color image processing 1
TRANSCRIPT
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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