Data compression

lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors - approximating methods (JPEG)

Grid graphics formats BMP – no compression PCX – lossless compression RLE PNG – lossless dictionary compression LZW GIF – lossless dictionary compression LZW + reduction to 256 color (adaptive palette) JPG – approximating compression JPEG

JFIF format (JPEG File Interchange format)‏  sequential, common known  progressive, more effective, for computer net transmissions  lossless, not known and not widely supported  hierarchic, more resolutions in one file, quick preview

Sequential JFIF encoding

Color model transofmation RGB → Y Cb Cr Y= 0,299*R + 0,587*G + 0,114*B (brightness)‏ Cb = - 0,1687*R - 0,3313*G + 0,5*B Cr = 0,5*R - 0,4187*G - 0,0813*B R = Y *(Cr-128)‏ G = Y *(Cb-128) *(Cr-128)‏ B = Y *(Cb-128)‏

Subsampling of Cb,Cr Computing of average value for the block 2x1 pixels (6 bits sample), –6 bits -> 4 bits (compression 67%) or 2x2 pixels (12 bits sample), –12 bits -> 6 bits (compression 50%)

DCT transformation

Example

DCT coefficients AC coefficient (= 8 x average brightness

Quantization matrix – example for 90% “qality”

Quantization matrices Defined by standardization committee JPEG. Separately for brightness and for color components. Defined matrices for quality 10% and 90%. For other values of quality between 10% and 90%obtained by linear interpolation. For values under 10% or over 90% extrapolation can be used but it is not recommended.

Coefficients after quantisation

AC coefficients Stored separately Not compressed Possibility used for quick preview

Huffman encoding

Example 0, -2, -1, -1, -1, 0, 0, -1, -1, 0, 0, 0…..

Reconstruction of DCT coeffients

After inverse DCT transformation

Table of differences