555 lines
18 KiB
HTML
555 lines
18 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html>
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<head>
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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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body {
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margin: 0 18px 0 18px;
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color: #333333;
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a {
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img {
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border: 0;
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}
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#xiphlogo {
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margin: 30px 0 16px 0;
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}
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#content p {
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line-height: 1.4;
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h1, h1 a, h2, h2 a, h3, h3 a {
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font-weight: bold;
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color: #ff9900;
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margin: 1.3em 0 8px 0;
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h1 {
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font-size: 1.3em;
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h2 {
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li {
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}
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#copyright {
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margin-top: 30px;
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line-height: 1.5em;
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text-align: center;
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clear: both;
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}
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</style>
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</head>
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<body>
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<div id="xiphlogo">
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<a href="http://www.xiph.org/"><img src="fish_xiph_org.png" alt="Fish Logo and Xiph.Org"/></a>
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</div>
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<h1>Programming with Xiph.Org <tt>libvorbis</tt></h1>
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<h2>Description</h2>
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<p>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.</p>
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<p>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.</p>
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<h2>Encoding Overview</h2>
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<h2>Decoding Overview</h2>
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<p>Decoding a bitstream with libvorbis follows roughly the following
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steps:</p>
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<ol>
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<li>Frame the incoming bitstream into pages</li>
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<li>Sort the pages by logical bitstream and buffer then into logical streams</li>
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<li>Decompose the logical streams into raw packets</li>
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<li>Reconstruct segments of the original data from each packet</li>
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<li>Glue the reconstructed segments back into a decoded stream</li>
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</ol>
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<h3>Framing</h3>
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<p>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.</p>
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<p>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).</p>
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<h3>Sorting</h3>
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<p>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).</p>
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<h3>Extracting packets</h3>
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<p>After submitting page[s] to a logical stream, read available packets
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using <tt>ogg_stream_packetout</tt>.</p>
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<h3>Decoding packets</h3>
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<h3>Reassembling data segments</h3>
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<h2>Ogg Bitstream Manipulation Structures</h2>
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<p>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.</p>
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<h3>ogg_packet</h3>
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<pre>
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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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</pre>
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<dl>
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<dt>packet:</dt>
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<dd>a pointer to the byte data of the raw packet</dd>
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<dt>bytes:</dt>
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<dd>the size of the packet' raw data</dd>
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<dt>b_o_s:</dt>
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<dd>beginning of stream; nonzero if this is the first packet of
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the logical bitstream</dd>
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<dt>e_o_s:</dt>
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<dd>end of stream; nonzero if this is the last packet of the
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logical bitstream</dd>
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<dt>granulepos:</dt>
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<dd>the absolute position of this packet in the original
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uncompressed data stream.</dd>
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</dl>
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<h4>encoding notes</h4>
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<p>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.</p>
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<h4>decoding notes</h4>
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<p><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.</p>
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<h3>ogg_page</h3>
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<pre>
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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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</pre>
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<dl>
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<dt>header:</dt>
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<dd>pointer to the page header data</dd>
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<dt>header_len:</dt>
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<dd>length of the page header in bytes</dd>
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<dt>body:</dt>
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<dd>pointer to the page body</dd>
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<dt>body_len:</dt>
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<dd>length of the page body</dd>
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</dl>
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<p>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.</p>
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<h2>Ogg Bitstream Manipulation Functions</h2>
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<h3>
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int ogg_page_bos(ogg_page *og);
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</h3>
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<p>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.</p>
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<p>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).</p>
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<h3>
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int ogg_page_continued(ogg_page *og);
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</h3>
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<p>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.</p>
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<p>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.</p>
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<h3>
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int ogg_page_eos(ogg_page *og);
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</h3>
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<p>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.</p>
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<p>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.</p>
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<h3>
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size64 ogg_page_granulepos(ogg_page *og);
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</h3>
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<p>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.</p>
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<p>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.</p>
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<h3>
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int ogg_page_pageno(ogg_page *og);
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</h3>
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<p>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.</p>
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<h3>
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int ogg_page_serialno(ogg_page *og);
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</h3>
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<p>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.</p>
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<h3>
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int ogg_page_version(ogg_page *og);
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</h3>
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<p>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.</p>
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<h3>
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int ogg_stream_clear(ogg_stream_state *os);
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</h3>
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<p>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.</p>
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<p><tt>ogg_stream_clear</tt> does not call <tt>free()</tt> on the pointer
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<tt>os</tt>, allowing use of this call on stream structures in static
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or automatic storage. <tt>ogg_stream_destroy</tt>is a complimentary
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function that frees the pointer as well.</p>
