ITU-T Recommendations

The computational complexity of the block-coding algorithm of Rec. ITU-T T.800 | ISO/IEC 15444-1 can be a challenge in some applications.

Rec. ITU-T T.814 | ISO/IEC 15444-15 specifies a high-throughput (HT) block-coding algorithm that can be used in place of the block-coding algorithm specified in Rec. ITU-T T.800 | ISO/IEC 15444-1.

The HT block-coding algorithm increases decoding and encoding throughput and allows mathematically lossless transcoding to and from the block-coding algorithm specified in Rec. ITU-T T.800 | ISO/IEC 15444-1. This is achieved at the expense of some loss in coding efficiency and substantial elimination of quality scalability.

The HT block-coding algorithm adopts a coding pass structure like that of the block-coding algorithm of Rec. ITU-T T.800 | ISO/IEC 15444-1. No more than three coding passes are required for any given code-block in the final codestream, and arithmetic coding is replaced with a combination of variable length coding tools, adaptive run-length coding and simple bit-packing. The algorithm involves three passes: a significance propagation pass (HT SigProp coding pass), a magnitude refinement pass (HT MagRef coding pass) and a cleanup pass (HT cleanup coding pass).

The HT MagRef coding pass is identical to that of the block-coding algorithm of Rec. ITU-T T.800 | ISO/IEC 15444-1, operating in the bypass mode, except that code bits are packed into bytes with a little-endian bit order. That is, the first code bit in a byte appears in its LSB, as opposed to its MSB.

The HT SigProp coding pass is also very similar to that of the block-coding algorithm of Rec. ITU-T T.800 | ISO/IEC 15444-1, operating in the BYPASS mode, with the following two differences:

• code bits are again packed into bytes of the raw bit-stream with a little-endian bit order, instead of big-endian bit packing order; and

• the significance bits associated with a set of four stripe columns are emitted first, followed by the associated sign bits, before advancing to the next set of stripe columns, instead of inserting any required sign bit immediately after the same sample's magnitude bit.

The HT cleanup coding pass is, however, significantly different from that of the block-coding algorithm of Rec. ITU-T T.800 | ISO/IEC 15444-1, and most of ITU-T T.814 | ISO/IEC 15444-15 is devoted to its description.

Aside from the block-coding algorithm itself and the parsing of packet headers, the HT block-coding algorithm preserves the syntax and semantics of other parts of the codestream specified in Rec. ITU-T T.800 | ISO/IEC 15444-1.

Recommendation ITU-T T.814 (2019) is a common text with ISO/IEC 15444-15:2019, both in their first edition.