processing of colour images - urząd miasta Łodzielectromagnetic spectrum 10 24 10 22 10 20 10 18...
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Processing of colour imagesimages
�Colour representation schemes
� Filtering of colour images
© Strzelecki, Strumiłło
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Electromagnetic spectrum
10181024 1016 1014 1012 1010 108 106 104 1021022 1020
frequency, Hz
gamma rays radio waves
X rays microwaves
Infrared wavesultravioletVisibleVisible lightlight
400 500 600 700 [nm]
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Eye sensitivity to colour components
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Commission Internationale de l’Eclairage (CIE)
Chromaticity Diagram
Point of equalenergy
(saturation zero)
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Chromaticity diagram and a typical
colour „gamut” (the triangle) for a
typical display CRTCRT device
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R
GB
RRGGB B colourcolour imagesimages
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RGB colour space
Each colour
compoment (R,G,B) is
registered and digitized
in a separate video
channel
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RGB additive primaries
© Addison-Wesley
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YIQ (YUV) colour space
Y is a luminance component and is a linear combination of (R,G,B)
(I,Q) componets define a colour.
NTSC system
Y – luminance,
I – inphase,
Q – quadrature
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There exist a one-to-one mapping betweenRGB and YIQ systems.
Y=0.299R + 0.587G + 0.114B R=Y + 0.956I + 0.621Q
I = 0.596R - 0.274G - 0.322B G=Y - 0.272I + 0.647Q
Q=0.211R - 0.523G + 0.312B B=Y - 1.106I + 1.703Q
For a human eye perception a better approach is is to code
separately luminance and chrominance components (SVHS, 8mm)
Equivalence of colour spaces
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I
Y
Q
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H - hue, S - saturation, I - intensity
•Well suited for a
human visual
perception system
• Difficult for hardware
implementation
HSI colour system
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Hue
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Saturation
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Intensity
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CMY colour
components are
substractive colours
(as opposed to RBG
components that are
additive colour
components)
A black component is added (CMY+K),in order to obtain a better imagecontrast
CMY colour system
−
=
B
G
R
Y
M
C
1
1
1
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© Addison-Wesley
CMY subtractive primaries
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cyan magenta
yellow black
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RGB colours produced by CRT
device (black triangle) and CMY
colour printers (magenta tringle)
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Colour coding in SVGA graphics boards
7 0 7 0 7 0R BG
Digital to Analog Converter (DAC) 3 x 8 bits
screen
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Colour resolutions in PC computers
• True-colour: 224 colours -> 16777216
• High-colour: 216 colours -> 65536 (R5, G6, B5)
• 256 colours (indexed colours from look-up-table)
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Source
image
Output
image
h(x,y)
f(x,y) g(x,y)
B
Filtering of colour images in spatial domain
*
*
*
R
G
B
R
G
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Low-pass filteringin colour space
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High pass filteringin colour space
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Filtering of colour images in spatial domain
1. Convert from RGB to YIQ
2. Filter the Y (luminance) component andkeep the colours (I, Q) unchanged
3. Convert back to RGB
Another possible approach:
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%Matlab
h=fspecial(‘unsharp’);
ys=filter2(h,y);
High pass filtering of Y component