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| #### 9.6 Dequantization Indices {#h-09-06} |
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| 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 Chapter 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 Chapter 14). |
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| 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. Following is pseudo code for reading the indices: |
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| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| yac_qi = L(7); /* Y ac index always specified */ |
| ydc_delta = F? delta(): 0; /* Y dc delta specified if |
| flag is true */ |
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| 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 */ |
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| 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 */ |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| {:lang="c"} |
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| Where `delta()` is the process to read 5 bits from the bitstream to determine a signed delta value: |
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| | Index | Description |
| | --------- | ------------------------------- |
| | `L(4)` | Magnitude of delta |
| | `L(1)` | Sign of delta, `0` for positive and `1` for negative |
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