JPEG XT

Key takeaways

JPEG XT belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect JPEG XT to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of JPEG XT from memory before moving on to harder problems.

Reference excerpt

JPEG XT (ISO/IEC 18477) is an image compression standard which specifies backward-compatible extensions of the base JPEG standard (ISO/IEC 10918-1 and ITU Rec. T.81). JPEG XT extends JPEG with support for higher integer bit depths, high dynamic range imaging and floating-point coding, lossless coding, alpha channel coding, and an extensible file format based on JFIF. It also includes reference software implementation and conformance testing specification. JPEG XT extensions are backward compatible with base JPEG/JFIF file format - existing software is forward compatible and can read the JPEG XT binary stream, though it would only decode the base 8-bit lossy image.

The JPEG XT standard JPEG standards are formally named as Information technology – Scalable compression and coding of continuous-tone still images. ISO/IEC 18477 consists of the following parts:

Overview The core Part 1 of the standard defines the JPEG specifications in common use today, such as ISO/IEC 10918-1 (base format), 10918-5 JPEG File Interchange Format (JFIF), and 10918-6 (printing applications). It restricts the JPEG coding modes to baseline, sequential, and progressive Huffman, and includes JFIF definitions of Rec. 601 color space transformations with YCbCr chroma subsampling. The first specification was authored by Thomas Richter from Germany, Tim Bruylants and Peter Schelkens from Belgium, and Swiss-Iranian engineer Touradj Ebrahimi. Part 3 Box file format defines an extensible format which is backward-compatible with JFIF. Extensions are based on 'boxes' - 64 KB chunks tagged by application marker 11 ('APP11'), containing enhancement data layers and additional binary metadata describing how to combine them with the base 8-bit layer to form full-precision image. Part 3 builds on the ISO base media file format used by JPEG 2000; similar arrangement was employed in the earlier JPEG-HDR format from Dolby Labs, which is standardized in JPEG XT Part 2. Part 7 includes floating-point HDR coding tools which produce an enhancement image layer from full-precision image and gamma-corrected tone-mapped 8-bit base image layer. These tools are intended for high dynamic range imaging with multiple photo exposures and computer-generated images which exceed linear 16-bit integer precision. It defines three main algorithms for reconstructing the HDR image: Profile A uses a common logarithmic scale factor for inverse tone-mapping of the base layer; Profile B uses a divisor image extension layer scaled by the common exposure value; Profile C is similar to A but uses per-component scaling factors and logarithmic space with piece-wise linear functions, which allows lossless encoding. Profile A is based on the Radiance RGBE image format and Profile B is based on the XDepth format from Trellis Management. Profile D uses a simple algorithm which does not generate an enhancement image – the enhancement layer is used to store extended precision of discrete cosine transform (DCT) transfer coefficients, and non-gamma transfer function is applied to increase dynamic range to 12 bits. Backward compatibility is limited because legacy decoders do not understand new EOTF curves and produce undersaturated colors. Profile D is not implemented in reference software. JPEG XT also allows mixing of various elements from different profiles in the code stream, allowing extended DCT precision and lossless encoding in all profiles (the 'Full Profile'). Part 6, Integer coding of Intermediate Dynamic Range (IDR) images, is an extension for coding 9 to 16-bit integer samples typical for RAW sensor data; its coding tools are identical to Part 7 Profile C. Part 2 defines a HDR imaging implementation based on JPEG-HDR format from Dolby. It uses RGBE image format defined by Part 7 Profile A, supporting both integer and floating point samples; file format is based on Part 3 but uses proprietary text-based metadata syntax. Part 8 Lossless coding is an extension of integer and floating point coding based on Part 7 Profile C, allowing for scalable lossy to lossless compression. For 10 and 12-bit precision, lossless integer-to-integer DCT is used, which replaces each rotation space with three shearings (similar to wavelet transform in JPEG2000). For 16 bit precision, a lossy fixed-point DCT approximation is specified by the standard and is required for decoders to implement. This makes it possible for the encoder to predict coding errors and store them in the enhancement layer, allowing lossless reconstruction. The error residuals in the enhancement layer can be either uncompressed, or compressed with lossless integer-to-integer DCT. Compression and image quality performance of Part 8 is comparable to PNG. Part 9 Alpha channel extension allows lossy and lossless coding of transparent images and arbitrarily shaped images. It uses an opacity (transparency) layer, encoded with integer or floating point precision, and metadata to specify if content was pre-multiplied with alpha, or pre-multiplied and blended with background color. In the future, privacy protection and security extensions would allow encoding of private image regions (or entire images) with reduced resolution, with digitally encrypted enhancement layers to restore full-resolution image only to those having the private decryption key. Only the public regions will be visible to those not having the key.

JPEG-HDR JPEG XT Part 2 HDR coding is based on Dolby JPEG-HDR format, created in 2005 by Greg Ward from BrightSide Technologies and Maryann Simmons from Walt Disney Feature Animation as a way to store high dynamic range images inside a standard JPEG file. BrightSide Technologies was acquired by Dolby Laboratories in 2007. The image encoding is based on two-layer RGBE image format used by Radiance renderer, both of which were also created by Ward. Reduction in filesize is achieved by first converting the image into a tone mapped version, then storing a reconstructive multiplier image in APP11 markers in the same JPEG/JFIF file. Ordinary viewing software will ignore the multiplier image allowing anyone to see the tone mapped version of the image presented in a standard dynamic range and color gamut. JPEG-HDR file format is similar to JPEG XT Part 3 Box file format but uses text-based metadata. Programs that support JPEG-HDR include Photosphere by Greg Ward and pfstools.

Reference software

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with JPEG XT

Start with the simplest possible case. Write down what JPEG XT claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to JPEG XT before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about JPEG XT ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of JPEG XT

In research: JPEG XT appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses JPEG XT in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: JPEG XT is common in secondary-school and first-year university syllabi. It links to neighbouring topics Belgian inventions, Computer-related introductions in 2015, German inventions, so understanding it makes those chapters shorter.
In everyday life: Look for JPEG XT outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study JPEG XT in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what JPEG XT means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain JPEG XT out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is JPEG XT in simple terms?

JPEG XT (ISO/IEC 18477) is an image compression standard which specifies backward-compatible extensions of the base JPEG standard (ISO/IEC 10918-1 and ITU Rec. T.81).

Why does JPEG XT matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study JPEG XT?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on JPEG XT.