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<p>Returns zero on success and non-zero on failure. This function always
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succeeds.</p>
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<h3>
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int ogg_stream_destroy(ogg_stream_state *os);
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</h3>
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<p>Clears and deallocates the internal storage of the given Ogg stream,
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then frees the storage associated with the pointer <tt>os</tt>.</p>
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<p><tt>ogg_stream_clear</tt> does not call <tt>free()</tt> on the pointer
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<tt>os</tt>, allowing use of that call on stream structures in static
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or automatic storage.</p>
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<p>Returns zero on success and non-zero on failure. This function always
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succeeds.</p>
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<h3>
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int ogg_stream_init(ogg_stream_state *os,int serialno);
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</h3>
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<p>Initialize the storage associated with <tt>os</tt> for use as an Ogg
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stream. This call is used to initialize a stream for both encode and
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decode. The given serial number is the serial number that will be
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stamped on pages of the produced bitstream (during encode), or used as
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a check that pages match (during decode).</p>
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<p>Returns zero on success, nonzero on failure.</p>
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<h3>
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int ogg_stream_packetin(ogg_stream_state *os, ogg_packet *op);
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</h3>
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<p>Used during encoding to add the given raw packet to the given Ogg
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bitstream. The contents of <tt>op</tt> are copied;
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<tt>ogg_stream_packetin</tt> does not retain any pointers into
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<tt>op</tt>'s storage. The encoding proccess buffers incoming packets
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until enough packets have been assembled to form an entire page;
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<tt>ogg_stream_pageout</tt> is used to read complete pages.</p>
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<p>Returns zero on success, nonzero on failure.</p>
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<h3>
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int ogg_stream_packetout(ogg_stream_state *os,ogg_packet *op);
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</h3>
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<p>Used during decoding to read raw packets from the given logical
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bitstream. <tt>ogg_stream_packetout</tt> will only return complete
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packets for which checksumming indicates no corruption. The size and
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contents of the packet exactly match those given in the encoding
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process. </p>
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<p>Returns zero if the next packet is not ready to be read (not buffered
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or incomplete), positive if it returned a complete packet in
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<tt>op</tt> and negative if there is a gap, extra bytes or corruption
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at this position in the bitstream (essentially that the bitstream had
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to be recaptured). A negative value is not necessarily an error. It
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would be a common occurence when seeking, for example, which requires
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recapture of the bitstream at the position decoding continued.</p>
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<p>If the return value is positive, <tt>ogg_stream_packetout</tt> placed
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a packet in <tt>op</tt>. The data in <tt>op</tt> points to static
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storage that is valid until the next call to
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<tt>ogg_stream_pagein</tt>, <tt>ogg_stream_clear</tt>,
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<tt>ogg_stream_reset</tt>, or <tt>ogg_stream_destroy</tt>. The
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pointers are not invalidated by more calls to
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<tt>ogg_stream_packetout</tt>.</p>
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<h3>
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int ogg_stream_pagein(ogg_stream_state *os, ogg_page *og);
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</h3>
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<p>Used during decoding to buffer the given complete, pre-verified page
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for decoding into raw Ogg packets. The given page must be framed,
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normally produced by <tt>ogg_sync_pageout</tt>, and from the logical
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bitstream associated with <tt>os</tt> (the serial numbers must match).
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The contents of the given page are copied; <tt>ogg_stream_pagein</tt>
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retains no pointers into <tt>og</tt> storage.</p>
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<p>Returns zero on success and non-zero on failure.</p>
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<h3>
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int ogg_stream_pageout(ogg_stream_state *os, ogg_page *og);
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</h3>
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<p>Used during encode to read complete pages from the stream buffer. The
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returned page is ready for sending out to the real world.</p>
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<p>Returns zero if there is no complete page ready for reading. Returns
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nonzero when it has placed data for a complete page into
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<tt>og</tt>. Note that the storage returned in og points into internal
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storage; the pointers in <tt>og</tt> are valid until the next call to
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<tt>ogg_stream_pageout</tt>, <tt>ogg_stream_packetin</tt>,
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<tt>ogg_stream_reset</tt>, <tt>ogg_stream_clear</tt> or
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<tt>ogg_stream_destroy</tt>.</p>
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<h3>
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int ogg_stream_reset(ogg_stream_state *os);
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</h3>
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<p>Resets the given stream's state to that of a blank, unused stream;
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this may be used during encode or decode.</p>
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<p>Note that if used during encode, it does not alter the stream's serial
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number. In addition, the next page produced during encoding will be
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marked as the 'initial' page of the logical bitstream.</p>
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<p>When used during decode, this simply clears the data buffer of any
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pending pages. Beginning and end of stream cues are read from the
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bitstream and are unaffected by reset.</p>
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<p>Returns zero on success and non-zero on failure. This function always
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succeeds.</p>
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<h3>
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char *ogg_sync_buffer(ogg_sync_state *oy, long size);
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</h3>
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<p>This call is used to buffer a raw bitstream for framing and
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verification. <tt>ogg_sync_buffer</tt> handles stream capture and
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recapture, checksumming, and division into Ogg pages (as required by
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<tt>ogg_stream_pagein</tt>).</p>
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<p><tt>ogg_sync_buffer</tt> exposes a buffer area into which the decoder
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copies the next (up to) <tt>size</tt> bytes. We expose the buffer
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(rather than taking a buffer) in order to avoid an extra copy many
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uses; this way, for example, <tt>read()</tt> can transfer data
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directly into the stream buffer without first needing to place it in
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temporary storage.</p>
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<p>Returns a pointer into <tt>oy</tt>'s internal bitstream sync buffer;
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the remaining space in the sync buffer is at least <tt>size</tt>
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bytes. The decoder need not write all of <tt>size</tt> bytes;
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<tt>ogg_sync_wrote</tt> is used to inform the engine how many bytes
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were actually written. Use of <tt>ogg_sync_wrote</tt> after writing
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into the exposed buffer is mandantory.</p>
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<h3>
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int ogg_sync_clear(ogg_sync_state *oy);
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</h3>
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<p><tt>ogg_sync_clear</tt>
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clears and deallocates the internal storage of the given Ogg sync
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buffer. After clearing, the sync structure is not initialized for
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use; <tt>ogg_sync_init</tt> must be called to reinitialize for use.
