Fundamentals of Multimedia
Chapter 4
Color in Image and Video
Ze-Nian Li and Mark S. Drew
건국대학교 인터넷미디어공학부
임창훈
Outline
4.1 Color Science (skip)
4.2 Color Models in Images
4.3 Color Models in Video
Chap 4 Color in Image and Video
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4.1 Color Models in Images
Fig. 4.15: RGB and CMY color cubes
Chap 4 Color in Image and Video
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Additive and Subtractive Color
 Additive color:
When two light beams impinge on a target,
their colors add.
When two phosphors on a CRT screen are turned on,
their colors add. (red + green = yellow)
 Subtractive color:
For ink on paper, the opposite situation holds:
yellow ink subtracts blue from white illumination,
but reflects red and green; it appears yellow.
(white – blue = yellow)
Chap 4 Color in Image and Video
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Subtractive Color: CMY Color Model
 Instead of red, green, and blue primaries, we need
primaries that amount to -red, -green, and -blue.
I.e., we need to subtract R, or G, or B from White (W).
 These subtractive color primaries are
Cyan (C), Magenta (M) and Yellow (Y ) inks.
C = W – R, (0, 1, 1) = (1, 1, 1) – (1, 0, 0)
M = W – G, (1, 0, 1) = (1, 1, 1) – (0, 1, 0)
Y = W – B, (1, 1, 0) = (1, 1, 1) – (0, 0, 1)
Chap 4 Color in Image and Video
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Transformation from RGB to CMY
 Simplest model we can invent to specify what ink
density to lay down on paper, to make a certain desired
RGB color:
 Then the inverse transform is:
Chap 4 Color in Image and Video
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Fig. 4.16: Additive and subtractive color.
(a): RGB is used to specify additive color.
(b): CMY is used to specify subtractive color
Chap 4 Color in Image and Video
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Undercolor Removal: CMYK System
 Undercolor removal
• Sharper and cheaper printer colors:
• Calculate that part of the CMY mix that would be
black, remove it from the color proportions, and add
it back as real black (K).
 The new specification of inks is thus:
Chap 4 Color in Image and Video
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4.2 Color Models in Video
 Largely derive from older analog methods of coding
color for TV.
Luminance is separated from color information.
 YIQ is used to transmit TV signals in North America
and Japan (NTSC).
 In Europe, video tape uses the PAL or SECAM,
which are based on TV that uses a matrix transform
called YUV.
 Digital video mostly uses a matrix transform called
YCbCr that is closely related to YUV
Chap 4 Color in Image and Video
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YUV Color Model
 YUV codes a luminance (luma) signal equal to Y’
(for gamma-corrected signals)
 Chrominance (chroma) refers to the difference
between color and luminance.
color differences U, V
U = B’ – Y’, V = R’ – Y’
Chap 4 Color in Image and Video
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YUV Color Model
 For composite video,
 Chrominance signal C in composite video
Chap 4 Color in Image and Video
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YUV Color Model
original color image
Y’
Chap 4 Color in Image and Video
U
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YIQ Color Model
 YIQ is used in analog NTSC color TV broadcasting
 I and Q are rotated version of U and V
Chap 4 Color in Image and Video
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YCbCr Color Model
 Digital video uses YCbCr model – closely related to YUV
 YUV is changed to YCbCr by scaling.
Cb and Cr are shifted between 0 and 1.
Chap 4 Color in Image and Video
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YCbCr Color Model
 In practice, in 8-bit coding, with a maximum Y’ value of
only 219, and a minimum of +16.
Cb and Cr have a range of 112 and offset of +128.
If R’, G’, B’ are floats in [0, 1], then we obtain
Y’, Cb, Cr in [0, 255] via the transform:
 The YCbCr transform is used in JPEG image
compression and MPEG video compression.
Chap 4 Color in Image and Video
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