blob: d60234ac1af0b309dd19977ab480c4c6948c7d45 [file] [log] [blame]
#### 9.6 Dequantization Indices {#h-09-06}
All residue signals are specified via a quantized 4x4 DCT applied to
the Y, U, V, or Y2 subblocks of a macroblock. As detailed in
Section 14, before inverting the transform, each decoded coefficient
is multiplied by one of six dequantization factors, the choice of
which depends on the plane (Y, chroma = U or V, Y2) and coefficient
position (DC = coefficient 0, AC = coefficients 1-15). The six
values are specified using 7-bit indices into six corresponding fixed
tables (the tables are given in Section 14).
The first 7-bit index gives the dequantization table index for
Y-plane AC coefficients, called yac_qi. It is always coded and acts
as a baseline for the other 5 quantization indices, each of which is
represented by a delta from this baseline index. Pseudocode for
reading the indices follows:
yac_qi = L(7); /* Y ac index always specified */
ydc_delta = F? delta(): 0; /* Y dc delta specified if
flag is true */
y2dc_delta = F? delta(): 0; /* Y2 dc delta specified if
flag is true */
y2ac_delta = F? delta(): 0; /* Y2 ac delta specified if
flag is true */
uvdc_delta = F? delta(): 0; /* chroma dc delta specified
if flag is true */
uvac_delta = F? delta(): 0; /* chroma ac delta specified
if flag is true */
Where `delta()` is the process to read 5 bits from the bitstream to
determine a signed delta value:
| Index | Description
| --------- | -------------------------------
| `L(4)` | Magnitude of delta
| `L(1)` | Sign of delta, `0` for positive and `1` for negative