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  • Ronald S. Bultje's avatar
    [WIP] Add column-based tiling. · 1407bdc2
    Ronald S. Bultje authored
    This patch adds column-based tiling. The idea is to make each tile
    independently decodable (after reading the common frame header) and
    also independendly encodable (minus within-frame cost adjustments in
    the RD loop) to speed-up hardware & software en/decoders if they used
    multi-threading. Column-based tiling has the added advantage (over
    other tiling methods) that it minimizes realtime use-case latency,
    since all threads can start encoding data as soon as the first SB-row
    worth of data is available to the encoder.
    There is some test code that does random tile ordering in the decoder,
    to confirm that each tile is indeed independently decodable from other
    tiles in the same frame. At tile edges, all contexts assume default
    values (i.e. 0, 0 motion vector, no coefficients, DC intra4x4 mode),
    and motion vector search and ordering do not cross tiles in the same
    t log
    Tile independence is not maintained between frames ATM, i.e. tile 0 of
    frame 1 is free to use motion vectors that point into any tile of frame
    0. We support 1 (i.e. no tiling), 2 or 4 column-tiles.
    The loopfilter crosses tile boundaries. I discussed this briefly with Aki
    and he says that's OK. An in-loop loopfilter would need to do some sync
    between tile threads, but that shouldn't be a big issue.
    Resuls: with tiling disabled, we go up slightly because of improved edge
    use in the intra4x4 prediction. With 2 tiles, we lose about ~1% on derf,
    ~0.35% on HD and ~0.55% on STD/HD. With 4 tiles, we lose another ~1.5%
    on derf ~0.77% on HD and ~0.85% on STD/HD. Most of this loss is
    concentrated in the low-bitrate end of clips, and most of it is because
    of the loss of edges at tile boundaries and the resulting loss of intra
    - more tiles (perhaps allow row-based tiling also, and max. 8 tiles)?
    - maybe optionally (for EC purposes), motion vectors themselves
      should not cross tile edges, or we should emulate such borders as
      if they were off-frame, to limit error propagation to within one
      tile only. This doesn't have to be the default behaviour but could
      be an optional bitstream flag.
    Change-Id: I5951c3a0742a767b20bc9fb5af685d9892c2c96f