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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-15"/>
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<title>Ogg Vorbis Documentation</title>
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h1, h1 a, h2, h2 a, h3, h3 a {
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line-height: 1.5em;
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text-align: center;
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</style>
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</head>
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<body>
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Programming with Xiph.Org <tt>libvorbis</tt>
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Description
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Libvorbis is the Xiph.Org Foundation's portable Ogg Vorbis CODEC
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implemented as a programmatic library. Libvorbis provides primitives
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to handle framing and manipulation of Ogg bitstreams (used by the
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Vorbis for streaming), a full analysis (encoding) interface as well as
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packet decoding and synthesis for playback.
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The libvorbis library does not provide any system interface; a
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full-featured demonstration player included with the library
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distribtion provides example code for a variety of system interfaces
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as well as a working example of using libvorbis in production code.
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Encoding Overview
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Decoding Overview
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Decoding a bitstream with libvorbis follows roughly the following
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steps:
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Frame the incoming bitstream into pages
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Sort the pages by logical bitstream and buffer then into logical streams
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Decompose the logical streams into raw packets
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Reconstruct segments of the original data from each packet
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Glue the reconstructed segments back into a decoded stream
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Framing
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An Ogg bitstream is logically arranged into pages, but to decode
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the pages, we have to find them first. The raw bitstream is first fed
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into an <tt>ogg_sync_state</tt> buffer using <tt>ogg_sync_buffer()</tt>
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and <tt>ogg_sync_wrote()</tt>. After each block we submit to the sync
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buffer, we should check to see if we can frame and extract a complete
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page or pages using <tt>ogg_sync_pageout()</tt>. Extra pages are
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buffered; allowing them to build up in the <tt>ogg_sync_state</tt>
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buffer will eventually exhaust memory.
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The Ogg pages returned from <tt>ogg_sync_pageout</tt> need not be
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decoded further to be used as landmarks in seeking; seeking can be
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either a rough process of simply jumping to approximately intuited
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portions of the bitstream, or it can be a precise bisection process
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that captures pages and inspects data position. When seeking,
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however, sequential multiplexing (chaining) must be accounted for;
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beginning play in a new logical bitstream requires initializing a
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synthesis engine with the headers from that bitstream. Vorbis
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bitstreams do not make use of concurent multiplexing (grouping).
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Sorting
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The pages produced by <tt>ogg_sync_pageout</tt> are then sorted by
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serial number to seperate logical bitstreams. Initialize logical
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bitstream buffers (<tt>og_stream_state</tt>) using
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<tt>ogg_stream_init()</tt>. Pages are submitted to the matching
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logical bitstream buffer using <tt>ogg_stream_pagein</tt>; the serial
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number of the page and the stream buffer must match, or the page will
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be rejected. A page submitted out of sequence will simply be noted,
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and in the course of outputting packets, the hole will be flagged
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(<tt>ogg_sync_pageout</tt> and <tt>ogg_stream_packetout</tt> will
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return a negative value at positions where they had to recapture the
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stream).
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Extracting packets
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After submitting page[s] to a logical stream, read available packets
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using <tt>ogg_stream_packetout</tt>.
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Decoding packets
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Reassembling data segments
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Ogg Bitstream Manipulation Structures
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Two of the Ogg bitstream data structures are intended to be
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transparent to the developer; the fields should be used directly.
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ogg_packet
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typedef struct {
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unsigned char *packet;
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long bytes;
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long b_o_s;
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long e_o_s;
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size64 granulepos;
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} ogg_packet;
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packet:
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a pointer to the byte data of the raw packet
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bytes:
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the size of the packet' raw data
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b_o_s:
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beginning of stream; nonzero if this is the first packet of
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the logical bitstream
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e_o_s:
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end of stream; nonzero if this is the last packet of the
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logical bitstream
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granulepos:
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the absolute position of this packet in the original
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uncompressed data stream.
