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# YUV Color Model

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```Fundamentals of Multimedia
Chapter 4
Color in Image and Video
Ze-Nian Li and Mark S. Drew
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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
Li & Drew; мќён„°л„·лЇёл””м–ґкіµн•™л¶Ђ мћ„м°Ѕн›€
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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
Li & Drew; мќён„°л„·лЇёл””м–ґкіµн•™л¶Ђ мћ„м°Ѕн›€
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When two light beams impinge on a target,
When two phosphors on a CRT screen are turned on,
their colors add. (red + green = yellow)
п‚§ 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 вЂ“ blue = yellow)
Chap 4 Color in Image and Video
Li & Drew; мќён„°л„·лЇёл””м–ґкіµн•™л¶Ђ мћ„м°Ѕн›€
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Subtractive Color: CMY Color Model
п‚§ 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).
п‚§ These subtractive color primaries are
Cyan (C), Magenta (M) and Yellow (Y ) inks.
C = W вЂ“ R, (0, 1, 1) = (1, 1, 1) вЂ“ (1, 0, 0)
M = W вЂ“ G, (1, 0, 1) = (1, 1, 1) вЂ“ (0, 1, 0)
Y = W вЂ“ B, (1, 1, 0) = (1, 1, 1) вЂ“ (0, 0, 1)
Chap 4 Color in Image and Video
Li & Drew; мќён„°л„·лЇёл””м–ґкіµн•™л¶Ђ мћ„м°Ѕн›€
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Transformation from RGB to CMY
п‚§ Simplest model we can invent to specify what ink
density to lay down on paper, to make a certain desired
RGB color:
п‚§ Then the inverse transform is:
Chap 4 Color in Image and Video
Li & Drew; мќён„°л„·лЇёл””м–ґкіµн•™л¶Ђ мћ„м°Ѕн›€
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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
Li & Drew; мќён„°л„·лЇёл””м–ґкіµн•™л¶Ђ мћ„м°Ѕн›€
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Undercolor Removal: CMYK System
п‚§ Undercolor removal
вЂў Sharper and cheaper printer colors:
вЂў 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).
п‚§ The new specification of inks is thus:
Chap 4 Color in Image and Video
Li & Drew; мќён„°л„·лЇёл””м–ґкіµн•™л¶Ђ мћ„м°Ѕн›€
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4.2 Color Models in Video
п‚§ Largely derive from older analog methods of coding
color for TV.
Luminance is separated from color information.
п‚§ YIQ is used to transmit TV signals in North America
and Japan (NTSC).
п‚§ In Europe, video tape uses the PAL or SECAM,
which are based on TV that uses a matrix transform
called YUV.
п‚§ Digital video mostly uses a matrix transform called
YCbCr that is closely related to YUV
Chap 4 Color in Image and Video
Li & Drew; мќён„°л„·лЇёл””м–ґкіµн•™л¶Ђ мћ„м°Ѕн›€
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YUV Color Model
п‚§ YUV codes a luminance (luma) signal equal to YвЂ™
(for gamma-corrected signals)
п‚§
Chrominance (chroma) refers to the difference
between color and luminance.
color differences U, V
U = BвЂ™ вЂ“ YвЂ™, V = RвЂ™ вЂ“ YвЂ™
Chap 4 Color in Image and Video
Li & Drew; мќён„°л„·лЇёл””м–ґкіµн•™л¶Ђ мћ„м°Ѕн›€
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YUV Color Model
п‚§ For composite video,
п‚§ Chrominance signal C in composite video
Chap 4 Color in Image and Video
Li & Drew; мќён„°л„·лЇёл””м–ґкіµн•™л¶Ђ мћ„м°Ѕн›€
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YUV Color Model
original color image
YвЂ™
Chap 4 Color in Image and Video
U
Li & Drew; мќён„°л„·лЇёл””м–ґкіµн•™л¶Ђ мћ„м°Ѕн›€
V
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YIQ Color Model
п‚§ YIQ is used in analog NTSC color TV broadcasting
п‚§ I and Q are rotated version of U and V
Chap 4 Color in Image and Video
Li & Drew; мќён„°л„·лЇёл””м–ґкіµн•™л¶Ђ мћ„м°Ѕн›€
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YCbCr Color Model
п‚§ Digital video uses YCbCr model вЂ“ closely related to YUV
п‚§ YUV is changed to YCbCr by scaling.
Cb and Cr are shifted between 0 and 1.
Chap 4 Color in Image and Video
Li & Drew; мќён„°л„·лЇёл””м–ґкіµн•™л¶Ђ мћ„м°Ѕн›€
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YCbCr Color Model
п‚§ In practice, in 8-bit coding, with a maximum YвЂ™ value of
only 219, and a minimum of +16.
Cb and Cr have a range of 112 and offset of +128.
If RвЂ™, GвЂ™, BвЂ™ are floats in [0, 1], then we obtain
YвЂ™, Cb, Cr in [0, 255] via the transform:
п‚§ The YCbCr transform is used in JPEG image
compression and MPEG video compression.
Chap 4 Color in Image and Video
Li & Drew; мќён„°л„·лЇёл””м–ґкіµн•™л¶Ђ мћ„м°Ѕн›€
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