2.6. PRIMARY COLOURS
It is a useful fact that the huge variety of colours that can
be perceived by humans can all be produced simply by adding together
appropriate amounts of red, blue and green colours. These colours are known as
the primary colours. Thus in most image processing applications, colours are
represented by specifying separate intensity values for red, green and blue
components. This representation is commonly referred to as RGB.
The primary colour phenomenon results from the fact that
humans have three different sorts of colour receptors in their retinas which
are each most sensitive to different visible light wavelengths.
The primary colours used in painting (red, yellow and blue)
are different. When paints are mixed, the `addition' of a new colour paint
actually subtracts wavelengths from the reflected visible light.
Colour Images
It is possible to construct (almost) all visible colours by
combining the three primary colours red, green and blue, because the human eye
has only three different colour receptor, each of them sensible to one of the
three colours. Different combinations in the stimulation of the receptors
enable the human eye to distinguish approximately 350,000 colours. A
RGB colour image is a multi-spectral image with one band for each colour red,
green and blue, thus producing a weighted combination of the three primary
colours for each pixel.
A full 24-bit colour image contains one 8-bit value for each
colour, thus being able to display different colours. However, it is
computationally expensive and often not necessary to use the full 24-bit to
store the colour for each pixel. Therefore, the colour for each pixel is often
encoded in a single byte, resulting in a 8-bit colour image. The process of
reducing the colour representation from 24-bits to 8-bits, known as colour
quantization, restricts the number of possible colours to 256. However, there
is normally no visible difference between a 24-colour image and the same image
displayed with 8 bits. A 8-bit colour images are based on colourmaps, which are
look up tables taking the 8 bit pixel value as index and providing an
output value for each colour.
RGB and Colourspaces
A colour perceived by the human eye can be defined by a linear
combination of the three primary colours red, green and blue. These three
colours form the basis for the RGB-colourspace. Hence, each perceivable colour
can be defined by a vector in the 3-dimensional colourspace. The intensity is
given by the length of the vector, and the actual colour by the two angles
describing the orientation of the vector in the colourspace.
The RGB-space can also be transformed into other coordinate
systems, which might be more useful for some applications. In this coordinate
system, a colour is described with its intensity, hue (average wavelength) and
saturation (the amount of white in the colour). This colour space makes it
easier to directly derive the intensity and colour of perceived light and is
therefore more likely to be used by human beings.
24-bit Colour Images
Full RGB colour requires that the intensities of three colour
components be specified for each and every pixel. It is common for each
component intensity to be stored as an 8-bit integer, and so each pixel
requires 24 bits to completely and accurately specifies its colour. Image
formats that store full 24 bits to describe the colour of each and every pixel
are therefore known as 24-bit colour images.
Using 24 bits to encode colour information allows different
colours to be represented, and this is sufficient to cover the full range of
human colour perception fairly well.[14]
The term 24-bit is also used to describe monitor displays that
use 24 bits per pixel in their display memories, and which are hence capable of
displaying a full range of colours.
There are also some disadvantages to using 24-bit images.
Perhaps the main one is that it requires three times as much memory, disk space
and processing time to store and manipulate 24-bit colour images as compared to
8-bit colour images. In addition, there is often not much point in being able
to store all those different colours if the final output device (e.g.
screen or printer) can only actually produce a fraction of them. Since it is
possible to use colourmaps to produce 8-bit colour images that look almost as
good, at the time of writing 24-bit displays are relatively little used.
However it is to be expected that as the technology becomes cheaper, their use
in image processing will grow.
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