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libsdl2_mixer/external/libvorbis-1.3.5/doc/02-bitpacking.tex

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+% -*- mode: latex; TeX-master: "Vorbis_I_spec"; -*-
+%!TEX root = Vorbis_I_spec.tex
+% $Id$
+\section{Bitpacking Convention} \label{vorbis:spec:bitpacking}
+
+\subsection{Overview}
+
+The Vorbis codec uses relatively unstructured raw packets containing
+arbitrary-width binary integer fields.  Logically, these packets are a
+bitstream in which bits are coded one-by-one by the encoder and then
+read one-by-one in the same monotonically increasing order by the
+decoder.  Most current binary storage arrangements group bits into a
+native word size of eight bits (octets), sixteen bits, thirty-two bits
+or, less commonly other fixed word sizes.  The Vorbis bitpacking
+convention specifies the correct mapping of the logical packet
+bitstream into an actual representation in fixed-width words.
+
+
+\subsubsection{octets, bytes and words}
+
+In most contemporary architectures, a 'byte' is synonymous with an
+'octet', that is, eight bits.  This has not always been the case;
+seven, ten, eleven and sixteen bit 'bytes' have been used.  For
+purposes of the bitpacking convention, a byte implies the native,
+smallest integer storage representation offered by a platform.  On
+modern platforms, this is generally assumed to be eight bits (not
+necessarily because of the processor but because of the
+filesystem/memory architecture.  Modern filesystems invariably offer
+bytes as the fundamental atom of storage).  A 'word' is an integer
+size that is a grouped multiple of this smallest size.
+
+The most ubiquitous architectures today consider a 'byte' to be an
+octet (eight bits) and a word to be a group of two, four or eight
+bytes (16, 32 or 64 bits).  Note however that the Vorbis bitpacking
+convention is still well defined for any native byte size; Vorbis uses
+the native bit-width of a given storage system. This document assumes
+that a byte is one octet for purposes of example.
+
+\subsubsection{bit order}
+
+A byte has a well-defined 'least significant' bit (LSb), which is the
+only bit set when the byte is storing the two's complement integer
+value +1.  A byte's 'most significant' bit (MSb) is at the opposite
+end of the byte. Bits in a byte are numbered from zero at the LSb to
+$n$ ($n=7$ in an octet) for the
+MSb.
+
+
+
+\subsubsection{byte order}
+
+Words are native groupings of multiple bytes.  Several byte orderings
+are possible in a word; the common ones are 3-2-1-0 ('big endian' or
+'most significant byte first' in which the highest-valued byte comes
+first), 0-1-2-3 ('little endian' or 'least significant byte first' in
+which the lowest value byte comes first) and less commonly 3-1-2-0 and
+0-2-1-3 ('mixed endian').
+
+The Vorbis bitpacking convention specifies storage and bitstream
+manipulation at the byte, not word, level, thus host word ordering is
+of a concern only during optimization when writing high performance
+code that operates on a word of storage at a time rather than by byte.
+Logically, bytes are always coded and decoded in order from byte zero
+through byte $n$.
+
+
+
+\subsubsection{coding bits into byte sequences}
+
+The Vorbis codec has need to code arbitrary bit-width integers, from
+zero to 32 bits wide, into packets.  These integer fields are not
+aligned to the boundaries of the byte representation; the next field
+is written at the bit position at which the previous field ends.
+
+The encoder logically packs integers by writing the LSb of a binary
+integer to the logical bitstream first, followed by next least
+significant bit, etc, until the requested number of bits have been
+coded.  When packing the bits into bytes, the encoder begins by
+placing the LSb of the integer to be written into the least
+significant unused bit position of the destination byte, followed by
+the next-least significant bit of the source integer and so on up to
+the requested number of bits.  When all bits of the destination byte
+have been filled, encoding continues by zeroing all bits of the next
+byte and writing the next bit into the bit position 0 of that byte.
+Decoding follows the same process as encoding, but by reading bits
+from the byte stream and reassembling them into integers.
+
+
+
+\subsubsection{signedness}
+
+The signedness of a specific number resulting from decode is to be
+interpreted by the decoder given decode context.  That is, the three
+bit binary pattern 'b111' can be taken to represent either 'seven' as
+an unsigned integer, or '-1' as a signed, two's complement integer.
+The encoder and decoder are responsible for knowing if fields are to
+be treated as signed or unsigned.
+
+
+
+\subsubsection{coding example}
+
+Code the 4 bit integer value '12' [b1100] into an empty bytestream.
