example



Object appears red

7



the wavelength

8



unchanged


(eg window)

9






colored glass)





10






11





12



(green)


to be reflected




product of



+

Human vision

13

HUMAN VISION




14


OF THE EYE


spectrum


wavelength


Green

15




16

R G and B cones and


curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21


light



4





5




LIGHT








6

In this example



Object appears red

7



the wavelength

8



unchanged


(eg window)

9






colored glass)





10






11





12



(green)


to be reflected




product of



+

Human vision

13

HUMAN VISION




14


OF THE EYE


spectrum


wavelength


Green

15




16

R G and B cones and


curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21





5




LIGHT








6

In this example



Object appears red

7



the wavelength

8



unchanged


(eg window)

9






colored glass)





10






11





12



(green)


to be reflected




product of



+

Human vision

13

HUMAN VISION




14


OF THE EYE


spectrum


wavelength


Green

15




16

R G and B cones and


curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21


LIGHT








6

In this example



Object appears red

7



the wavelength

8



unchanged


(eg window)

9






colored glass)





10






11





12



(green)


to be reflected




product of



+

Human vision

13

HUMAN VISION




14


OF THE EYE


spectrum


wavelength


Green

15




16

R G and B cones and


curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21

In this example



Object appears red

7



the wavelength

8



unchanged


(eg window)

9






colored glass)





10






11





12



(green)


to be reflected




product of



+

Human vision

13

HUMAN VISION




14


OF THE EYE


spectrum


wavelength


Green

15




16

R G and B cones and


curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21



the wavelength

8



unchanged


(eg window)

9






colored glass)





10






11





12



(green)


to be reflected




product of



+

Human vision

13

HUMAN VISION




14


OF THE EYE


spectrum


wavelength


Green

15




16

R G and B cones and


curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21



unchanged


(eg window)

9






colored glass)





10






11





12



(green)


to be reflected




product of



+

Human vision

13

HUMAN VISION




14


OF THE EYE


spectrum


wavelength


Green

15




16

R G and B cones and


curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21



10






11





12



(green)


to be reflected




product of



+

Human vision

13

HUMAN VISION




14


OF THE EYE


spectrum


wavelength


Green

15




16

R G and B cones and


curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21




11





12



(green)


to be reflected




product of



+

Human vision

13

HUMAN VISION




14


OF THE EYE


spectrum


wavelength


Green

15




16

R G and B cones and


curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21





12



(green)


to be reflected




product of



+

Human vision

13

HUMAN VISION




14


OF THE EYE


spectrum


wavelength


Green

15




16

R G and B cones and


curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21


product of



+

Human vision

13

HUMAN VISION




14


OF THE EYE


spectrum


wavelength


Green

15




16

R G and B cones and


curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21

HUMAN VISION




14


OF THE EYE


spectrum


wavelength


Green

15




16

R G and B cones and


curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21


OF THE EYE


spectrum


wavelength


Green

15




16

R G and B cones and


curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21




16

R G and B cones and


curve

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21

IMAGE FORMATION

MODEL

17

C(λ) Color signal

C(λ ) = E(λ )S(λ )

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21

CAMERA SYSTEMS





and stored






18

GAMMA CORRECTION






19


FROM SCREEN



20


21

GAMMA CORRECTION






19


FROM SCREEN



20


21


FROM SCREEN



20


21


21

lecture iii: color in image & video · most sources used for illumination emit white, or...

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