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Use <tt>ogg_sync_reset</tt> to reset the sync state and buffer to a
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fresh, intiialized state.</p>
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<p><tt>ogg_sync_clear</tt> does not call <tt>free()</tt> on the pointer
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<tt>oy</tt>, allowing use of this call on sync structures in static
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or automatic storage. <tt>ogg_sync_destroy</tt>is a complimentary
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function that frees the pointer as well.</p>
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|
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<p>Returns zero on success and non-zero on failure. This function always
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succeeds.</p>
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<h3>
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int ogg_sync_destroy(ogg_sync_state *oy);
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</h3>
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|
<p>Clears and deallocates the internal storage of the given Ogg sync
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|
buffer, then frees the storage associated with the pointer
|
|
<tt>oy</tt>.</p>
|
|
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|
<p>An alternative function,<tt>ogg_sync_clear</tt>, does not call
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|
<tt>free()</tt> on the pointer <tt>oy</tt>, allowing use of that call on
|
|
stream structures in static or automatic storage.</p>
|
|
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|
<p>Returns zero on success and non-zero on failure. This function always
|
|
succeeds.</p>
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<h3>
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int ogg_sync_init(ogg_sync_state *oy);
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</h3>
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|
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<p>Initializes the sync buffer <tt>oy</tt> for use.</p>
|
|
|
|
<p>Returns zero on success and non-zero on failure. This function always
|
|
succeeds.</p>
|
|
|
|
<h3>
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|
int ogg_sync_pageout(ogg_sync_state *oy, ogg_page *og);
|
|
</h3>
|
|
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|
<p>Reads complete, framed, verified Ogg pages from the sync buffer,
|
|
placing the page data in <tt>og</tt>.</p>
|
|
|
|
<p>Returns zero when there's no complete pages buffered for
|
|
retrieval. Returns negative when a loss of sync or recapture occurred
|
|
(this is not necessarily an error; recapture would be required after
|
|
seeking, for example). Returns positive when a page is returned in
|
|
<tt>og</tt>. Note that the data in <tt>og</tt> points into the sync
|
|
buffer storage; the pointers are valid until the next call to
|
|
<tt>ogg_sync_buffer</tt>, <tt>ogg_sync_clear</tt>,
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|
<tt>ogg_sync_destroy</tt> or <tt>ogg_sync_reset</tt>.</p>
|
|
|
|
<h3>
|
|
int ogg_sync_reset(ogg_sync_state *oy);
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|
</h3>
|
|
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|
<p><tt>ogg_sync_reset</tt> resets the sync state in <tt>oy</tt> to a
|
|
clean, empty state. This is useful, for example, when seeking to a
|
|
new location in a bitstream.</p>
|
|
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|
<p>Returns zero on success, nonzero on failure.</p>
|
|
|
|
<h3>
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|
int ogg_sync_wrote(ogg_sync_state *oy, long bytes);
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|
</h3>
|
|
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|
<p>Used to inform the sync state as to how many bytes were actually
|
|
written into the exposed sync buffer. It must be equal to or less
|
|
than the size of the buffer requested.</p>
|
|
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|
<p>Returns zero on success and non-zero on failure; failure occurs only
|
|
when the number of bytes written were larger than the buffer.</p>
|
|
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|
<div id="copyright">
|
|
The Xiph Fish Logo is a
|
|
trademark (™) of Xiph.Org.<br/>
|
|
|
|
These pages © 1994 - 2005 Xiph.Org. All rights reserved.
|
|
</div>
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</body>
|
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</html>
|