Image compression with multiple JPEG

Pao Yue-kong Library Electronic Theses Database

Image compression with multiple JPEG


Author: Poon, Pak-lok
Title: Image compression with multiple JPEG
Degree: M.Sc.
Year: 1997
Subject: Image compression -- Data processing
Image processing -- Digital techniques
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Pages: ix, 93, [22] leaves : ill. ; 30 cm
Language: English
InnoPac Record:
Abstract: The objective of this dissertation is to study the mechanism of recompression using lossy JPEG baseline model and highlight the areas which contribute to additional image distortion during recompression. Baseline JPEG was selected to be the compression mechanism in this dissertation since it is very widely adopted for still images and it is the de facto standard of still visual material format, say in the Internet. In this dissertation, two different types of lossy JPEG recompression have been studied. In the first type, the compression parameter which determines the amount of information that would be lost, hence the quality of the restored image and the compression ratio was not changed throughout subsequent recompression. The compressed images were decoded back to the point where pixel value has been reconstructed, then the images were recompressed. This kind of experiment emulate the situation of decompressing a compressed picture for picture modifications, such as cropping, and then to be compressed again for transfer or storage. As FDCT (Forward Discrete Cosine Transformation) and IDCT (Inverse Discrete Cosine Transformation) introduces round-off errors, accumulation of this error in consecutive recompression may not be masked off by quantisation error. Study was conducted on the relationship between the coarseness of quantiser and its mask-off ability to round-off errors of recompression. In the second type, the parameter was changed in each subsequent recompression with the purpose of achieving additional data reduction. In order to eliminate the round-off errors from DCT operations, image was only partially decoded up to the point where the DCT coefficient has been reconstructed. The relationship between the quality of recompressed image and the ratio of two consecutive compression quantiser was investigated. An distortion reduction algorithm with the aim to preserve the originality of the image against recompression was presented. Statistical information from a typical image was collected which was then used as the threshold in the run time criteria of the algorithm. The algorithm works with the approach of adjusting the DCT coefficient to correct the requantisation error. As only the DCT coefficient was adjusted, the algorithm did not alter any requirment of the JPEG standrard. The usage of Correction Bias Table and the requantisation error probability table will also be illustrated. The algorithm was found to be working satisfactory for typical images.

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