digital image processing color processing

Digital Image Processing
DAY 3- 24/6/2020
TODAYS TOPICS:
• Colour image fundamentals
• RGB
• HSI models

Main types of digital
image
Colour image (RGB, CMYK)
•An RGB image has 24-bit information (millions of
colours)
•Channels Red, Green and Blue have 256 colours
each.
•For screen images.
•A CMYK image has a 32-bit information
•Channels Cyan, Magenta, Yellow, and Black have
256 colours each.
•For images used in printing offices.
Indexed colour
•8-bit image information. Availability of 256 colours.
•Suits well for coloured drawing graphics, but not
for photographs.

Main types of digital image CONT…..
Line-art, bitmap
•An image element consists of 1 bit; 1 = white
and 0 = black.
•Suitable for the presentation of drawings, for
instance, drawing ink works, black and white line
graphics, and texts.
•Requires little saving space due to the lack of
colours, but requires high resolution in order to
show details accurately.
Grayscale image
•The image has 8 bits and 256 tones of grey;
• 1 = black and 255 = white.
•Requires 8 times more saving space than a line-
art image.
•Suitable for presenting black and white

Colorimetric
Color Perception involve Hue, Saturation, and Lightness
Hue :Distinguish among colors such as red, green, and
purple.
Saturation :Refer to how color far from gray.white light
mixed with Hue
Lightness: The perceived intensity of a reflecting object.
Brightness :Refer to the perceived intensity of self-luminous.
White “Pure”
color
Grays
Shades
Tones
Black
© 1992–2008 R.C. Gonzalez & R. E. Woods
Artists Terms
Tint: results of adding white pigments
pure pigments
Shade: comes from adding black pigments
to pure pigments
Tone:results of adding both black and white
pigments to pure pigments

Color Image Processing
Green
Color Models:
The Newton Color Circle
Cyan
Y
ellow
Blue
Red
Magenta
•The Newton color circle provides a convenient way to perceive
the additive mixing properties of colors.
•The R,G,B and their complementary colors C,M,Y are placed on
the circle in the order of the wavelengths of the corresponding
spectral colors.

Digital Image Processing, 3rd ed.
Gonzalez & Woods
www.ImageProcessingPlace.com
Chapter 6
• The separation of colors by a prism expose a continuous range
of spectral colors
© 1992–2008 R. C. Gonzalez & R. E. Woods

Gonzalez & Woods
Chapter 6
• A spectral color is composed of a single wavelength
• The helium-neon laser monochromatic light is red (632 nm).
• Most colored objects give off a range of wavelengths and the
characterization of color is much more than the statement of
wavelength.

Gonzalez & Woods
Chapter 6

CIE Chromatic Diagram
Chromatic values depend on dominant
wavelength and saturation and independent
of luminous energy.
Consider a color C, the we can Write C =
XX + YY + ZZ
Normalize Against X+Y+Z
The XYZ space
x = X/(X+Y+Z) y = Y/(X+Y+Z) z =
Z/(X+Y+Z)
We know that x+y+z = 1, and the
luminance information usually in Y (Y
cef.), thus we can recover X,Y,Z
X = Y (x/y) ; Y = Y ; Z = Y(1-x-y)/y
Plotting these parameters

cessing
When two color A and B are added together new color C
lies on the line connects both colors.
In the side Figure, B defines the dominant wavelength,
and the ratio AC to BC expressed as a percent of the
excitation purity of A.
The closer A to C the more light A includes.
Complementary colors are those that can be mixed to
produce white light. D and E on the side Figure are
complementary colors.
Nonspectral color are those that can not be defined by
dominant wavelength such as F. Color gamuts or color
ranges is the effect of adding colors together

.

RGB Color Model
The color range (gamut) RGB
model is defined by the CRT’s
phosphor.
C = RR + GG + BB
Let us look at these colors in
XYZ space RGB NTSC
CIE Monitor
R (0.67, 0.33)
G (0.21, 0.71)
(0.73, 0.26) (0.62, 0.34)
(0.27, 0.71) (0.26, 0.59)
B (0.14, 0.08) (0.16, 0.01) (0.15, 0.07)

Generating Image of Three Color Cross sectional Colour Plane (127,G,B)

CMY Color Model
Cyan, magenta, and blue are the complements of red, green, blue.
CMY is important when dealing with hardcopy that deposit color
pigments onto paper.
 C  1  R
M   1  G
     
 Y  1 B
 R 1  C 
1
    
B 1 Y 
G  
1
G

 M

In various color processing
applications it is possible to
use the corresponding Red,
Green, and Blue channels

Color Coding for
visualization

Digital Image Processing.
Color Transformation

digital image processing color processing

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