text_src/14.01__vp8-bitstream__dequantization.txt

#### 14.1 Dequantization                                   {#h-14-01}


After decoding the DCTs/WHTs as described above, each (quantized)
coefficient in each subblock is multiplied by one of six
dequantization factors, the choice of factor depending on the plane
(Y2, Y, or chroma) and position (DC = coefficient zero, AC = any
other coefficient).  If the current macroblock has overridden the
quantizer level (as described in Section 10), then the six factors
are looked up from two dequantization tables with appropriate scaling
and clamping using the single index supplied by the override.
Otherwise, the frame-level dequantization factors (as described in
Section 9.6) are used.  In either case, the multiplies are computed
and stored using 16-bit signed integers.

The two dequantization tables, which may also be found in the
reference decoder file `dequant_data.h` (Section 20.3), are as follows.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
static const int dc_qlookup[QINDEX_RANGE] =
{
    4,   5,   6,   7,   8,   9,  10,  10,   11,  12,  13,  14,  15,
   16,  17,  17,  18,  19,  20,  20,  21,   21,  22,  22,  23,  23,
   24,  25,  25,  26,  27,  28,  29,  30,   31,  32,  33,  34,  35,
   36,  37,  37,  38,  39,  40,  41,  42,   43,  44,  45,  46,  46,
   47,  48,  49,  50,  51,  52,  53,  54,   55,  56,  57,  58,  59,
   60,  61,  62,  63,  64,  65,  66,  67,   68,  69,  70,  71,  72,
   73,  74,  75,  76,  76,  77,  78,  79,   80,  81,  82,  83,  84,
   85,  86,  87,  88,  89,  91,  93,  95,   96,  98, 100, 101, 102,
   104, 106, 108, 110, 112, 114, 116, 118, 122, 124, 126, 128, 130,
   132, 134, 136, 138, 140, 143, 145, 148, 151, 154, 157,
};

static const int ac_qlookup[QINDEX_RANGE] =
{
    4,   5,   6,   7,   8,   9,  10,  11,  12,  13,  14,  15,  16,
   17,  18,  19,  20,  21,  22,  23,  24,  25,  26,  27,  28,  29,
   30,  31,  32,  33,  34,  35,  36,  37,  38,  39,  40,  41,  42,
   43,  44,  45,  46,  47,  48,  49,  50,  51,  52,  53,  54,  55,
   56,  57,  58,  60,  62,  64,  66,  68,  70,  72,  74,  76,  78,
   80,  82,  84,  86,  88,  90,  92,  94,  96,  98, 100, 102, 104,
  106, 108, 110, 112, 114, 116, 119, 122, 125, 128, 131, 134, 137,
  140, 143, 146, 149, 152, 155, 158, 161, 164, 167, 170, 173, 177,
  181, 185, 189, 193, 197, 201, 205, 209, 213, 217, 221, 225, 229,
  234, 239, 245, 249, 254, 259, 264, 269, 274, 279, 284,
};
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}


Lookup values from the above two tables are directly used in the DC
and AC coefficients in Y1, respectively.  For Y2 and chroma, values
from the above tables undergo either scaling or clamping before the
multiplies.  Details regarding these scaling and clamping processes
can be found in related lookup functions in `dixie.c`(Section 20.4).