JPEG XR

History

Description

Capabilities

Container format

Compression algorithm

• 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 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 by:
• While JPEG uses 8 × 8 blocks for its frequency transformation, JPEG XR primarily uses 4 × 4 block transforms.
• 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, resembles a 4 × 4 DCT but is lossless. 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, before each of its 4 × 4 core transform PCT stages. The filter operates on 4 × 4 blocks which are offset by 2 samples in each direction from the 4 × 4 core transform blocks. Its purpose is to improve compression capability and reduce block-boundary artifacts at low bitrates. At high bitrates, where such artifacts are typically not a problem, the prefiltering can be omitted to reduce encoding and decoding time. The overlap filtering is constructed using integer operations following a lifting scheme, so that it is also lossless. When appropriately combined, the POT and the PCT in JPEG-XR form a lapped transform.
• 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, macroblock-level AC coefficients, and lower-level AC coefficients. 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, 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.

Software support

• Adobe AIR 3.3 and later
• Adobe Flash Player 11.3 and later
• Integrated Performance Primitives
• , the reference implementation library
• Windows Imaging Component

Licensing