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7.5 JPEG 2000
JPEG 2000 (ISO 15444) was developed as a more
modern replacement for JPEG. Its most important features are greater
compression for equal quality of resulting imaging and improved lossless
compression.
The improvement in compression, while present, is not great. [SantaCruz] comments that “there have
not been any truly significant [advances] in image compression efficiency in
the past decade”. The improvements in compression are mainly seen at very
high and very low bitrates (i.e. the number of bits to encode a pixel, or bpp).
At the ‘near visual lossless’ performance, JPEG 2000 is only around
20% better than JPEG. The trade-off is that computational complexity for JPEG 2000
is significantly higher. In 2000, JPEG codecs were around 3 times faster than
JPEG 2000 codecs and, while it was expected that some improvement would be
made, this difference was never expected to close.
[Jakulin] describes the
artifacts introduced by JPEG 2000 and compares them (using the same images)
with the original image and JPEG. The conclusion was that, although JPEG 2000
solves some of JPEG’s problems at lower bit rates, it introduces others.
The author recommends that JPEG 2000 is more appropriate for images that do not
have noisy textures. He goes on to suggest that between 2.5 and 3 bpp (bits per
pixel), JPEG 2000 is superior, and above 3 bpp, there is no difference. For
black and white images JPEG 2000 is superior to JPEG, but not as good as a
lossless black and white compression (e.g 16 colour greyscale PNG).
[Gormish] concluded that JPEG 2000 “is unlikely to
replace JPEG in low complexity applications at bitrates in the range where JPEG
performs well”.
The standard consists of a large number of parts:
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Part 1: Core coding system. Published
as an international standard in 2000 and revised in September 2004. Intended as
a royalty and license free core with maximum capability to interchange between
implementations. Includes the file format (JP2).
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Part 2: Extensions. Published
as an international standard in May 2004. Extensions for circumstances where interchange
is less important than other requirements (e.g. ability to handle a particular
type of data). It is not expected that JPEG 2000 files using the part 2
extensions will be handled by part 1 decoders. Since interchange is critically
important in an archival setting, the part 2 extensions should not be accepted
without consideration of their archival impact. Includes a new file format
(JPX) that supports multiple compositing layers, animation, and extended colour
spaces
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Part 3: Motion JPEG 2000. Published
as an international standard in November 2004, and currently under review. This
defines a file format (MJ2 or MJP2) for storing motion sequences of images.
Inter frame coding is not supported. This has now been effectively superseded
by Part 12.
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Part 4: Conformance testing. Published
as an international standard in 2002 and a revised in December 2004. Compliance
tests for JPEG 2000 Part 1. Includes a set of compliance classes for decoders.
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Part 5: Reference software. Reference
implementations in C and Java for Part 1. Published as an international
standard in November 2003.
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Part 6: Compound image file
format. Published as an international standard in 2003. Compound
images include both text and continuous tone photographs. This part defines the
JPM file format, which uses the Mixed Raster Content model of ISO 16485. It
supports multi-page documents with many objects per page. The objects can be
compressed using different algorithms (e.g. JBIG for b/w images). JPM is an
extension of JP2, and uses content defined in JPX (Part 2).
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Part 7: Not produced.
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Part 8: JPSEC.
This part examines security aspects of JPEG 2000. It includes encryption of the
image or metadata, source authentication, data integrity, conditional access,
and ownership protection. This part is currently a Draft International
Standard.
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Part 9: Interactivity tools,
APIs and protocols. Published as an international standard in
November 2005. Interactive protocols and API. This part defines a protocol
(JPIP) which sits on top of HTTP and allows access to images (or portions of
images). It is designed to avoid retransmission of image portions. It is primarily
focussed on access to Part 1 images, but provides access to some of the format
extensions in Part 2. It also allows selection amongst multiple codestreams in
JPX (Part 2), MJ2 (Part 3), and JPM (Part 6).
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Part 10: JP3D.
This part is concerned with the compression of 3 dimensional
‘images’ (actually data arrays).
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Part 11: JPWL. This
part provides additional error detection and correction to allow for
transmission of Part 1 images over an error prone wireless connection. In
particular it attempts to protect the header. Since this only adds additional
information to an existing image during transmission, it should not be an issue
for a long term preservation format.
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Part 12: ISO Base Media File
Format. Published as an international standard in 2004 and revised
in April 2005. This is a common format for both JPEG 2000 and MPEG-4 files. It
is intended to provide a base file format for future applications.
VERS currently only accepts images encoded using Part 1 of the
standard. This is primarily due to the lack of time required to investigate the
other Parts, but also due to the immaturity of most of the remaining parts.
Restricting acceptance of JPEG 2000 images to those conforming
to Part 1 means that the images must be encoded using the JP2 format. Other
parts of the standard define a variety of other formats:
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JPX. Defined in Part 2 to support the extensions defined in that
part. It appears that the extensions can either be represented in a way that is
upwardly compatible with JP2, or in an incompatible way.
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MJ2/MPJ2. Defined in Part 3 to support sequences of frames
encoded in JPEG 2000. Appear to be superseded by the ISO Base Media File
Format.
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JPM. An extension of JP2 for representing compound documents. A
JPM file contains a hierarchy of page collections, pages, layout objects, masks
and image objects. A JP2 decoder should ignore the components it does not
recognise.
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ISO Base Media File Format. This is a common format for JPEG 2000
and MPEG-4 files. Does not appear (yet) to be widely used.
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