Page 15 - DCAP504_Computer Graphics

Computer Graphics

Did you know? A 1000×1000 true color image can take up three million bytes. In addition, even if
every pixel in that image has a different color, there would almost be a million colors
in the image.
In most of the applications, the total number of colors that appear in any one particular image is much
less. Therefore, the ability of 24-bit representation to have 16.7 million different colors appear all
together in a single image seems to be overloaded.
1.5 Lookup Table

In computer graphics, an important mechanism is used for converting a variety of colors into another
variety of colors. This mechanism is referred to as a lookup table, also called as a Color Look Up Table
(CLUT). Lookup table is generally used for partitioning the pixel values into the colors that are
displayed, and the colors that are not displayed. Generally, the operating system initializes this table to
particular default values. The lookup table size relies on the number of colors that the graphics system
can display simultaneously.

The representation of image with the help of a lookup table can be viewed as a negotiation between the
desire to have a lower storage necessity and the requirement to support a reasonably sufficient number
of simultaneous colors. The main aspect of this approach is that there will be no direct coding of colors
with pixel values. Instead, the pixel values are addresses of color values in a table. The color of a certain
pixel is concluded by the color value in the table entry, which the pixel value refers to.
The figure 1.3 depicts the lookup table with 256 entries.

Figure 1.3: 24-Bit Lookup Table with 256 Entries

In the figure 1.3, the entries have addresses ranging from 0 to 255. Each entry has a 24 bit RGB color
value. Pixel values are now 8 bit or 1 byte quantity. The color of a pixel, whose value is i, where 0 < i <
255 is determined by the color value in the table entry, whose address is i. The above lookup table
representation is often referred as the 8 bit format. It decreases the storage requirement of a 1000×1000
image to one million bytes plus 768 bytes for the color values in the lookup table. It selects 256
simultaneous colors from 16.7 million possible colors.

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