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| #### 9.3 Segment-Based Adjustments {#h-09-03} |
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| This subsection contains probability and value information for |
| implementing segment adaptive adjustments to default decoder |
| behavior. The data in this subsection is used in the decoding of the |
| ensuing per-segment information and applies to the entire frame. |
| When segment adaptive adjustments are enabled, each macroblock will |
| be assigned a segment ID. Macroblocks with the same segment ID |
| belong to the same segment and have the same adaptive adjustments |
| over default baseline values for the frame. The adjustments can be |
| quantizer level or loop filter strength. |
| |
| The context for decoding this feature at the macroblock level is |
| provided by a subsection in the frame header, which contains: |
| |
| 1. A `segmentation_enabled` flag that enables the feature for this |
| frame if set to `1`, and disables it if set to `0`. The following |
| fields occur if the feature is enabled. |
| |
| 2. `L(1)` indicates if the segment map is updated for the current |
| frame (`update_mb_segmentation_map`). |
| |
| 3. `L(1)` indicates if the segment feature data items are updated for |
| the current frame (`update_segment_feature_data`). |
| |
| 4. If Item 3 above (`update_segment_feature_data`) is `1`, the following |
| fields occur: |
| |
| a. `L(1)`, the mode of segment feature data |
| (`segment_feature_mode`), can be absolute-value mode (`0`) or |
| delta value mode (`1`). |
| |
| b. Segment feature data items are decoded segment by segment for |
| each segment feature. For every data item, a one-bit flag |
| indicates whether the item is `0`, or a non-zero value to be |
| decoded. If the value is non-zero, then the value is decoded |
| as a magnitude `L(n)`, followed by a one-bit sign (`L(1)` -- `0` |
| for positive and `1` for negative). The length `n` can be looked |
| up from a pre-defined length table for all feature data. |
| |
| 5. If the `L(1)` flag as noted in Item 2 above is set to `1`, the |
| probabilities of the decoding tree for the segment map are |
| decoded from the bitstream. Each probability is decoded with a |
| one-bit flag indicating whether the probability is the default |
| value of `255` (flag is set to `0`), or an 8-bit value, `L(8)`, from |
| the bitstream. |
| |
| The layout and semantics supporting this feature at the macroblock |
| level are described in Section 10. |
| |
