POWERPOINT PLUS 11/17/07 Class Notes

WHAT IS A PIXEL A 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 numbers (0) (1) (2) ◦ and so on (255)

Three-byte color pixels In 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 bytes My 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 COMPRESSION 1. 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  ( )  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 (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