+Bytestream result:
+
+\begin{Verbatim}[commandchars=\\\{\}]
+              |
+              V
+
+        7 6 5 4 3 2 1 0
+byte 0 [0 0 0 0 1 1 0 0]  <-
+byte 1 [               ]
+byte 2 [               ]
+byte 3 [               ]
+             ...
+byte n [               ]  bytestream length == 1 byte
+
+\end{Verbatim}
+
+
+Continue by coding the 3 bit integer value '-1' [b111]:
+
+\begin{Verbatim}[commandchars=\\\{\}]
+        |
+        V
+
+        7 6 5 4 3 2 1 0
+byte 0 [0 1 1 1 1 1 0 0]  <-
+byte 1 [               ]
+byte 2 [               ]
+byte 3 [               ]
+             ...
+byte n [               ]  bytestream length == 1 byte
+\end{Verbatim}
+
+
+Continue by coding the 7 bit integer value '17' [b0010001]:
+
+\begin{Verbatim}[commandchars=\\\{\}]
+          |
+          V
+
+        7 6 5 4 3 2 1 0
+byte 0 [1 1 1 1 1 1 0 0]
+byte 1 [0 0 0 0 1 0 0 0]  <-
+byte 2 [               ]
+byte 3 [               ]
+             ...
+byte n [               ]  bytestream length == 2 bytes
+                          bit cursor == 6
+\end{Verbatim}
+
+
+Continue by coding the 13 bit integer value '6969' [b110 11001110 01]:
+
+\begin{Verbatim}[commandchars=\\\{\}]
+                |
+                V
+
+        7 6 5 4 3 2 1 0
+byte 0 [1 1 1 1 1 1 0 0]
+byte 1 [0 1 0 0 1 0 0 0]
+byte 2 [1 1 0 0 1 1 1 0]
+byte 3 [0 0 0 0 0 1 1 0]  <-
+             ...
+byte n [               ]  bytestream length == 4 bytes
+
+\end{Verbatim}
+
+
+
+
+\subsubsection{decoding example}
+
+Reading from the beginning of the bytestream encoded in the above example:
+
+\begin{Verbatim}[commandchars=\\\{\}]
+                      |
+                      V
+
+        7 6 5 4 3 2 1 0
+byte 0 [1 1 1 1 1 1 0 0]  <-
+byte 1 [0 1 0 0 1 0 0 0]
+byte 2 [1 1 0 0 1 1 1 0]
+byte 3 [0 0 0 0 0 1 1 0]  bytestream length == 4 bytes
+
+\end{Verbatim}
+
+
+We read two, two-bit integer fields, resulting in the returned numbers
+'b00' and 'b11'.  Two things are worth noting here:
+
+\begin{itemize}
+\item Although these four bits were originally written as a single
+four-bit integer, reading some other combination of bit-widths from the
+bitstream is well defined.  There are no artificial alignment
+boundaries maintained in the bitstream.
+
+\item The second value is the
+two-bit-wide integer 'b11'.  This value may be interpreted either as
+the unsigned value '3', or the signed value '-1'.  Signedness is
+dependent on decode context.
+\end{itemize}
+
+
+
+
+\subsubsection{end-of-packet alignment}
+
+The typical use of bitpacking is to produce many independent
+byte-aligned packets which are embedded into a larger byte-aligned
+container structure, such as an Ogg transport bitstream.  Externally,
+each bytestream (encoded bitstream) must begin and end on a byte
+boundary.  Often, the encoded bitstream is not an integer number of
+bytes, and so there is unused (uncoded) space in the last byte of a
+packet.
+
+Unused space in the last byte of a bytestream is always zeroed during
+the coding process.  Thus, should this unused space be read, it will
+return binary zeroes.
+
+Attempting to read past the end of an encoded packet results in an
+'end-of-packet' condition.  End-of-packet is not to be considered an
+error; it is merely a state indicating that there is insufficient
+remaining data to fulfill the desired read size.  Vorbis uses truncated
+packets as a normal mode of operation, and as such, decoders must
+handle reading past the end of a packet as a typical mode of
+operation. Any further read operations after an 'end-of-packet'
+condition shall also return 'end-of-packet'.
+
+
+
+\subsubsection{reading zero bits}
+
+Reading a zero-bit-wide integer returns the value '0' and does not
+increment the stream cursor.  Reading to the end of the packet (but
+not past, such that an 'end-of-packet' condition has not triggered)
+and then reading a zero bit integer shall succeed, returning 0, and
+not trigger an end-of-packet condition.  Reading a zero-bit-wide
+integer after a previous read sets 'end-of-packet' shall also fail
+with 'end-of-packet'.
+
+
+
+
+
+