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encoding notes
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The encoder is responsible for setting all of
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the fields of the packet to appropriate values before submission to
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<tt>ogg_stream_packetin()</tt>; however, it is noted that the value in
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<tt>b_o_s</tt> is ignored; the first page produced from a given
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<tt>ogg_stream_state</tt> structure will be stamped as the initial
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page. <tt>e_o_s</tt>, however, must be set; this is the means by
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which the stream encoding primitives handle end of stream and cleanup.
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decoding notes
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<tt>ogg_stream_packetout()</tt> sets the fields
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to appropriate values. Note that granulepos will be >= 0 only in the
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case that the given packet actually represents that position (ie, only
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the last packet completed on any page will have a meaningful
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<tt>granulepos</tt>). Intervening frames will see <tt>granulepos</tt> set
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to -1.
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ogg_page
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typedef struct {
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unsigned char *header;
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long header_len;
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unsigned char *body;
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long body_len;
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} ogg_page;
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header:
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pointer to the page header data
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header_len:
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length of the page header in bytes
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body:
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pointer to the page body
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body_len:
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length of the page body
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Note that although the <tt>header</tt> and <tt>body</tt> pointers do
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not necessarily point into a single contiguous page vector, the page
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body must immediately follow the header in the bitstream.
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Ogg Bitstream Manipulation Functions
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int ogg_page_bos(ogg_page *og);
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Returns the 'beginning of stream' flag for the given Ogg page. The
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beginning of stream flag is set on the initial page of a logical
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bitstream.
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Zero indicates the flag is cleared (this is not the initial page of a
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logical bitstream). Nonzero indicates the flag is set (this is the
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initial page of a logical bitstream).
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int ogg_page_continued(ogg_page *og);
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Returns the 'packet continued' flag for the given Ogg page. The packet
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continued flag indicates whether or not the body data of this page
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begins with packet continued from a preceeding page.
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Zero (unset) indicates that the body data begins with a new packet.
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Nonzero (set) indicates that the first packet data on the page is a
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continuation from the preceeding page.
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int ogg_page_eos(ogg_page *og);
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Returns the 'end of stream' flag for a give Ogg page. The end of page
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flag is set on the last (terminal) page of a logical bitstream.
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Zero (unset) indicates that this is not the last page of a logical
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bitstream. Nonzero (set) indicates that this is the last page of a
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logical bitstream and that no addiitonal pages belonging to this
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bitstream may follow.
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size64 ogg_page_granulepos(ogg_page *og);
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Returns the position of this page as an absolute position within the
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original uncompressed data. The position, as returned, is 'frames
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encoded to date up to and including the last whole packet on this
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page'. Partial packets begun on this page but continued to the
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following page are not included. If no packet ends on this page, the
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frame position value will be equal to the frame position value of the
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preceeding page. If none of the original uncompressed data is yet
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represented in the logical bitstream (for example, the first page of a
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bitstream consists only of a header packet; this packet encodes only
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metadata), the value shall be zero.
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The units of the framenumber are determined by media mapping. A
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vorbis audio bitstream, for example, defines one frame to be the
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channel values from a single sampling period (eg, a 16 bit stereo
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bitstream consists of two samples of two bytes for a total of four
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bytes, thus a frame would be four bytes). A video stream defines one
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frame to be a single frame of video.
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int ogg_page_pageno(ogg_page *og);
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Returns the sequential page number of the given Ogg page. The first
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page in a logical bitstream is numbered zero; following pages are
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numbered in increasing monotonic order.
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int ogg_page_serialno(ogg_page *og);
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Returns the serial number of the given Ogg page. The serial number is
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used as a handle to distinguish various logical bitstreams in a
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physical Ogg bitstresm. Every logical bitstream within a
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physical bitstream must use a unique (within the scope of the physical
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bitstream) serial number, which is stamped on all bitstream pages.
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int ogg_page_version(ogg_page *og);
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Returns the revision of the Ogg bitstream structure of the given page.
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Currently, the only permitted number is zero. Later revisions of the
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bitstream spec will increment this version should any changes be
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incompatable.
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int ogg_stream_clear(ogg_stream_state *os);
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Clears and deallocates the internal storage of the given Ogg stream.
