Page 36 - DCAP313_LAB_ON_COMPUTER_GRAPHICS

Lab on Computer Graphics

Notes 2.7.1 N-bit Colour Frame Buffer
Colour or gray scales are included into a frame buffer raster graphics device by using additional
bit planes. The intensity of each pixel on the CRT is controlled by a corresponding pixel location
in each of the N bit planes. The binary value from each of the N bit planes is loaded into
corresponding locations in a register. The resulting binary number is interpreted as an intensity
level between 0 (dark) and 2 -1 (full intensity). This is exchanged into an analogue voltage
n
between 0 and the maximum voltage of the electron gun by the DAC. A total of 2 intensity
N
levels are possible. Show illustrates a system with 3 bit planes for a total of 8 (2 ) intensity
3
levels. Each bit plane requires the full complement of memory for a given raster resolution;
e.g., a 3-bit plane frame buffer for a 1024 X 1024 raster requires 3,145,728 (3 X 1024 X 1024)
memory bits.

Figure 2.16: N-bit Plane Gray Level Frame Buffer

An increase in the number of available intensity levels is achieved for a reserved increase in
required memory by using a lookup table. Upon reading the bit planes in the frame buffer,
the resulting number is used as an index into the lookup table. The look up table must contain
2 entries. Each entry in the lookup table is W bit wise. W may be greater than N. When this
N
occurs, 2 intensities are available; but only 2 different intensities are available at one time. To
N
W
get additional intensities, the lookup table must be changed.
n
Figure 2.17: 2 -bit Plane Gray Level Frame Buffer

Because there are three primary colours, a simple colour frame buffer is implemented with three
bit planes, one for each primary colour. Each bit plane drives an individual colour gun for each

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