Rate control and bit allocations for JPEG transcoding

An image transcoder that produces a baseline JPEG file from a baseline JPEG input is developed. The goal is to produce a high quality image while accurately meeting a filesize target and keeping computational complexity-especially the memory usage and number of passes at the input image--low. Building upon the work of He and Mitra, the JPEG transcoder exploits a linear relationship between the number of zero-valued quantized DCT coefficients and the bitrate. Using this relationship and a histogram of coefficients, it is possible to determine an effective way to scale the quantization tables of an image to approach a target filesize. As the image is being transcoded, an intra-image process makes minor corrections, saving more bits as needed throughout the transcoding of the image. This intra-image process decrements specific coefficients, minimizing the change in value (and hence image quality) while maximizing the savings in bitrate. The result is a fast JPEG transcoder that reliably achieves a target filesize and preserves as much image quality as possible. The proposed transcoder and several variations were tested on a set of twenty-nine images that gave a fair representation of typical JPEG photos. The evaluation metric consisted of three parts: first, the accuracy and precision of the output filesize with respect to the target filesize; second, the PSNR of the output image with respect to the original image; and third, the subjective visual image quality.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Electrical Engineering and Computer Science.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2007

Author dc:creator

• Nguyen, Ricky D. (Ricky Do)

Advisor dc:contributor.advisor

• Shizhong Liu and Vivek K. Goyal.

• Electrical Engineering and Computer Science.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/41667

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/41667