Page 35 - DCAP313_LAB_ON_COMPUTER_GRAPHICS

Unit 2: Graphics Systems

image files dedicate 8 bits to each colour pixel in the image. In 8-bit images the 256 colours that Notes
make up the image are stored in an array called a “palette” or an “index.” The colour palette
may also be referred to as a “colour lookup table” (CLUT). As mentioned above, 8-bit images
can never contain more than 256 exclusive colours:

Figure 2.14: 256 Colour Palette

True-colour, or 24-bit, images are typically much larger than 8-bit images in their uncompressed
state, because 24-bits of memory are dedicated to each pixel, typically arranged in three
monochrome layers red, green, and blue:

2.7 Frame Buffer

A frame buffer is a huge, close part of computer memory. At a least there is one memory bit
for each pixel in the rater; this amount of memory is called a bit plane. The picture is built up
in the frame buffer one bit at a time. You know that a memory bit has only two states, and then
a single bit plane yields a black-and white display. You know that a frame buffer is a digital
device and the CRT is an analogue device. Therefore, a change from a digital representation to an
analogue signal must take place when information is read from the frame buffer and presented on
the raster CRT graphics device. For this you can use a digital to analogue converter
(DAC). Each pixel in the frame buffer must be accessed and converted before it is visible on
the raster CRT.

Figure 2.15: A Single Bit-pack White Frame Buffer Raster Graphic Device

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