1. SWE 423: Multimedia Systems
Chapter 7: Data Compression (6)

2. Outline
JPEG
Introduction
JPEG Requirements
JPEG Modes and Procedure

3. Introduction
JPEG (Joint Photographic Experts Group) is the result of a joint project between ISO and CCITT
ISO (International Organization for Standardization)
Founded in 1947
An international standard-setting body composed of representatives from national standards bodies.
CCITT (Comité Consultatif International Téléphonique et Télégraphique)
i.e. International Telegraph and Telephone Consultative Committee
Since 1992 onwards known as ITU-T (International Telecommunication Union - ITU Telecommunication Standardization Sector)
Under UN
Developed many standards
Group 3 and Group 4 protocols for sending faxes.
V.34 and V.90 standards for sending and receiving data from full duplex fax modems
JPEG became an ISO standard in 1992.

4. Introduction JPEG applies to color and gray-scaled still images.
Motion JPEG handles video sequences through a fast coding and decoding of still images.
Currently, implementations of parts of JPEG are available as s/w only packages or using special hardware support.
Most products support the absolutely necessary algorithms.
The commercially available JPEG includes the base mode with certain processing restrictions

5. JPEG Requirements
These were put to ensure widespread distribution and application of JPEG.
Independence from image size
Applicability to any image aspect ratio and any pixel aspect ratio
Independence of the color space and the number of colors used
Unlimited complexity of image content
Currency regarding the compression factor and image quality
Platform independence of software solutions and major complexity reductions for h/w solutions
Support for sequential and progressive decoding
Support for lossless hierarchical coding with different resolutions

6. JPEG Modes

7. JPEG Modes JPEG defines four modes Lossy sequential DCT-based mode
Must be supported by every JPEG decoder
Expanded lossy DCT-based mode
Provides a set of enhancements for the base mode
Lossless mode
Low compression ratio and perfect reconstruction of images
Hierarchical mode
Accommodates images of different resolutions by using algorithms defined for the other three modes

8. JPEG: Image Preparation
JPEG specifies a general image model that can describe most commonly used still image representations
The mapping between coded color values and the colors they represent is not coded
Which requirements the above two properties satisfy?
An image consists of at least one and at most N = 255 components or planes

9. JPEG: Image Preparation
An image consists of at least one and at most N = 255 components or planes
Planes: RGB, YIQ, YUV
Gray-scale images will consist of
RGB color images will consist of ...
YUV color images will consist of ...

10. JPEG: Image Preparation

11. JPEG: Image Preparation
Each pixel is represented by p bits
Values in the range of ....
Lossy modes of JPEG use p = 8 or 12 bits/pixel
Lossless modes can use 2 to 12 bits/pixel.
Applications must conform to the standards above (if needed, it must transform the image to conform to the above)

12. JPEG: Image Preparation
Compressed data includes values of X (maximum of all Xi’s) and Y (maximum of all Yi’s) as well as factors Hi and Vi for each plane representing the relative horizontal and vertical resolutions with respect to the minimal horizontal and vertical resolutions.
Hi and Vi are integers ranging between 1 and 4
Example: 512  512 image consisting of 3 planes with the following factors:
Plane 0: H0 = 4 and V0 = 1
Plane 1: H1 = 2 and V1 = 2
Plane 2: H2 = 1 and V2 = 1
leads to ....
The image is divided into data units.
Lossless mode: 1 pixel = 1 data unit
Lossy mode: 8  8 pixels = 1 data unit (block)
Consequence of DCT which always transforms connected blocks.

13. JPEG: Image Preparation
Within each component, the data units are processed from left to right, as shown below (non-interleaved data ordering).

14. JPEG: Image Preparation
Interleaved processing order of data units of different components

15. JPEG: Lossy Sequential DCT Mode

16. JPEG: Lossy Sequential DCT Mode
After image preparation, the uncompressed image samples are grouped into data units of 8  8 pixels.
The order is defined by the MCUs
Each sample is encoded using p=8bit.
Each pixel is an integer between 0 and 255
Image processing is carried out as follows
DCT-based transformation coding is carried out
Pixel values are shifted into (-128, 127) interval
A forward DCT (FDCT) is applied to each transformed pixel value
For later reconstruction, the decoder uses the IDCT
Note that if the FDCT and IDCT computations were exact, it would be possible to reproduce the original 64 pixel values exactly. In practice, precision is limited, and therefore, the technique is lossy.
JPEG does not specify a standard precision. Therefore, two different decoders may yield different images as output of the same compressed data.

17. JPEG: Lossy Sequential DCT Mode
Image processing is followed by the quantization of all DCT coefficients
Lossy process.
Specific frequencies can be given more importance than others
Tables are used for the quantization and dequantization
Must use the same tables for both processes
Image quality may decrease due to quantization

18. JPEG: Lossy Sequential DCT Mode

19. JPEG: Lossy Sequential DCT Mode
Quantization is followed by Entropy Encoding (using Huffman Coding only)
DC coefficients are encoded by subtracting the DC coefficient of the previous data unit
Since changes are little in DC values of neighboring data units, the differences are stored instead of the values
Huffman coding is chosen because it is free (not patented)
However, coding tables must be provided by the application (one for DC and one for AC coefficients)
AC-Coefficients are processed using the zig-zag sequence

20. JPEG: Lossy Sequential DCT Mode

21. JPEG: Lossy Sequential DCT Mode

22. JPEG: Expanded Lossy DCT Mode
Image preparation here differs from that of lossy sequential using p = 12 instead of p = 8 bits per pixel.
The image processing step is analogue to that of lossy sequential.
JPEG also provides progressive coding, in addition to sequential coding, where the first decoding run produces a rough unsharp image that is refined during successive runs.
Arithmetic entropy coding can be used in addition to Huffman coding in expanded lossy DCT-based mode.

23. JPEG: Expanded Lossy DCT Mode

24. JPEG: Lossless Mode Use predictive technique (as explained earlier)
One of eight
predictors is
selected.

25. JPEG: Hierarchical Mode
The main feature here is the encoding of an image at different resolutions.
i.e. the compressed image contains images at several resolutions
The process is done as follows
The prepared image is reduced by a factor of 2n and compressed
The original image is then reduced by a factor of 2n – 1 vertically and horizontally. The previously compressed image is subtracted from this one and the result is once again compressed
This process repeats until the image with full resolution is compressed
Can use both lossy DCT-based techniques or lossless compression techniques
Computationally intensive and requires considerable storage space.
The advantage of this mode is that applications working with lower resolution do not need to first decode the whole image and then reduce the resolution.