AN1368 datasheet(2/34 Pages) MICROCHIP

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AN1368

DS01368A-page 2

 2011 Microchip Technology Inc.

In general, to divide 16 bits among Red, Green and

Blue, two schemes are used:

• Scheme 1 (R<5> G<6> B<5>): In this scheme,

there are 5 bits of Red, followed by 6 bits of Green

and 5 bits of Blue. Green is given more bits

because of the property of the human eye, which

can distinguish more shades of Green than Red

and Blue. Figure 2 illustrates these 64K colors.

• Scheme 2 (T<1> R<5> G<5> B<5>): In this

scheme, there is one transparent bit, followed by

5 bits each of Red, Green and Blue. The

transparent bit indicates if the color should be

used or not.

Currently, the Microchip Graphics Library (Version 2.11)

supports only Scheme 1.

FIGURE 2:

COLORS IN 16-BIT

REPRESENTATION

Grayscale is usually represented in a byte, with 0 as

Black, 1-254 as the shades of Black, getting lighter as

the number increases, and 255 as White, as shown in

Figure 3. Sometimes, only 4 or 2 bits are used to

represent 16 or 4 shades of Black, respectively. If only

one bit is used to represent either the on or off of a

color, then it is called ‘Monochrome’.

The number of bits required to represent a color is

called the ‘Color Depth’. For example, a color depth of

16 bits means it requires 16 bits to represent a color,

and therefore, we can represent 216 different colors.

FIGURE 3:

GRAYSCALE VALUES OF

0 TO 225

Alternatively, color may be represented using a Color

Look-up Table (CLUT), also called a palette table, where

the color is specified by the index of the table, as shown

in Figure 4. Depending on the size of the table, the bits

used to represent the index will vary as 256 entries of

RGB (8-bit index), 16 entries of RGB (4-bit index),

4 entries of RGB (2-bit index) and 2 entries of RGB (1-bit

index). This scheme is mainly used to save memory. For

more information on this scheme, see Appendix A:

“Color Look-up Table (CLUT)”

FIGURE 4:

COLOR LOOK-UP TABLE

(CLUT)

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