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After clearing, the stream structure is not initialized for use;
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<tt>ogg_stream_init</tt> must be called to reinitialize for use.
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Use <tt>ogg_stream_reset</tt> to reset the stream state
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to a fresh, intiialized state.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
<tt>ogg_stream_clear</tt> does not call <tt>free()</tt> on the pointer
|
|
Packit |
06404a |
<tt>os</tt>, allowing use of this call on stream structures in static
|
|
Packit |
06404a |
or automatic storage. <tt>ogg_stream_destroy</tt>is a complimentary
|
|
Packit |
06404a |
function that frees the pointer as well.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Returns zero on success and non-zero on failure. This function always
|
|
Packit |
06404a |
succeeds.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
int ogg_stream_destroy(ogg_stream_state *os);
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Clears and deallocates the internal storage of the given Ogg stream,
|
|
Packit |
06404a |
then frees the storage associated with the pointer <tt>os</tt>.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
<tt>ogg_stream_clear</tt> does not call <tt>free()</tt> on the pointer
|
|
Packit |
06404a |
<tt>os</tt>, allowing use of that call on stream structures in static
|
|
Packit |
06404a |
or automatic storage.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Returns zero on success and non-zero on failure. This function always
|
|
Packit |
06404a |
succeeds.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
int ogg_stream_init(ogg_stream_state *os,int serialno);
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Initialize the storage associated with <tt>os</tt> for use as an Ogg
|
|
Packit |
06404a |
stream. This call is used to initialize a stream for both encode and
|
|
Packit |
06404a |
decode. The given serial number is the serial number that will be
|
|
Packit |
06404a |
stamped on pages of the produced bitstream (during encode), or used as
|
|
Packit |
06404a |
a check that pages match (during decode).
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Returns zero on success, nonzero on failure.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
int ogg_stream_packetin(ogg_stream_state *os, ogg_packet *op);
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Used during encoding to add the given raw packet to the given Ogg
|
|
Packit |
06404a |
bitstream. The contents of <tt>op</tt> are copied;
|
|
Packit |
06404a |
<tt>ogg_stream_packetin</tt> does not retain any pointers into
|
|
Packit |
06404a |
<tt>op</tt>'s storage. The encoding proccess buffers incoming packets
|
|
Packit |
06404a |
until enough packets have been assembled to form an entire page;
|
|
Packit |
06404a |
<tt>ogg_stream_pageout</tt> is used to read complete pages.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Returns zero on success, nonzero on failure.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
int ogg_stream_packetout(ogg_stream_state *os,ogg_packet *op);
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Used during decoding to read raw packets from the given logical
|
|
Packit |
06404a |
bitstream. <tt>ogg_stream_packetout</tt> will only return complete
|
|
Packit |
06404a |
packets for which checksumming indicates no corruption. The size and
|
|
Packit |
06404a |
contents of the packet exactly match those given in the encoding
|
|
Packit |
06404a |
process.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Returns zero if the next packet is not ready to be read (not buffered
|
|
Packit |
06404a |
or incomplete), positive if it returned a complete packet in
|
|
Packit |
06404a |
<tt>op</tt> and negative if there is a gap, extra bytes or corruption
|
|
Packit |
06404a |
at this position in the bitstream (essentially that the bitstream had
|
|
Packit |
06404a |
to be recaptured). A negative value is not necessarily an error. It
|
|
Packit |
06404a |
would be a common occurence when seeking, for example, which requires
|
|
Packit |
06404a |
recapture of the bitstream at the position decoding continued.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
If the return value is positive, <tt>ogg_stream_packetout</tt> placed
|
|
Packit |
06404a |
a packet in <tt>op</tt>. The data in <tt>op</tt> points to static
|
|
Packit |
06404a |
storage that is valid until the next call to
|
|
Packit |
06404a |
<tt>ogg_stream_pagein</tt>, <tt>ogg_stream_clear</tt>,
|
|
Packit |
06404a |
<tt>ogg_stream_reset</tt>, or <tt>ogg_stream_destroy</tt>. The
|
|
Packit |
06404a |
pointers are not invalidated by more calls to
|
|
Packit |
06404a |
<tt>ogg_stream_packetout</tt>.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
int ogg_stream_pagein(ogg_stream_state *os, ogg_page *og);
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Used during decoding to buffer the given complete, pre-verified page
|
|
Packit |
06404a |
for decoding into raw Ogg packets. The given page must be framed,
|
|
Packit |
06404a |
normally produced by <tt>ogg_sync_pageout</tt>, and from the logical
|
|
Packit |
06404a |
bitstream associated with <tt>os</tt> (the serial numbers must match).
