| |
| |
| ### 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 */ |
| } |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| {:lang="c"} |
| |
| |
| ... 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`. |
| |
