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### Section 4: Overview of Compressed Data Format {#h-04-00}
The input to a VP8 decoder is a sequence of compressed frames whose
order matches their order in time. Issues such as the duration of
frames, the corresponding audio, and synchronization are generally
provided by the playback environment and are irrelevant to the
decoding process itself; however, to aid in fast seeking, a start
code is included in the header of each key frame.
The decoder is simply presented with a sequence of compressed frames
and produces a sequence of decompressed (reconstructed) YUV frames
corresponding to the input sequence. As stated in the Introduction,
the exact pixel values in the reconstructed frame are part of VP8's
specification. This document specifies the layout of the compressed
frames and gives unambiguous algorithms for the correct production of
reconstructed frames.
The first frame presented to the decompressor is of course a key
frame. This may be followed by any number of interframes; the
correct reconstruction of each frame depends on all prior frames up
to the key frame. The next key frame restarts this process: The
decompressor resets to its default initial condition upon reception
of a key frame, and the decoding of a key frame (and its ensuing
interframes) is completely independent of any prior decoding.
At the highest level, every compressed frame has three or more
pieces. It begins with an uncompressed data chunk comprising
10 bytes in the case of key frames and 3 bytes for interframes. This
is followed by two or more blocks of compressed data (called
_partitions_). These compressed data partitions begin and end on byte
The first compressed partition has two subsections:
1. Header information that applies to the frame as a whole.
2. Per-macroblock information specifying how each macroblock is
predicted from the already-reconstructed data that is available
to the decompressor.
As stated above, the macroblock-level information occurs in raster-
scan order.
The rest of the partitions contain, for each block, the DCT/WHT
coefficients (quantized and logically compressed) of the residue
signal to be added to the predicted block values. It typically
accounts for roughly 70% of the overall datarate. VP8 supports
packing the compressed DCT/WHT coefficients' data from macroblock
rows into separate partitions. If there is more than one partition
for these coefficients, the sizes of the partitions -- except the
last partition -- in bytes are also present in the bitstream right
after the above first partition. Each of the sizes is a 3-byte data
item written in little endian format. These sizes provide the
decoder direct access to all DCT/WHT coefficient partitions, which
enables parallel processing of the coefficients in a decoder.
The separate partitioning of the prediction data and coefficient data
also allows flexibility in the implementation of a decompressor: An
implementation may decode and store the prediction information for
the whole frame and then decode, transform, and add the residue
signal to the entire frame, or it may simultaneously decode both
partitions, calculating prediction information and adding in the
residue signal for each block in order. The length field in the
frame tag, which allows decoding of the second partition to begin
before the first partition has been completely decoded, is necessary
for the second "block-at-a-time" decoder implementation.
All partitions are decoded using separate instances of the boolean
entropy decoder described in Section 7. Although some of the data
represented within the partitions is conceptually "flat" (a bit is
just a bit with no probabilistic expectation one way or the other),
because of the way such coders work, there is never a direct
correspondence between a "conceptual bit" and an actual physical bit
in the compressed data partitions. Only in the 3- or 10-byte
uncompressed chunk described above is there such a physical
A related matter is that seeking within a partition is not supported.
The data must be decompressed and processed (or at least stored) in
the order in which it occurs in the partition.
While this document specifies the ordering of the partition data
correctly, the details and semantics of this data are discussed in a
more logical fashion to facilitate comprehension. For example, the
frame header contains updates to many probability tables used in
decoding per-macroblock data. The per-macroblock data is often
described before the layouts of the probabilities and their updates,
even though this is the opposite of their order in the bitstream.