DATA COMPRESSION LOSSY COMPRESSION METHODS

What it is… A compression of information that is acceptable in pictures or videos, but not texts or programs. Methods of Lossy Data Compression are: Joint Photographic Experts Group (JPEG) Motion Picture Experts Group (MPEG)

Why use it? Cost effective Time Efficient

How It’s Done! Change the picture into a linear set of numbers that reveals the redundancies. Redundancies can be removed. The set of bits is simplified. Each block of 64 pixels goes through the discrete cosine transform (DCT) P(x,y)T(x,y) Value of each pixel before DCT Value after DCT

Operation: If a block has two different sections, each uniform in its respective section, you can decrease the amount of operations performed to display the image by 4800 times. How? By doing the DCT operation: x c(x) x c(y) x Σ Σ x P(x,y) m=0 n=0 If there is no change to any one pixel out of the 64 in the chart, then 0’s are created for the spot.

Quantization After all that math, there’s more math… Divide the number of bits needed for compression by a constant Drop the fraction # of bits and # of 0’s are now optimized for compression

Compression After quantization, redundant 0’s are removed Done by zigzag linearization rather than row by row, to conceal fine changes.

MPEG’s Means spatially compressing each frame while temporally compressing a set of frames Types: I-frames—intracoded frame= an independent frame from a set P-frames—predicted frame= related to the previous frame, without the possibility for changes being recorded B-frames—bidirectional frame= relative to preceding and future frames, never related to another B-frame

Summation All received data need not be an exact replica of the sent data MPEG is used to compress video bits JPEG is a method to compress pics and graphics Spatial compression is similar to JPEG, compressing extraneous bits DCT, quantization, and compression are tools for compression Temporal compression removes redundant frames