**ISO/IEC 10918-1 : 1993(E)**

**Annex G**

**Progressive DCT-based mode of operation**

(This annex forms an integral part of this Recommendation |

International Standard)

This annex provides a **functional specification** of the following coding processes for the progressive DCT-based mode

of operation:

1)

spectral selection only, Huffman coding, 8-bit sample precision;

2)

spectral selection only, arithmetic coding, 8-bit sample precision;

3)

full progression, Huffman coding, 8-bit sample precision;

4)

full progression, arithmetic coding, 8-bit sample precision;

5)

spectral selection only, Huffman coding, 12-bit sample precision;

6)

spectral selection only, arithmetic coding, 12-bit sample precision;

7)

full progression, Huffman coding, 12-bit sample precision;

8)

full progression, arithmetic coding, 12-bit sample precision.

For each of these, the encoding process is specified in G.1, and the decoding process is specified in G.2. The functional

specification is presented by means of specific flow charts for the various procedures which comprise these coding

processes.

NOTE

There is **no requirement** in this Specification that any encoder or decoder which embodies one of the above-named

processes shall implement the procedures in precisely the manner specified by the flow charts in this annex. It is necessary only that an

encoder or decoder implement the **function** specified in this annex. The sole criterion for an encoder or decoder to be considered in

compliance with this Specification is that it satisfy the requirements given in clause 6 (for encoders) or clause 7 (for decoders), as

determined by the compliance tests specified in Part 2.

The number of Huffman or arithmetic conditioning tables which may be used within the same scan is four.

Two complementary progressive procedures are defined, spectral selection and successive approximation.

In spectral selection the DCT coefficients of each block are segmented into frequency bands. The bands are coded in

separate scans.

In successive approximation the DCT coefficients are divided by a power of two before coding. In the decoder the

coefficients are multiplied by that same power of two before computing the IDCT. In the succeeding scans the precision of

the coefficients is increased by one bit in each scan until full precision is reached.

An encoder or decoder implementing a full progression uses spectral selection within successive approximation. An

allowed subset is spectral selection alone.

Figure G.1 illustrates the spectral selection and successive approximation progressive processes.

**G.1**

**Progressive DCT-based encoding processes**

**G.1.1**

**Control procedures and coding models for progressive DCT-based procedures**

**G.1.1.1 Control procedures for progressive DCT-based encoders**

The control procedures for encoding an image and its constituent parts the frame, scan, restart interval and MCU are

given in Figures E.1 through E.5.

The control structure for encoding a frame is the same as for the sequential procedures. However, it is convenient to

calculate the FDCT for the entire set of components in a frame before starting the scans. A buffer which is large enough to

store all of the DCT coefficients may be used for this progressive mode of operation.

The number of scans is determined by the progression defined; the number of scans may be much larger than the number

of components in the frame.

**CCITT Rec. T.81 (1992 E)**

**119**