color image processing 1

8/8/2019 Color Image Processing 1

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Color Image

Processing


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Outline

Image processing flowchart in a DSC

Sensor,Aperture,and Lens

Preprocessing

White Balance

Demosaicking

Color Transformation

Postprocessing

Display/archive


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Image processing flowchart in a DSC(1/2)


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Image processing flowchart in a DSC(2/2)

(a) Raw image

(b) Preprocessing/ White balance

(c) Demosaicking

(d) Trans. ISO-RGB

(e) Trans. sRGB


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Sensor,Aperture,and Lens(1/6)


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Sensor,Aperture,and Lens(2/6)

CFA(Color Filter Array)

Exposure control

Focus control


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Sensor,Aperture,and Lens(3/6)CFA(Color Filter Array)

Bayer array


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Sensor,Apertu

re,and Lens(4/6)Exposure control

(a) Underexposed image

(b) Overexposed image

(c) Well-exposed image


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Sensor,Apertu

re,and Lens(6/6)Focus control

(a) Out-of-focus

(b) Lens position closer to

the required focal length

(c) An in-focus image block

(d) A plot of the focal measureversus the lens position


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Preprocessing(1/5)


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Preprocessing(2/5)

Linearization

Dark current compensation

Flare compensation


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Preprocessing(3/5)Linearization

Some cameras require that data be linearized since the

captured data resides in a nonlinear space

Cameras that include correction for nonlinear data usean opto-electronic conversion function (OECF)


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Preprocessing(4/5)Dark current compensation

A dark current signal is recored,which is due to thermally

generated electrons in the sensor substrate

Place an opaque mask along the adges ofthe sensorto givean estimate of intensity due to dark current alone

capture a drak image forthe given exposure time


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Preprocessing(5/5)Flare compensation

To subtract from the whole image a percentage ofthe

mean measured signal energy in a channel

To subtract a fixed percentage ofthe mean signal energyin the pixels neighborhood


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White Balance(1/3)


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White Balance White balance (WB) is the process of removing unrealistic color

casts, so that objects which appear white in person are renderedwhite in your photo. Proper camera white balance has to take intoaccount the "color temperature" of a light source, which refers to the

relative warmth or coolness of white light. Our eyes are very good atjudging what is white under different light sources, but digitalcameras often have great difficulty with auto white balance (AWB) --and can create unsightly blue, orange, or even green color casts.Understanding digital white balance can help you avoid these colorcasts, thereby improving your photos under a wider range of lightingconditions.


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Incorrect White Balance


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Correct white balance

More@ http://www.cambridgeincolour.com/tutorials/white-balance.htm


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White Balance(2/3)

gray world

BKGRK br ,,

maxmaxmax ,, BBGGRR

!

!

meanmeanb

meanmeanr

BG

RG

K

K


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Demosaicking(1/2)


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DemosaicingA demosaicing algorithm is a digital image

process used to reconstruct a full color

image from the incomplete color samplesoutput from an image sensor overlaid with

a color filter array (CFA). Also known as

CFA interpolation or color reconstruction,another common spelling is demosaicking.


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The Bayer arrangement of color filters on the pixel array of an image

sensor. Each two-by-two cell contains two green, one blue, and one red

filter.


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Demosaicking(2/2)

Raw image -> full color image

Non-Adaptive algorithm

Adaptive algorithm


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Color Transformation(1/2)


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Color Transformation(2/2)

Unrended Color Spaces

convenient storage or calculation

CIEXYZ (ISO-RGB , RIMM-RGB)

Rended Color Spaces

are designed for output purpose

sRGB


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Postprocessing(2/2)

Edge enhancement

the demosaicking step may introduce a zipper

artifact along intensity edges

Coring(thresholding)

Is used to remove detail information that has no significant

contribution to image detail and behaves much like noise


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Display/archive(1/2)


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Display/archive(2/2)

Display

CRT - additive color system

printer - CMYK color space

Archive

TIFF/EP

EXIF,JPEG

JPEG2000

color image processing 1

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