Image Compression

Topics: JPEG, Data compression, Huffman coding Pages: 13 (3038 words) Published: April 11, 2012
Version 2 ECE IIT, Kharagpur

Still Image
Standards: JBIG
and JPEG
Version 2 ECE IIT, Kharagpur

Instructional Objectives
At the end of this lesson, the students should be able to:
1. Explain the need for standardization in image transmission and reception. 2. Name the coding standards for fax and bi-level images and state their characteristics.
3. Present the block diagrams of JPEG encoder and decoder.
4. Describe the baseline JPEG approach.
5. Describe the progressive JPEG approach through spectral selection. 6. Describe







7. Describe the hierarchical JPEG approach.
8. Describe the lossless JPEG approach.
9. Convert YUV images from RGB.
10. Illustrate the interleaved and non-interleaved ordering for color images.

16.0 Introduction
With the rapid developments of imaging technology, image compression and coding tools and techniques, it is necessary to evolve coding standards so that there is compatibility and interoperability between the image communication and storage products manufactured by different vendors. Without the availability of standards, encoders and decoders can not communicate with each other; the service providers will have to support a variety of formats to meet the needs of the customers and the customers will have to install a number of decoders to handle a large number of data formats. Towards the objective of setting up coding standards, the international standardization agencies, such as International Standards Organization (ISO), International Telecommunications Union (ITU), International Electro-technical Commission (IEC) etc. have formed expert groups and solicited proposals from industries, universities and research laboratories. This has resulted in establishing standards for bi-level (facsimile) images and continuous tone (gray scale) images. In this lesson, we are going to discuss the highlighting features of these standards. These standards use the coding and compression techniques – both lossless and lossy which we have already studied in the previous lessons.

The first part of this lesson is devoted to the standards for bi-level image coding. Modified Huffman (MH) and Modified Relative Element Address Designate (MREAD) standards are used for text-based documents, but more recent Version 2 ECE IIT, Kharagpur

standards like JBIG1 and JBIG2, proposed by the Joint bi-level experts’ group (JBIG) can efficiently encode handwritten characters and binary halftone images. The latter part of this lesson is devoted to the standards for continuous tone images. We are going to discuss in details about the Joint Photographic Experts Group (JPEG) standard and its different modes, such as baseline (sequential), progressive, hierarchical and lossless. The more recent and advanced coding standard – the JPEG-2000 will be discussed in the next lesson (lesson-17).

16.1 Coding Standards for Fax and Bi-level Images
Consider an A4-sized (8.5 in x 11 in) scanned page having 200 dots/in. An uncompressed image would require transmission of 3,740,000 bits for this scanned page. It is however seen that most of the information on the scanned page is highly correlated along the scan lines, which proceed in the direction of left to right in top to bottom order and also in between the scan lines. The coding standards have exploited this redundancy to compress bi-level images. The coding standards proposed for bi-level images are:

(a) Modified Huffman (MH): This algorithm performs one-dimensional run length coding of scan lines, along with special end-of-line (EOL), end-ofpage (EOP) and synchronization codes. The MH algorithm on an average achieves a compression ratio of 20:1 on simple text documents. (b) Modified Relative Element Address Designate (MREAD): This algorithm uses a two-dimensional run length coding to take advantage of vertical...
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