Colour Image and Video Representations

• A black and white image is a 2-D array of integers.

• A colour image is a 2-D array of (R,G,B) integer triplets. These triplets encode how much the corresponding phosphor should be excited in devices such as a monitor.

• Example is shown in Fig 6.23.

  

  Display of a Colour Cube

Beside the RGB representation, YIQ and YUV are the two commonly used in video.

YIQ Colour Space

• YIQ is used in colour TV broadcasting, it is downward compatible with B/W TV.

• Y (luminance) is the CIE Y primary.

  Y = 0.299R + 0.587G + 0.114B

• the other two vectors:

  I = 0.596R - 0.275G - 0.321B Q = 0.212R - 0.528G + 0.311B

• The YIQ transform:

  

  YIQ Transform of a colour image

• I is red-orange axis, Q is roughly orthogonal to I.

• Eye is most sensitive to Y, next to I, next to Q. In NTSC, 4 MHz is allocated to Y, 1.5 MHz to I, 0.6 MHz to Q.

• An Example YIQ Decomposition is shown in Fig. 6.24.

  

  Example YIQ Decomposition

CCIR 601 (YUV)

• Established in 1982 to build digital video standard

• Video is represented by a sequence of fields (odd and even lines). Two fields make a frame.

• Works in PAL (50 fields/sec) or NTSC (60 fields/sec)

• Uses the Y, Cr, Cb colour space (also called YUV) Y = 0.299R + 0.587G + 0.114B Cr = R - Y Cb = B - Y

• The YCrCb (YUV) Transform:

  $\left[ \begin{array} {c}Y\ U\ V\end{array} \right] = \left[ \begin{array} {r... ...081 \end{array} \right] \left[ \begin{array} {c}R\ G\ B\end{array} \right]$

• CCIR 601 also defines other image parameters, e.g. for NTSC, Luminance (Y) image size = 720 x 243 at 60 fields per second Chrominance image size = 360 x 243 at 60 fields per second

• An example YCrCb Decomposition is shown in Fig. 6.25

  

  YCrCb Decomposition of a colour image

The CMY Colour Model

• Cyan, Magenta, and Yellow (CMY) are complementary colours of RGB (Fig. 6.26). They can be used as Subtractive Primaries.

• CMY model is mostly used in printing devices where the colour pigments on the paper absorb certain colours (e.g., no red light reflected from cyan ink).

The RGB and CMY Cubes