Method for transcoding a JPEG2000 compressed image
View Patent ↗A JPEG2000 compressed image is transcoded to a lower bit-rate or lower resolution, or both, without having to decompress the initial JPEG2000 image and then recompress it to a lower bit-rate and/or resolution. Instead, arithmetic decoding is performed only to the nearest higher bit-rate layer, up to the desired resolution, before performing rate-distortion optimization to produce the transcoded image.
1. A method for transcoding a JPEG2000 compressed image comprised of a collection of sub-bands encoded in one or more coding passes into a compressed bit-stream of quantized sub-band coefficients partitioned into codeblocks of each sub-band and aggregated into layers, said method comprising the steps of:
(a) parsing a JPEG2000 compressed image to extract header information and packets;
(b) calculating the number of bytes associated with each layer at a user-specified desired resolution to produce a layer byte-count table;
(c) estimating the number of layers of the JPEG2000 compressed image that should be decoded up to the user-specified desired resolution for transcoding to a user specified desired bit-rate;
(d) extracting packets belonging to resolutions up to the user-specified desired resolutions from the layers to be decoded as determined in step (c), and extracting compressed data corresponding to each codeblock that contributed to the packets;
(e) concatenating the compressed data corresponding to each codeblock, extracted in step (d), to produce a compressed codeblock bit-stream for each codeblock;
(f) arithmetically decoding the compressed codeblock bit-stream corresponding to each codeblock to produce a quantizer index for each subband coefficient and a coding pass length table, which contains an estimate of the length of the compressed data corresponding to each decoded coding pass;
(g) producing an operational rate-distortion table for each codeblock from the quantizer indices and the coding pass length table produced in step (f);
(h) producing coding pass inclusion information, which represents the coding passes that should be included in the transcoded bit-stream, using the operational rate-distortion table produced in step (g), the desired bit-rate and a plurality of user-specified visual weights; and
(i) producing a transcoded JPEG2000 bit-stream from the compressed codeblock bit-streams produced in step (e), the header information extracted in step (a) and the coding pass inclusion information produced in step (h).
2. A method for transcoding a JPEG2000 compressed image comprised of a collection of sub-bands encoded in one or more coding passes into a compressed bit-stream of quantized sub-band coefficients partitioned into codeblocks of each sub-band and aggregated into layers, said method comprising the steps of:
(a) parsing a JPEG2000 compressed image to extract header information and packets;
(b) calculating the number of bytes associated with each layer at a user-specified desired resolution to produce a layer byte-count table;
(c) estimating the number of layers of the JPEG2000 compressed image that should be decoded up to the user-specified desired resolution for transcoding to a user specified desired bit-rate;
(d) extracting packets belonging to resolutions up to the user-specified desired resolutions from the layers to be decoded as determined in step (c), thereby extracting compressed data corresponding to each codeblock that contributed to the packets;
(e) concatenating the compressed data corresponding to each codeblock, obtained from the packets extracted in step (d), to produce a compressed codeblock bit-stream for each codeblock;
(f) arithmetically decoding the compressed codeblock bit-stream corresponding to each codeblock to produce a coding pass length table, which contains an estimate of the length of the compressed data corresponding to each decoded coding pass;
(g) providing a rate-control function that uses the desired bit-rate from step (c) and the coding pass length tables produced in step (f) to produce coding pass inclusion information, which represents the coding passes that should be included in the transcoded bit-stream, for each codeblock; and
(h) producing a transcoded JPEG2000 bit-stream from the compressed codeblock bit-streams produced in step (e), header information extracted in step (a) and the coding pass inclusion information produced in step (g).