Mumbai university > Electronics and telecommunication Engineering > Sem 7 > Data compression and Encryption

Marks: 4

Years: May 2016

1. Joint Picture Expert Group (JPEG) is a lossy/ lossless compression method for colour and gray scale still images.

2. The compression steps involved in JPEG are:

   • Colour images are transformed from RGB to YCrCbcolour space. This step is not required for gray scale images.

   • Colour images are down sampled by creating low resolution pixels from original ones. This is used only when hierarchical compression is needed and it is always skipped for gray scale images.

   • The down sampling is not done for luminance component but only for Cr and Cb components.

   • Down sampling is done either at the ration of 2:1 both horizontally and vertically called 4:1:1 sampling or at the ratio of 2:1 horizontally and 1:1 vertically called as 4:2:2 sampling.

3. Organization into group: The pixels of each colour component are originated in groups of 8x8 pixels. If the no. of rows or columns is not a multiple of 8, then the bottom row and rightmost column are duplicated as many times as necessary.

4. The DCT is then applied to each 8x8 block of data to get the 64 DCT co-efficient. They represent the average pixel value and successive higher frequency changes within the block of data.

5. Due to limited precision of arithmetic, some amount of loss is introduced due to DCT.

6. Quantization: Each of the 64 frequency components are divided by a separate number called quantization co-efficient (Q) and then rounded to an integer. This causes permanent loss of information.

7. Large Q’s causes more loss and hence higher frequency components typically have large Q’s.

8. Each of the 64 Q’s is a parameter defined by JPEG standard. The 64 quantized frequency co-efficient which are now integers are encoded using a combination of RLE and Huffman coding.

9. Adding header: In the final step, header information and all parameters used are added.

10. Decoding: The JPEG decoder performs the reverse operation since JPEG is a symmetric compression procedure.