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Lecture 5

Computer Science 1033A/B Lecture Notes - Lecture 5: Internet Protocol Suite, Lossless Compression, Image File Formats

Computer Science
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Picture Compression and the Internet
2 types of compression:
compresses original bits and byte, but does not lose the original information about
the picture
when you reopen the file, all the info about the picture is still there... no info is lost
When compressed, the original information is lost
The least necessary parts of the image are the parts of the image that are
Different types of images use algorisms for compression
Common compressed image file formats are: GIF, JPG, PNG
Uncompressed files are "raw"
Types of compression ideas:
-use of co-ordinates for pictures
-pattern decoding
Huffman Coding -> more common letters use less bytes (E,O,S,T,R,etc.); less common letters
use the larger byte codes (Q,X,Z,etc.)
Humans notice changes in brightness more than changes in colour
-lossy discards the slight difference in colour rather than brightness
Only supports 8 bits (256 colours)
Used primarily for cartoons, not good enough quality for photographs
Lossless compression, except for the first time that a JPG is getting compressed to a GIF
Benefits of GIF:
-Can chose a colour to be transparent (can't do that with a JPG)
-Allows for simple animations, work in all browsers
-Allows for dithering
can put two colours together to make more colour
like very small checkerboard, alternating colours so they blend together
ex. if you need 257 pictures
can store palette of only the colours you need
Supports 24 bit colour (16 million colours - 2^24)
File size is very large
Uses lossy compression -> discards information about colour, not brightness
Good for gradients, skies, etc.
Not so good for well defined, crisp images
No transparent, no animation, no dithering
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