Colour Depth Colour depth describes the number of
of memory that are used to store the colour information about each
in a bitmap image.
With 1 bit colour depth the number of bits used to store the information about each
is
. This allows
colours, represented by 0 or 1.
With 2 bit colour depth the number of
used to store the information about each pixel is 2. This allows
colours, represented by the binary codes 00, 01, 10 or
With 3 bit colour depth the number of bits used to store the information about each pixel is
. This allows
colours, represented by the binary codes 000, 001, 010, 011,
, 101, 110 or 111.
The number of colours in this GIF image suggest that it is using
bit colour depth.
SUMMARY: The greater the colour depth of a bitmap image, the greater the Colour mapping, palette tables and Direct colour: Colour mapping - With low colour depths (up to 8 bit) it is practical to map every colour to a
code because there are not too many colours involved. This is called colour mapping and an example might be the binary code 110 representing yellow.
Palette tables - The GIF file format uses 8 bits to store the
colour data so each pixel can have
different colours. However, every GIF image can have a
different set of 256 colours because the set that is used is stored as part of the file in a
table. This table stores the 256 colours using 24 bit direct colour giving 16,777,216 possible colours. Each of the 256 colours in the palette is indexed using
bits so it is the 8 bit
value that is actually stored for each pixel in a GIF file.
Direct colour - As the colour depth increases, colour mapping and palettes become impractical due to the number of possible
that the image can display. In higher colour depths (8 bit and above) a system called
colour is used instead. In this the bits allocated to each pixel seperatly encode the relative proportions of Red, Green and
to specify what is called an RGB colour.
For example, if 24 bit direct colour was used then the code for a yellow pixel (maximum red and green and zero
) would be;
-
255 0 in denary
- 11111111 11111111 00000000 in
(note the use of 24 bits)
- FF FF 00 in