fundamentals of multimedia chapter 4 color in image and video ze-nian li and mark s. drew

Fundamentals of Multimedia

Chapter 4

Color in Image and Video

Ze-Nian Li and Mark S. Drew

건국대학교 인터넷미디어공학부임 창 훈

Chap 4 Color in Image and Video Li & Drew; 인터넷미디어공학부 임창훈2

Outline

4.1 Color Science (skip)

4.2 Color Models in Images

4.3 Color Models in Video


4.1 Color Models in Images

Fig. 4.15: RGB and CMY color cubes


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)

Additive and Subtractive Color


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)

Subtractive Color: CMY Color Model


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:


Fig. 4.16: Additive and subtractive color. (a): RGB is used to specify additive color. (b): CMY is used to specify subtractive color


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:


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


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’


YUV Color Model

For composite video,

Chrominance signal C in composite video


YUV Color Model

original color

image

Y’ U V


YIQ Color Model

YIQ is used in analog NTSC color TV broadcasting

I and Q are rotated version of U and V


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.


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.

fundamentals of multimedia chapter 4 color in image and video ze-nian li and mark s. drew

Documents