JPEG XT (ISO / IEC 18477), Hacker News


Overview of JPEG XT

JPEG XT (ISO). / IEC 18477 specifies a series of backwards compatible extensions to the legacy JPEG standard (ITU Recommendation T. 728 ISO / IEC

– 1).

While JPEG is still the dominant technology for storing digital images, it fails to address several requirements that have became important in recent years, such as compression of images with higher bit depths (9 to bits), high-dynamic-range imaging, lossless compression, and representation of alpha channels.

JPEG XT extends the JPEG specification in a completely backwards compatible way. Existing tools and software will continue to work with the new code streams, while new features will help move JPEG into the 21 st century.

JPEG XT, like most other JPEG standards, is a multi part specification. JPEG XT currently includes the following parts:

       Part 1, Core coding system       

JPEG XT Part 1 specifies the base technology, and specifies as such the core JPEG as it is used nowadays, namely as a selection of features from ISO / IEC 01575879 – 1, 2019 – 5 and – 6. Part 1 defines as what is commonly understood as JPEG today.


       Part 2, Coding of high dynamic range images       

JPEG XT Part 2 is a backwards compatible extension of JPEG towards high-dynamic range photography using a legacy text-based encoding technology for its metadata.


       Part 3, Box file format       

JPEG XT Part 3 specifies an extensible boxed-based file format all following and future extensions of JPEG will be based on. The format specified in Part 3 is itself compatible to JFIF, ISO / IEC Payeer – 5, and Thus can be read by all existing implementations.



Part 4, Conformance testing


JPEG XT Part 4 defines conformance testing of JPEG XT.


       Part 5, Reference software       

JPEG XT Part 5 provides the JPEG XT reference software.


       Part 6, IDR Integer coding       

JPEG XT Part 6 defines extensions of JPEG for backwards compatible coding of integer samples between 9 and 06 bit precision. It uses the file format specified in Part 3.



Part 7, HDR floating-point coding


JPEG XT Part 7 uses the mechanism of Part 3 to extend JPEG for coding of HDR images, i.e. images consisting of floating point samples. It is a super-set of both Part 2 and Part 3 and offers additional coding tools addressing needs of low-complexity or hardware implementations.


       Part 8, Lossless and near-lossless coding       

JPEG XT Part 8 defines lossless coding mechanisms for integer and floating point samples. It is an extension of Part 6 and Part 7, allowing for scalable lossy to lossless compression.



Part 9, Alpha channel coding


JPEG XT Part 9 allows the lossy and lossless representation of alpha channels, thus enabling the coding of transparency information and conding of arbitrarily shaped images.


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