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  • Yaowu Xu's avatar
    WebM Experimental Codec Branch Snapshot · 6035da54
    Yaowu Xu authored
    This is a code snapshot of experimental work currently ongoing for a
    next-generation codec.
    The codebase has been cut down considerably from the libvpx baseline.
    For example, we are currently only supporting VBR 2-pass rate control
    and have removed most of the code relating to coding speed, threading,
    error resilience, partitions and various other features.  This is in
    part to make the codebase easier to work on and experiment with, but
    also because we want to have an open discussion about how the bitstream
    will be structured and partitioned and not have that conversation
    constrained by past work.
    Our basic working pattern has been to initially encapsulate experiments
    using configure options linked to #IF CONFIG_XXX statements in the
    code. Once experiments have matured and we are reasonably happy that
    they give benefit and can be merged without breaking other experiments,
    we remove the conditional compile statements and merge them in.
    Current changes include:
    * Temporal coding experiment for segments (though still only 4 max, it
      will likely be increased).
    * Segment feature experiment - to allow various bits of information to
      be coded at the segment level. Features tested so far include mode
      and reference frame information, limiting end of block offset and
      transform size, alongside Q and loop filter parameters, but this set
      is very fluid.
    * Support for 8x8 transform - 8x8 dct with 2nd order 2x2 haar is used
      in MBs using 16x16 prediction modes within inter frames.
    * Compound prediction (combination of signals from existing predictors
      to create a new predictor).
    * 8 tap interpolation filters and 1/8th pel motion vectors.
    * Loop filter modifications.
    * Various entropy modifications and changes to how entropy contexts and
      updates are handled.
    * Extended quantizer range matched to transform precision improvements.
    There are also ongoing further experiments that we hope to merge in the
    near future: For example, coding of motion and other aspects of the
    prediction signal to better support larger image formats, use of larger
    block sizes (e.g. 32x32 and up) and lossless non-transform based coding
    options (especially for key frames). It is our hope that we will be
    able to make regular updates and we will warmly welcome community
    Please be warned that, at this stage, the codebase is currently slower
    than VP8 stable branch as most new code has not been optimized, and
    even the 'C' has been deliberately written to be simple and obvious,
    not fast.
    The following graphs have the initial test results, numbers in the
    tables measure the compression improvement in terms of percentage. The
    build has  the following optional experiments configured:
    --enable-experimental --enable-enhanced_interp --enable-uvintra
    --enable-high_precision_mv --enable-sixteenth_subpel_uv
    CIF Size clips:
    HD size clips:
    (stable_20120309 represents encoding results of WebM master branch
    build as of commit#7a159071)
    They were encoded using the following encode parameters:
    --good --cpu-used=0 -t 0 --lag-in-frames=25 --min-q=0 --max-q=63
    --end-usage=0 --auto-alt-ref=1 -p 2 --pass=2 --kf-max-dist=9999
    --kf-min-dist=0 --drop-frame=0 --static-thresh=0 --bias-pct=50
    --minsection-pct=0 --maxsection-pct=800 --sharpness=0
    --arnr-maxframes=7 --arnr-strength=3(for HD,6 for CIF)
    Change-Id: I5c62ed09cfff5815a2bb34e7820d6a810c23183c