JPEG XR

Compressed image file format

JPEG XR (JPEG extended range) is an image compression standard for continuous tone photographic images, based on the HD Photo (formerly Windows Media Photo) specifications that Microsoft originally developed and patented. It supports both lossy and lossless compression, and is the preferred image format for Ecma 388 Open XML Paper Specification documents. Support for the format was made available in Adobe Flash Player 11.0, Adobe AIR 3.0, Sumatra PDF 2.1, Windows Imaging Component, NET Framework 3.0, Windows Vista, Windows 7, Windows 8, Internet Explorer 9, Internet Explorer 10, Internet Explorer 11, Pale Moon 27.2. As of January 2021, there were still no cameras that shoot photos in the JPEG XR (. JXR) format.

Microsoft first announced Windows Media Photo at WinHEC 2006, and then renamed it to HD Photo in November of that year. In July 2007, the Joint Photographic Experts Group and Microsoft announced HD Photo to be under consideration to become a JPEG standard known as JPEG XR. On 16 March 2009, JPEG XR was given final approval as ITU T Recommendation T.832 and starting in April 2009, it became available from the ITU T in "pre published" form. The ITU T updated its publication with a corrigendum approved in December 2009,

In 2010, after completion of the image coding specification, the ITU T and ISO/IEC also published a motion format specification (ITU T T.833 | ISO/IEC 29199 3), a conformance test set (ITU T T.834 | ISO/IEC 29199 4), and reference software (ITU T T.835 | ISO/IEC 29199 5) for JPEG XR. In 2011, they published a technical report describing the workflow architecture for the use of JPEG XR images in applications (ITU T T. Sup2 | ISO/IEC TR 29199 1).

One file container format that can be used to store JPEG XR image data is specified in Annex A of the JPEG XR standard. It is a TIFF like format using a table of Image File Directory (IFD) tags. A JPEG XR file contains image data, optional alpha channel data, metadata, optional XMP metadata stored as RDF/XML, and optional Exif metadata, in IFD tags. The image data is a contiguous self contained chunk of data. The optional alpha channel, if present, can be compressed as a separate image record, enabling decoding of the image data independently of transparency data in applications which do not support transparency. (Alternatively, JPEG XR also supports an "interleaved" alpha channel format in which the alpha channel data is encoded together with the other image data in a single compressed codestream.) Being TIFF based, this format inherits all of the limitations of the TIFF format including the 4 GB file size limit, which according to the HD Photo specification "will be addressed in a future update". New work has been started in the JPEG committee to enable the use of JPEG XR image coding within the JPX file storage format – enabling use of the JPIP protocol, which allows interactive browsing of networked images.

JPEG XR's design is conceptually very similar to JPEG: the source image is optionally converted to a luma chroma colorspace, the chroma planes are optionally subsampled, each plane is divided into fixed size blocks, the blocks are transformed into the frequency domain, and the frequency coefficients are quantized and entropy coded. Major differences include the following:

JPEG supports bit depths of 8 and 12 bits; JPEG XR supports bit depths of up to 32 bits. JPEG XR also supports lossless and lossy compression of floating point image data (by representing the floating point values in an IEEE 754 like format, and encoding them as though they were integers) and RGBE imagery. JFIF and other typical image encoding practices specify a linear transformation from RGB to YCbCr, which is slightly lossy in practice because of roundoff error. JPEG XR specifies a lossless colorspace transformation, namely YCoCg R, given (for RGB) by:

While JPEG uses 8 × 8 blocks for its frequency transformation, JPEG XR primarily uses 4 × 4 block transforms. (2 × 4 and 2 × 2 transformations are also defined for special cases involving chroma subsampling; encoder options include YUV 444, YUV 422, YUV 420, and a monochrome Y only.) While JPEG uses a single transformation stage, JPEG XR applies its 4 × 4 core transform in a two level hierarchical fashion within 16 × 16 macroblock regions. This gives the transform a wavelet like multi resolution hierarchy and improves its compression capability. The DCT, the frequency transformation used by JPEG, is slightly lossy because of roundoff error. JPEG XR uses a type of integer transform employing a lifting scheme. The required transform, called the Photo Core Transform (PCT), resembles a 4 × 4 DCT but is lossless (exactly invertible). In fact, it is a particular realization of a larger family of binary friendly multiplier less transforms called the binDCT. JPEG XR allows an optional overlap prefiltering step, called the Photo Overlap Transform (POT), before each of its 4 × 4 core transform PCT stages. In JPEG, the image DC coefficients of the DCT blocks are predicted by applying DC prediction from the left neighbor transform block, and no other coeffients are predicted. In JPEG XR, 4 × 4 blocks are grouped into macroblocks of 16 × 16 samples, and the 16 DC coefficients from the 4 × 4 blocks of each macroblock are passed through another level of frequency transformation, leaving three types of coefficients to be entropy coded: the macroblock DC coefficients (called DC), macroblock level AC coefficients (called "lowpass"), and lower level AC coefficients (called AC). Prediction of coefficient values across transform blocks is applied to the DC coefficients and to an additional row or column of AC coefficients as well. JPEG XR supports the encoding of an image by decomposing it into smaller individual rectangular tile area regions. Each tile area can be decoded independently from the other areas of the picture. This allows fast access to spatial areas of pictures without decoding the entire picture. JPEG XR's entropy coding phase is more adaptive and complex than JPEG's, involving a DC and AC coefficient prediction scheme, adaptive coefficient reordering (in contrast to JPEG's fixed zigzag ordering), and a form of adaptive Huffman coding for the coefficients themselves. JPEG uses a single quantization step size per DC/AC component per color plane per image. JPEG XR allows a selection of DC quantization step sizes on a tile region basis, and allows lowpass and AC quantization step sizes to vary from macroblock to macroblock. Because all encoding phases except quantization are lossless, JPEG XR is lossless when all quantization coefficients are equal to 1. This is not true of JPEG. JPEG defines a separate lossless mode which does not use the DCT, but it is not implemented by libjpeg and therefore not widely supported. The HD Photo bitstream specification claims that "HD Photo offers image quality comparable to JPEG 2000 with computational and memory performance more closely comparable to JPEG", that it "delivers a lossy compressed image of better perceptive quality than JPEG at less than half the file size", and that "lossless compressed images are typically 2.5 times smaller than the original uncompressed data".

Microsoft has patents on the technology in JPEG XR. A Microsoft representative stated in a January 2007 interview that in order to encourage the adoption and use of HD Photo, the specification is made available under the Microsoft Open Specification Promise, which asserts that Microsoft allows implementation of the specification for free, and will not file suits on the patented technology for its implementation, as reportedly stated by Josh Weisberg, director of Microsoft's Rich Media Group. As of 15 August 2010, Microsoft made the resulting JPEG XR standard available under its Community Promise. In July 2010, reference software to implement the JPEG XR standard was published as ITU T Recommendation T.835 and International Standard ISO/IEC 29199 5. Microsoft included these publications in the list of specifications covered by its Community Promise. This resolved any licensing issues with the library being implemented in software packages distributed under popular open source licences such as the GNU General Public License, with which the previously released "HD Photo Device Porting Kit" was incompatible.