|
|
Packit |
06404a |
The contents of the given page are copied; <tt>ogg_stream_pagein</tt>
|
|
Packit |
06404a |
retains no pointers into <tt>og</tt> storage.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Returns zero on success and non-zero on failure.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
int ogg_stream_pageout(ogg_stream_state *os, ogg_page *og);
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Used during encode to read complete pages from the stream buffer. The
|
|
Packit |
06404a |
returned page is ready for sending out to the real world.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Returns zero if there is no complete page ready for reading. Returns
|
|
Packit |
06404a |
nonzero when it has placed data for a complete page into
|
|
Packit |
06404a |
<tt>og</tt>. Note that the storage returned in og points into internal
|
|
Packit |
06404a |
storage; the pointers in <tt>og</tt> are valid until the next call to
|
|
Packit |
06404a |
<tt>ogg_stream_pageout</tt>, <tt>ogg_stream_packetin</tt>,
|
|
Packit |
06404a |
<tt>ogg_stream_reset</tt>, <tt>ogg_stream_clear</tt> or
|
|
Packit |
06404a |
<tt>ogg_stream_destroy</tt>.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
int ogg_stream_reset(ogg_stream_state *os);
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Resets the given stream's state to that of a blank, unused stream;
|
|
Packit |
06404a |
this may be used during encode or decode.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Note that if used during encode, it does not alter the stream's serial
|
|
Packit |
06404a |
number. In addition, the next page produced during encoding will be
|
|
Packit |
06404a |
marked as the 'initial' page of the logical bitstream.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
When used during decode, this simply clears the data buffer of any
|
|
Packit |
06404a |
pending pages. Beginning and end of stream cues are read from the
|
|
Packit |
06404a |
bitstream and are unaffected by reset.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Returns zero on success and non-zero on failure. This function always
|
|
Packit |
06404a |
succeeds.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
char *ogg_sync_buffer(ogg_sync_state *oy, long size);
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
This call is used to buffer a raw bitstream for framing and
|
|
Packit |
06404a |
verification. <tt>ogg_sync_buffer</tt> handles stream capture and
|
|
Packit |
06404a |
recapture, checksumming, and division into Ogg pages (as required by
|
|
Packit |
06404a |
<tt>ogg_stream_pagein</tt>).
|
|
Packit |
06404a |
|
|
Packit |
06404a |
<tt>ogg_sync_buffer</tt> exposes a buffer area into which the decoder
|
|
Packit |
06404a |
copies the next (up to) <tt>size</tt> bytes. We expose the buffer
|
|
Packit |
06404a |
(rather than taking a buffer) in order to avoid an extra copy many
|
|
Packit |
06404a |
uses; this way, for example, <tt>read()</tt> can transfer data
|
|
Packit |
06404a |
directly into the stream buffer without first needing to place it in
|
|
Packit |
06404a |
temporary storage.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Returns a pointer into <tt>oy</tt>'s internal bitstream sync buffer;
|
|
Packit |
06404a |
the remaining space in the sync buffer is at least <tt>size</tt>
|
|
Packit |
06404a |
bytes. The decoder need not write all of <tt>size</tt> bytes;
|
|
Packit |
06404a |
<tt>ogg_sync_wrote</tt> is used to inform the engine how many bytes
|
|
Packit |
06404a |
were actually written. Use of <tt>ogg_sync_wrote</tt> after writing
|
|
Packit |
06404a |
into the exposed buffer is mandantory.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
int ogg_sync_clear(ogg_sync_state *oy);
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
<tt>ogg_sync_clear</tt>
|
|
Packit |
06404a |
clears and deallocates the internal storage of the given Ogg sync
|
|
Packit |
06404a |
buffer. After clearing, the sync structure is not initialized for
|
|
Packit |
06404a |
use; <tt>ogg_sync_init</tt> must be called to reinitialize for use.
