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7.3 Inverse scan

Let the data at the output of the variable length decoder be denoted by QFS[n]. n is in the range 0 to 63.

This clause specifies the way in which the one-dimensional data, QFS[n], is converted into a two-dimensional array of coefficients denoted by QF[v][u]. u and v both lie in the range 0 to 7.

Two scan patterns are defined. The scan that shall be used shall be determined by alternate_scan which is encoded in the picture coding extension.

Figure 7-2 defines scan[alternate_scan][v][u] for the case that alternate_scan is zero. Figure 7-3 defines scan[alternate_scan][v][u] for the case that alternate_scan is one.

									u
		0	1	2	3	4	5	6	7
	0	0	1	5	6	14	15	27	28
	1	2	4	7	13	16	26	29	42
v	7	35	36	48	49	57	58	62	63

Figure 7-2. Definition of scan[0][v][u]

									u
		0	1	2	3	4	5	6	7
	0	0	4	6	20	22	36	38	52
	1	1	5	7	21	23	37	39	53
v	7	13	14	29	33	45	49	59	63

Figure 7-3. Definition of scan[1][v][u]

The inverse scan shall be any process equivalent to the following:

for (v=0; v<8; v++)

for (u=0; u<8; u++)

QF[v][u] = QFS[scan[alternate_scan][v][u]]

NOTE - The scan patterns defined here are often referred to as “zigzag scanning order”.

7.3.1 Inverse scan for matrix download

When the quantisation matrices are downloaded they are encoded in the bitstream in a scan order that is converted into the two-dimensional matrix used in the inverse quantiser in an identical manner to that used for coefficients.

For matrix download the scan defined by Figure 7-2 (i.e. scan[0][v][u]) shall always be used.

Let W[w][v][u] denote the weighting matrix in the inverse quantiser (see 7.4.2.1), and W’[w][n] denote the matrix as it is encoded in the bitstream. The matrix download shall then be equivalent to the following:

for (v=0; v<8; v++)

for (u=0; u<8; u++)

W[w][v][u] = W’[w][scan[0][v][u]]

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