AN1368 datasheet(28/34 Pages) MICROCHIP

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AN1368

DS01368A-page 28

 2011 Microchip Technology Inc.

APPENDIX A:

COLOR LOOK-UP

TABLE (CLUT)

A Recap of Basic Concepts

Before understanding the concept of a CLUT, image

representations and their data types should be

understood.

A digital image consists of pixels, also known as Pels.

This is a binary image (‘0’ or ‘1’) and is represented by

a simple on/off of a pixel. It is also called a

monochrome image, and for a 640x480 screen size,

the image size is (640x480/8) 37.5 Kbytes.

Image resolution refers to the number of pixels in an

image. Aspect ratio is the ratio of the column/row. In the

above example of 640x480, the aspect ratio is 4:3. This

above aspect ratio has been found to appear as a

natural image.

8-Bit Gray Level Image

An 8-bit image is an image where each pixel has an 8-bit

value (0-255) represented by a byte, which is also known

as Grayscale. Thus, the image can be a 2-dimensional

array of values, ranging between (0-255), which is also

referred to as a bit map.

For Example: An 8-bit Grayscale of VGA resolution

would be the size of 300 Kbytes.

Image Data Type for Color Images

The common data type for graphics and image file

formats is 24-bit color or 8-bit color. In a 24-bit color

image data type, each pixel is represented by three

bytes, one for the RGB of each primary color. The other

colors are represented as a combination of the RGB

values. Because each value is in the range of 0-255, it

provides a total of 16,777,216 possible colors; however,

this requires a huge storage memory (16 Mbytes).

For example, for a resolution of 640x480, a 24-bit color

image would require 900 Kbytes of memory (without

any compression).

If memory space is a concern (which is generally the

case), by quantizing the 24-bit color information,

reasonably accurate 8-bit color information can be

achieved. This also means that we have only

256 possible colors.

8-Bit Color Image Files

Image files use a special concept to store color

information in a CLUT. The image is not represented by

colors but a set of bytes. These bytes form the index to

a table, which has 3-byte values that specify the color

for a pixel. This means the user has to represent the

image by choosing the colors that best represent the

image and does not exceed the 256 color

combinations, as they are indexed by 8-bit values.

One important savings of 8-bit representation over 24-bit

representation is in storage space, which is 300 Kbytes

vs. 900 Kbytes (with no compression applied).

Figure A-1 can help in understanding the CLUT, which

is also known as a palette table.

FIGURE A-1:

COLOR LOOK-UP TABLE (CLUT)

1 Pixel

Color 0

Color 21

210 200

CLUT

Color Value

Value = 21

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