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

The marker and parameters shown in Figure B.7 are defined below. The size and allowed values of each parameter are

given in Table B.5.

**DHT:**

Define Huffman table marker Marks the beginning of Huffman table definition parameters.

**Lh:**

Huffman table definition length Specifies the length of all Huffman table parameters shown in Figure B.7

(see B.1.1.4).

**Tc:**

Table class 0 = DC table or lossless table, 1 = AC table.

**Th:**

Huffman table destination identifier Specifies one of four possible destinations at the decoder into which

the Huffman table shall be installed.

**L**

**i**

**:**

Number of Huffman codes of length i Specifies the number of Huffman codes for each of the 16 possible

lengths allowed by this Specification. L

i

's are the elements of the list BITS.

**V**

**i,j**

**:**

Value associated with each Huffman code Specifies, for each i, the value associated with each Huffman

code of length i. The meaning of each value is determined by the Huffman coding model. The V

i,j

's are the

elements of the list HUFFVAL.

**Table B.5 Huffman table specification parameter sizes and values**

Values

Parameter

Size (bits)

Sequential DCT

Progressive DCT

Lossless

Baseline

Extended

Lh

16

2

17

1

+

+

=

*t*

*n*

*t*

*m*

?

D

Tc

1

4

0, 1

0

Th

1

4

0, 1

0-3

L

i

1

8

0-255

V

i, j

1

8

0-255

The value n in Table B.5 is the number of Huffman tables specified in the DHT marker segment. The value m

t

is the

number of parameters which follow the 16 L

i

(t) parameters for Huffman table t, and is given by:

*m*

*L*

*t*

*i*

*i*

=

=

1

16

In general, m

t

is different for each table.

Once a Huffman table has been defined for a particular destination, it replaces the previous tables stored in that

destination and shall be used when referenced, in the remaining scans of the current image and in subsequent images

represented in the abbreviated format for compressed image data. If a table has never been defined for a particular

destination, then when this destination is specified in a scan header, the results are unpredictable.

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

**41**