POWERPOINT PLUSPOWERPOINT PLUS11/17/07 Class Notes11/17/07 Class Notes

WHAT IS A PIXELWHAT IS A PIXELA pixel is a number that represents

the intensity of light at a square spot in the picture.

Pixels can be constructed as gray-scale or color. In many cases, the pixel will be constructed as a byte. A byte is 8 bits or binary numbers.A bit can hold a single binary number, 1 or 0.

A byte can hold 8 binary A byte can hold 8 binary numbersnumbers0000 0000 (0)0000 0001 (1)0000 0010 (2)

◦and so on1111 1111 (255)

Three-byte color pixelsThree-byte color pixelsIn many systems a color pixel is

constructed using 3 bytes◦ To record color as a mixture of Red, Green

and Blue lights◦ Therefore, to make a 24-bit color, you need

3 8-bit bytesMy Favorite Brown is a mixture of red,

green and blue:◦ Red (200), Green (40), Blue (10)

In this 3-byte system pure black looks like:◦ Red (0), Green (0), Blue (0)

And pure white is this:◦ Red (255), Green (255), Blue (255)

IMAGE COMPRESSIONIMAGE COMPRESSION1. GIF Format

◦ Many images have a "run" of the same color pixel 234, 234, 234,... (300 times)

◦ More efficient: record the run, not every pixel (300 - 234) This is known as "run length encoding“ GIF uses a form of run length encoding

◦ Another fact: GIF is a "lossless encoding“ When it's restored the image will be exactly as it

was before compression

IMAGE COMPRESSION IMAGE COMPRESSION (cont’d)(cont’d)2. JPEG Format

Very sophisticated (mathematically speaking) Recording waves of information These waves have different frequencies

JPEG loses information when it encodes high-frequency waves In pictures, this corresponds to sharp, high-contrast boundaries

between regions of different colors This loss of information shows up as a shadow (or

echo) artifact Therefore, JPEG is a "lossy" compression method

The restored image is different than the original However, for "smooth" photographic images, the loss in not

noticeable In JPEG, you can "dial" the amount of compression

More compression gives less quality when the image is restored to a bitmap display

Less compression gives better quality, although the file size will be somewhat larger