1. JPG picture compresion1

2. Terminology‏ JFIF = JPEF File Interchange Format JFIF Variants
JPG, JPEG = Join Photograph Expert Group
JFIF = JPEF File Interchange Format
JFIF Variants
Sequential, most used JPG
Progressive – for network transmiting
Lossles
Hierarchical – more resolution in one file (similar to format TIFF) 2

3. Sequential JPG storing process
Original Picture RGB True Color]
Color model Changing]
Subsampling
User input
Discrete Cosine Transformation (DCT)
Quantisation
User input
Huffman Encoding
Picture in JFIF Format 3

4. Color Model Changing User input User input
Original Picture RGB True Color]
Color model Changing]
Subsampling
User input
Discrete Cosine Transformation (DCT)
Quantisation
User input
Huffman Encoding
Picture in JFIF Format 4

5. Color Model Changing
Brightness 5

6. Color Model Transformation
RGB → Y Cb Cr
Y= 0,299*R ,587*G + 0,114*B (brightness)‏
Cb = - 0,1687*R - 0,3313*G + 0,5*B
Cr = 0,5*R ,4187*G - 0,0813*B + 128
R = Y *(Cr-128)‏
G = Y *(Cb-128) *(Cr-128)‏
B = Y *(Cb-128)‏ 6

7. Color Model Changing User input User input
Original Picture RGB True Color]
Color model Changing]
Subsampling
User input
Discrete Cosine Transformation (DCT)
Quantisation
User input
Huffman Encoding
Picture in JFIF Format 7

8. Subsampling Cb,Cr Compresion Ratio 67% 2 pixels subsampling Y1 Cb1Cr1
6 Bytes
Y1 Y2 ØCb ØCr
4 Bytes
Compresion Ratio 67% 8

9. Subsampling Cb,Cr Compresion Ratio 50% 4 pixels subsampling Y1 Cb1Cr1
12 Bytes
Y3 Cb3Cr3
Y4 Cb4Cr4
Y1 Y2 Y3 Y4 ØCb ØCr
6 Bytes
Compresion Ratio 50% 9

10. Color Model Changing User input User input
Original Picture RGB True Color]
Color model Changing]
Subsampling
User input
Discrete Cosine Transformation (DCT)
Quantisation
User input
Huffman Encoding
Picture in JFIF Format 10

11. DCT transformation
where
for
for

12. DCT
Pro 8x8 pixels block 12

13. Example 13

14. DCT Coeficients
AC coeficient (= 8 times average brightness) 14

15. Quantisation User input User input Original Picture RGB True Color]
Color model Changing]
Subsampling
User input
Discrete Cosine Transformation (DCT)
Quantisation
User input
Huffman Encoding
Picture in JFIF Format 15

16. Quantisation Matrix (Brightnes, 90% quality)16

17. Quantisation Matrices
. Defined by JPEG
. For Brightness, for Cb, for Cr
Defined for (so called) quality 10% and 90%.
For quality values between 10% and 90% linear interpolation is used
For quality values under 10% or above 90% linear extrapolation can me mathematicaly used, but it is not recomanded by JPEG.

18. Quantisation Coeficients18

19. AC coeficients
Stored separately
No comprimation
Possible use for a quick picture prewiew

20. Quantisation User input User input Original Picture RGB True Color]
Color model Changing]
Subsampling
User input
Discrete Cosine Transformation (DCT)
Quantisation
User input
Huffman Encoding
Picture in JFIF Format 20

21. Huffmanov encoding
+ Run Lenghth Encoding 21

22. Example
0, -2, -1, -1, -1, 0, 0, -1, -1

23. Quantisation User input User input Original Picture RGB True Color]
Color model Changing]
Subsampling
User input
Discrete Cosine Transformation (DCT)
Quantisation
User input
Huffman Encoding
Picture in JFIF Format 23

24. Reconstructed DCT coefficients table

25. Inverse DCT

26. Differencies