|
|
Packit |
06404a |
Use <tt>ogg_sync_reset</tt> to reset the sync state and buffer to a
|
|
Packit |
06404a |
fresh, intiialized state.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
<tt>ogg_sync_clear</tt> does not call <tt>free()</tt> on the pointer
|
|
Packit |
06404a |
<tt>oy</tt>, allowing use of this call on sync structures in static
|
|
Packit |
06404a |
or automatic storage. <tt>ogg_sync_destroy</tt>is a complimentary
|
|
Packit |
06404a |
function that frees the pointer as well.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Returns zero on success and non-zero on failure. This function always
|
|
Packit |
06404a |
succeeds.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
int ogg_sync_destroy(ogg_sync_state *oy);
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Clears and deallocates the internal storage of the given Ogg sync
|
|
Packit |
06404a |
buffer, then frees the storage associated with the pointer
|
|
Packit |
06404a |
<tt>oy</tt>.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
An alternative function,<tt>ogg_sync_clear</tt>, does not call
|
|
Packit |
06404a |
<tt>free()</tt> on the pointer <tt>oy</tt>, allowing use of that call on
|
|
Packit |
06404a |
stream structures in static or automatic storage.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Returns zero on success and non-zero on failure. This function always
|
|
Packit |
06404a |
succeeds.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
int ogg_sync_init(ogg_sync_state *oy);
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Initializes the sync buffer <tt>oy</tt> for use.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Returns zero on success and non-zero on failure. This function always
|
|
Packit |
06404a |
succeeds.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
int ogg_sync_pageout(ogg_sync_state *oy, ogg_page *og);
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Reads complete, framed, verified Ogg pages from the sync buffer,
|
|
Packit |
06404a |
placing the page data in <tt>og</tt>.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Returns zero when there's no complete pages buffered for
|
|
Packit |
06404a |
retrieval. Returns negative when a loss of sync or recapture occurred
|
|
Packit |
06404a |
(this is not necessarily an error; recapture would be required after
|
|
Packit |
06404a |
seeking, for example). Returns positive when a page is returned in
|
|
Packit |
06404a |
<tt>og</tt>. Note that the data in <tt>og</tt> points into the sync
|
|
Packit |
06404a |
buffer storage; the pointers are valid until the next call to
|
|
Packit |
06404a |
<tt>ogg_sync_buffer</tt>, <tt>ogg_sync_clear</tt>,
|
|
Packit |
06404a |
<tt>ogg_sync_destroy</tt> or <tt>ogg_sync_reset</tt>.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
int ogg_sync_reset(ogg_sync_state *oy);
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
<tt>ogg_sync_reset</tt> resets the sync state in <tt>oy</tt> to a
|
|
Packit |
06404a |
clean, empty state. This is useful, for example, when seeking to a
|
|
Packit |
06404a |
new location in a bitstream.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Returns zero on success, nonzero on failure.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
int ogg_sync_wrote(ogg_sync_state *oy, long bytes);
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Used to inform the sync state as to how many bytes were actually
|
|
Packit |
06404a |
written into the exposed sync buffer. It must be equal to or less
|
|
Packit |
06404a |
than the size of the buffer requested.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
Returns zero on success and non-zero on failure; failure occurs only
|
|
Packit |
06404a |
when the number of bytes written were larger than the buffer.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
The Xiph Fish Logo is a
|
|
Packit |
06404a |
trademark (™) of Xiph.Org.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
These pages © 1994 - 2005 Xiph.Org. All rights reserved.
|
|
Packit |
06404a |
|
|
Packit |
06404a |
|
|
Packit |
06404a |
</body>
|
|
Packit |
06404a |
</html>
|