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 (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". Software support A reference software implementation of JPEG XR has been