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Data compression

Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

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Page 1: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

Data compression

Page 2: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

Data compression

• lossless – looking for unicolor areas or repeating patterns– Run length encoding– Dictionary compressions

• Lossy

– reduction of colors

- approximating methods (JPEG)

Page 3: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

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

Page 4: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

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

Page 5: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

Sequential JFIF encoding

Page 6: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

Color model transofmation

RGB → Y Cb CrY= 0,299*R + 0,587*G + 0,114*B (brightness)

Cb = - 0,1687*R - 0,3313*G + 0,5*B + 128

Cr = 0,5*R - 0,4187*G - 0,0813*B + 128

R = Y + 1.402*(Cr-128)

G = Y - 0.34414*(Cb-128) - 0.71414*(Cr-128)

B = Y + 1.772*(Cb-128)

Page 7: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

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%)

Page 8: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

DCT transformation

Page 9: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

Example

139 144 149 153 155 155 155 155144 151 153 156 159 156 156 156150 155 160 163 158 156 156 156159 161 162 160 160 159 159 159159 160 161 162 162 155 155 155161 161 161 161 160 157 157 157162 162 161 163 162 157 157 157162 162 161 161 163 158 158 158

Page 10: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

DCT coefficientsAC coefficient (= 8 x average brightness

Page 11: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

Quantization matrix – example for 90% “qality”

Page 12: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

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.

Page 13: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

Coefficients after quantisation

Page 14: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

AC coefficients

• Stored separately• Not compressed• Possibility used for quick preview

Page 15: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

Huffman encoding

Page 16: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

Example

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

Page 17: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

Reconstruction of DCT coeffients

Page 18: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

After inverse DCT transformation

Page 19: Data compression. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors

Table of differences