COLOUR FUNDAMENTALS AND COLOUR MODELING
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Presented by:THULASI RAM P
COLOUR FUNDAMENTALS AND COLOUR MODELING
Color is a powerful descriptor that simplifies object identification and extraction from a scene.
Characterization of light is central to the science of color.
Achromatic light is what viewers see on a black and white TV.
Gray-level is a scalar measure of intensity that ranges from black, to grays, and finally to white.
NEED FOR COLOUR:
>>> Information extraction is very easy when compared with black and white.
>>> Human eye can distinguish between thousands of colours, but, only 24 gray-level intensities can be distinguished.
Two major areas of CIP: Full colour image processing:
Processing an image acquired by full colour camera or full colour scanner.
Pseudo colour image processing:
Processing an image acquired by assigning certain colours to a range of gray-levels.
This is mainly for human interpretation.
when a beam of sunlight passes through a glass prism, the emerging beam is split into a spectrum of colours.
The colours that humans and most animals perceive in an object are determined by the nature of the light reflected from the object
For example, green objects reflect light with wave lengths primarily in the range of 500 – 570 nm while absorbing most of the energy at other wavelengths.
Chromatic light spans the electromagnetic spectrum from approximately 400 to 700 nm.
As we mentioned before human colour vision is achieved through 6 to 7 million cones in each eye.
ATTRIBUTES OF LIGHT:
These three correspond to quantity of light.
3 basic qualities are used to describe the quality of a chromatic light source:
Radiance: the total amount of energy that flows from the light source (measured in watts)
Luminance: the amount of energy an observer perceives from the light source (measured in lumens)
Note we can have high radiance, but low luminance
Brightness: a subjective (practically unmeasurable) notion that embodies the intensity of light
Primary colours: RGB
Out of seven, why only RGB?
>>> Cones and Rods.
>>> (6-7)million cone cells.
CIE specified three different wavelengths for three different colours.
Experimentally, it’s different.
Cones are sensitive to BLUE light means cones are sensitive to light of wave-length varying from (400-550)nm.
No single WL can specify a particular colour, only band of WLs, since, no clear cut boundary between two adjacent spectrum colours.
LIGHT AND PIGMENT
Mixing of primary colours gives secondary colours.
PIGMENT: The colour, perceived by observer after the object absorbs colours of some wavelength is called pigment.
The secondary colours of light are called the primary colours of pigment and vice-versa.
Appropriate mixing of RGB gives WHITE.
Appropriate mixing of CMY gives BLACK.
Concepts required for human perception:
Brightness: chromatic notion of intensity.
Hue: dominant wavelength in a mixture of colours(overall sensation of this mix colour is determined by the colour of dominant wavelength)
Saturation: purity of colour(amount of light that is added to a particular colour to make it a diluted one)
These are used by human to distinguish one colour to another colour.
• Since, no white is present in spectrum colours, we say, it is not diluted. So, they are fully saturated.
For example, pink is not a spectrum colour.
PINK(diluted) = RED(fully saturated) +WHITE
• HUE & SATURATION à colour sensitivity
• BRIGHTNESS à chromatic notion of intensity
Hue and saturation together, it is called as chromaticity.
Chromaticity is used to distinguish HS & I.
Amount of R,G,B to form a particular colour is called tristimulus.(X,Y,Z)
Chromatic coefficients: (x,y,z)
‘X’ is the amount of red light to be mixed to form a particular colour.
x+y+z=1 (normalized form)
As any colour can be specified by its chromatic coefficients, another way to specify a colour is CIE chromaticity diagram.
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