Multimedia Systems

1. Multimedia Systems Lecture - 5

2. Lecture Outline • Colour Fundamentals • Colour Science • Light and spectra • Colour Sensation • Colour Space/Models

3. Colour Science • The science of colour is sometimes called chromatics. It includes: • the perception of colour by the human eye and brain, • the origin of colour in materials, • colour theory in art, and • the physics of electromagnetic radiation in the visible range (commonly referred to simply as light).

4. Colour Science • Light is an electromagnetic wave. • Colour is characterized by the light’s wavelength within the narrow band of the electromagnetic spectrum (380 – 760nm) to which the human eye responds.

5. Colour Science – Light & Spectra (1)

6. Colour Science – Light & Spectra (2) • Red light has longer wavelength in the visible light & blue the shorter. • The shorter the wavelength, higher the vibration and energy. • Red photons carry around 1.8eV and blue 3.1eV

7. Colour Science – Light & Spectra (3) • Colour derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors. • Colour categories and physical specifications of colour are also associated with objects, materials, light sources, etc., based on their physical properties such as light absorption, reflection, or emission spectra.

8. Colour Science - Colour Sensation (1) • “Our sensations of colour are within us and colour can not exist unless there is an observer to perceive them. Colour does not exist even in the chain of events between the retinal receptors and the visual cortex, but only when the information is finally interpreted in the consciousness of the observer” Wright, ‘the rays are not coloured’ 1963, 1967

9. Colour Science - Colour Sensation (2) • Colour is a physical property of light, but colour perception is a human physiological activity. • Colour is the brains reaction to a specific visual stimulus. • Human eye is like a camera focusing image onto the retina. • The retina consists of rods and cones: • Cones are sensitive to colours • Rods are sensitive to intensity (only) – low light intensity produces images in gray

10. Colour Science - Colour Sensation (3) • The signals from the cones and rods are combined in the brain to give several different “sensations” of the colour. • These sensations have been defined by the CIE (Commission Internationale de L’Eclairage) • Brightness: the human sensation by which an area exhibits more or less light. • Hue: the human sensation according to which an area appears to be similar to one, or to proportions of two, of the perceived colours red, yellow, green and blue.

11. Colour Science - Colour Sensation (4) • Colourfulness: the human sensation according to which an area appears to exhibit more or less of its hue. • Lightness: the sensation of an area’s brightness relative to a reference white in the scene. • Chroma: the colourfulness of an area relative to the brightness of a reference white. • Saturation: the colourfulness of an area relative to its brightness.

12. Colour Science – Colour Space (1) • Colour space or colour model is a method by which we can specify, create and visualise colour. • As humans, we may define a colour by its attributes of brightness, hue and colourfulness. • A computer may describe a colour using the amounts of red, green and blue phosphor emission required to match a colour. • A printing press may produce a specific colour in terms of the reflectance and absorbance of cyan, magenta, yellow and black inks on the printing paper.

13. Colour Science – Colour Space (2) • Reasons for having more than one colour spaces: • Different colour spaces are suitable for different applications, for example some equipment limitations may allow a specific type of colour space. • Some colour spaces are perceptually linear, i.e. a 10 unit change in stimulus will produce the same change in perception wherever it is applied. Many colour spaces, particularly in computer graphics, are not linear in this way. • Some colour spaces are intuitive to use (easy for the user) while other spaces are confusing for the user with parameters with abstract relationships to the perceived colour. • Finally, some colour spaces are tied to a specific piece of equipment (i.e. are device dependent) while others are equally valid on whatever device they are used.

14. Colour Science – Colour Space (3) • Device dependent colour space • is a colour space where the colour produced depends on both the parameters used and the equipment used for display. • Device independent colour space • is a colour space where a set of parameters will produce the same colour on whatever equipment they are used.

15. RGB Colour Model • This is probably the most popular colour model used in computer graphics. • It is an additive system in which varying amount of the three primary colours, red, green and blue, are added to black to produce new colours.

16. CMY Colour Model • This model is based on the light absorbing quality of inks printed on paper. • Combining three primary colour pigments, Cyan, Magenta and Yellow, should absorb all light, thus resulting in black. • It is a subtractive model. • The value of each primary colour is assigned a percentage from the lightest (0%) to the darkest (100%).

17. Addition of Black – CMYK Model • Because all inks contain some impurities, the three inks actually produce a muddy brown, a black colour is added in printing process, thus CMYK model. + + + = ≈

18. Colour Models – RGB Vs CMY (1)

19. Colour Models – RGB Vs CMY (2) RGB & CMY Cubes

20. Colour Models – RGB Vs CMY (3) • Conversion From RGB to CMY • Conversion From CMY to RGB

21. Addition of Black - CMYK • Used especially in the printing of images + + + = ≈

22. HSB Colour Model • This model is based on the human perception of colour. The three fundamental characteristics of colours are: • Hue — is the wavelength of the light. Hue is often identified by the name of the colour. It is measured as a location on the standard colour wheel as a degree between 00 to3600. • Saturation — is the strength or purity of the colour. It represents the amount of gray in proportion to the hue and is measured as a percentage from 0%(gray) to 100% (fully saturated). • Brightness — is the relative lightness or darkness of the colour. It is measured as a percentage from 0%(black) to 100%(white).

23. YUV Colour Model • This model is widely used in encoding colour for use in television and video. • The theory behind this model is that human perception is more sensitive to brightness than any chrominance information, so a more suitable coding distinguishes between luminance and chrominance. • This also produces a system that is compatible with black-and-white TV systems. • The Y-signal encodes the brightness information. Black-and-white television system will use this channel only. • The U and V channels encode the chromatic information. The resolution of the U and V channels is often less than the Y channel for the reason of reducing the size.

24. Gamut • The gamut of a colour system is the range of colours that can be displayed or printed. • The spectrum of colours that can be viewed by human eye is wider than any method of reproducing colour. • Different colour models have different gamut. The CMYK model is smaller than RGB model. • On the right is a Chromaticity Diagram which illustrates gumat of RGB and CMYK colour systems.

25. Other Color Coordinate Systems • Most of these colour spaces are linear transforms from RGB and are device dependent. • HSL(Hue-Saturation-Lightness) • HSV(Hue-Saturation-Value) • HSI(Hue-Saturation-Intensity) • HCI(Hue-Chroma-Intensity) • HVC(Hue-Value-Intensity) • HSD(Hue-Saturation-Darkness)

26. Also read … • http://en.wikipedia.org/wiki/Color