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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
- 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 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)
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
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
DCT coefficientsAC 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