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### Section 10: Segment-Based Feature Adjustments {#h-10-00}
Every macroblock may optionally override some of the default
behaviors of the decoder. Specifically, VP8 uses segment-based
adjustments to support changing quantizer level and loop filter level
for a macroblock. When the segment-based adjustment feature is
enabled for a frame, each macroblock within the frame is coded with a
`segment_id`. This effectively segments all the macroblocks in the
current frame into a number of different segments. Macroblocks
within the same segment behave exactly the same for quantizer and
loop filter level adjustments.
If both the `segmentation_enabled` and `update_mb_segmentation_map` flags
in subsection B of the frame header take a value of `1`, the prediction
data for each (intra- or inter-coded) macroblock begins with a
specification of `segment_id` for the current macroblock. It is
decoded using this simple tree ...
const tree_index mb_segment_tree [2 * (4-1)] =
2, 4, /* root: "0", "1" subtrees */
-0, -1, /* "00" = 0th value, "01" = 1st value */
-2, -3 /* "10" = 2nd value, "11" = 3rd value */
... combined with a 3-entry probability table,
`mb_segment_tree_probs[3]`. The macroblock's `segment_id` is used later
in the decoding process to look into the `segment_feature_data` table
and determine how the quantizer and loop filter levels are adjusted.
The decoding of `segment_id`, together with the parsing of intra-
prediction modes (which is taken up next), is implemented in the
reference decoder file `modemv.c`.