Page 32 - DCAP313_LAB_ON_COMPUTER_GRAPHICS

Lab on Computer Graphics

Notes When a large number of pixels are packed together, each one a different colour, the eye is unable
to resolve where one pixel ends and the next one begins from a normal viewing distance. The
brain, however, must do something to bridge the gap between two adjacent differently coloured
pixels and will integrate average, ignore the blur, or otherwise adapt to the situation. For these
reasons and others, the eye typically perceives many fewer colours than are physically displayed
on the output device.
How a colour is created also plays an important role in how it is perceived. Normally, we think
of colours as being associated with a single wavelength of light. We know, however, that two
or more colours can be mixed together to produce a new colour. An example of this is mixing
green and red light to produce yellow light, or mixing yellow and blue pigments to produce
green pigment. This mixing of colours can also occur when an object is illuminated by light.
The colour of the object will always mix with the colour of the light to produce a third colour.
A blue object illuminated by white light appears blue, while the same object illuminated by red
light will appear violet in colour.
One implication of this is that the same image rendered to two different devices will look
different. Two different colour monitors, for example, seldom produce identically perceived
images, even if the monitors are the same make and model. Another implication is that images
rendered to different types of devices will look different. An example is the difference between an
image rendered to a monitor and one rendered to a colour hardcopy device. Although there are
numerous schemes designed to minimize colour-matching problems, none is wholly satisfactory.
For these and other reasons, the accurate rendition of colour is full of difficulties, and much work
continues to be done. Although a number of mechanical devices have recently appeared on the
market, they are for the most part designed to work with one type of output device. The ultimate
arbiter of colour quality will always be the person who views the image on the output device.
Colour monitors for desktop microcomputers are based on cathode ray tubes (CRTs) or back-
lighted flat-screen technologies. Because monitors transmit light, displays use the red-green-blue
(RGB) additive colour model. The RGB model is called “additive” because a combination of the
three pure colours red, green, and blue “adds up” to white light:

Figure 2.9: The RGB Model

The computer’s operating system organizes the display screen into a grid of x and y coordinates,
like a checkerboard. Each little box on the screen is called a “pixel” (short for “picture element”).
Current Macintosh and Windows displays are composed of these grids of pixels.
2.6.2 Pixels and Colour Depth
To control the colour of each pixel on the screen, the operating system must grant a small amount
of memory to each pixel. In collective this memory dedicated to the display screen is often

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