Page 51 - DCAP303_MULTIMEDIA_SYSTEMS

Unit 4: Image

Note that bitmaps are always orientated horizontally and vertically. Pixels should be considered notes
square although they may have other aspect ratios in practice. In the majority of situations bitmaps
are used to represent images on the computer. For example, the figure given below is a bitmap
which has 397 pixels horizontally, 294 pixels vertically, and each pixel contains a grey value from
a possible 256 different greys.
figure 4.2: Bitmap image in Different Greys

4.1.1 Colour Depth

Each pixel in a bitmap contains certain information, usually interpreted as colour information.
The information content is always the same for all the pixels in a particular bitmap. The amount
of colour information could be whatever the application requires but there are some standards,
the main ones are described below.
1 bit (black and white)
This is the smallest possible information content that can be held for each pixel. The resulting
bitmap is referred to as monochrome or black and white. The pixels with a 0 are referred to as
black and pixels with a 1 are referred to as white. Note that while only two states are possible
they could be interpreted as any two colours, 0 is mapped to one colour, 1 is mapped to another
colour.

8 bit greys
In this case each pixel takes 1 byte (8 bits) (See Figure 4.3) of storage resulting in 256 different
states. If these states are mapped onto a ramp of greys from black to white, the bitmap is referred
to as a greyscale image. By convention, 0 is normally black and 255 white. The grey levels are the
numbers in between, for example, in a linear scale 127 would be a 50% grey level.

figure 4.3: 8 Bit Greys

In any particular application, the range of grey values can be anything, it is most common to map
the levels 0-255 onto a 0-1 scale but some programs will map it onto a 0-65535 scale.

24 bit rGB
This is the next step from 8 bit grey, now there is 8 bits allocated to each red, green, and blue
component. In each component the value of 0 refers to no contribution of that colour, 255 refer to
fully saturated contribution of that colour. Since each component has 256 different states there
are a total of 16777216 possible colours (See Figure 4.4).

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