CGMA UNIT 6 part 4

Image Compression Standards: Types

Compression is a process to store an image in reduced bits. It allows to store images in less storage space and to transmit through limited bandwidth channels. Compression is two types:

1. Lossy Compression:- it is used in multimedia applications in which reconstructed image is dragradeted. Higher compression ratio is achieved in this type of compression. (PNG,TIFF)

2. Lossless compression:- It is an exact peplica of the original image after compression. It is useful in medical applications. Compression rate is modest. (JPEG)

In image compression , the correlation between the pixel is reduced if the values of one pixel and its adjacent pixels are very similar. 

1. Predictive coding, othogonal trnsformation, sub-band coding (Reduce correclation between pixels)

2. Quantization (lossless)

3. Entropy encoding

Types

1. JPEG baseline: Joint Photographic Experts Group is given by Wallace. It is the first international compression standard for continuous-tone still images. For image compression DCT(discrete cosine transform) is used. In JPEG huffman coding and arithmetic coding is used. Artihmetic coding produce better compression than huffman coding. 

2. JPEG 2000: It is given by Chistopoulos et al. It is compatible with multimedia technologies. The first step of encoding process of JPEG2000 is pre-processing. Pre-processing involves tiling, offset leveling and irrevertible color transform. Tiling is to divide the image as rectangular and non-overlapping blocks. The second step is to apply DCT on all the blocks of the image.

    JPEG supports 16 million possible colors using 8 bits for each color(red,green, and blue) in the RGB color space. This provides 256 values for each of the three colors, which combined allow for 256*256*256 or 16,777,216 colors. Three values of 0 produce pure black, while three values of 255 create pure white.

    File size is ten times reduced in JPEG compression algorithm and there i. s no degradation in quality. Although the compression algorithm used in JPEG is lossy. The lossy compression can cause an issue called artifacting in which parts of the image become blocky. GIF format is better alternative if we want to save small images abd images with a lot of text. Second disadvantage is that JPEG images dont support transparency. For trancparency PNG format is more better than JPEG.

Video Compression Standards
Two video compression  standards are there: 1. MPEG (Motion Photographic Expert Group) 2. H.26X

MPEG is for motion picture storage and communications and it is developed by the ISO.
H.26X focuses on videp=conferencing applications and proposed by ITU.

Video compression can be accomplished by using image compression techniques on consecutive video frames. MJPEG is ditrect application of JPEG on video sequences.  It uses JPEG copmression to encode individual picture in the video sequence separately. It is used in high-quality video application.

MPEG-1

The first public standard of the MPEG committee was the MPEG-1. MPEG-1 video compression includes techniques for efficient coding of a video sequence. This standard focuses on bit stream. It uses for storing data on CDs. In this the compression ratio is important than the picture quality. In MPEG-1 only the new parts of the video sequence is identified and coded. This is done while transmission of video sequence. This process has reduce the bandwidth of the signal to large extent.

MPEG-2 

MPEG-2 is the ―Generic Coding of Moving Pictures and Associated Audio‖. The MPEG-2 standard is used for TV transmission and other applications of high data rates. It gives very high picture quality. This is a compatible extension of MPEG-1 standard means MPEG-2 decoder can decode MPEG1 data streams. Combining of multiple audio, video and private data streams into a single multiplexed stream is specify by MPEG-2 standard. It supports wide range of broad cast, telecommunication and storage application. It enhances the better video quality at same bit rate. Using this techniques the DVD movies are compressed. 

MPEG-4 

The main feature of MPEG-4 is that it gives the support of lower bandwidth application as well as extremely high quality and unlimited bandwidth application. The one example of it is studio movies. The basic idea of this is not only related to video coding but it involves fully encoding only key frames through JPEG algorithm where minimal information is sent between every four to five frames. Compression ratio achieved by this is approximately 100:1.This standard encoder is very complex and places very heavy computational load for motion estimation whereas decoding is comparatively simple which can done using low cost decoder chips. The basic idea is to detect motion from frame to frame in temporal directions and then use DCT to organize redundancies. The DCT is done on 8*8 block and motion prediction is done on luminance (Y) of 16*16 blocks. The DCT coefficients are end up with zero. The quantization can change for every macro block, which is 16*16 of Y and the corresponding 8*8 in both U and V. The DCT coefficients, motion vectors and quantization parameters are Huffman coded using fixed tables. The motion vectors and the DCT components are DPCM coded.

Hypertext
Hypertext are link among textual items that takes the user to the referent text when clicked. These are often formatted with underlined and blue type. Hypertext very commonly used in internet or local computers. Normally text in books or in normal word document are sequential i.e. one after another.  Hypertext are not sequential, readers can jump from a text or a page to another.

Hypertext is a powerful cross-referencing tool meant for user-driven access to an ocean wealth of interconnected information either static or dynamic in an electronic format. Simply put, hypertext may refer to plain simple text that contains links to access other chunks of text within the same or different document. It provides a means to organize and present information in a way that is easily accessible to the end users. It’s more like a user-driven tool to represent textual information which are linked together to provide more flexibility and a greater degree of control. This allows users or readers to move from one location to another via hyperlinks or “go to” links. Links connect nodes to other documents and are usually activated when clicked upon by a mouse or other pointing device.

Hypermedia
Hypermedia are link among textual items that takes the user to the referent media when clicked. It is like hypertext but when it is clicked it links to media.

Hypermedia is an extension of hypertext that employs multiple forms of media such as text, graphics, audio or video sequences, still or moving graphics, etc. The structure of hypermedia is quite similar to that of a hypertext, except it’s not constrained to be just text-based. It extends the capabilities of hypertext systems by creating clickable links within web pages to create a network of interconnected non-linear information which user can both access and interact with for a better multimedia experience. The most common hypermedia type is image links which are often linked to other web pages. It is used in a variety of applications from problem solving and qualitative research to electronic studying and sophisticated learning.

In lossless compression, the reconstructed image is an exact replica of

the original image after compression. Only the modest compression rate

can be achieved and used in medical applications. The categorization of

compression is shown in Fig. 1