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diff --git a/doc/programming.html b/doc/programming.html new file mode 100644 index 0000000..4b54347 --- /dev/null +++ b/doc/programming.html @@ -0,0 +1,554 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<html> +<head> + +<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-15"/> +<title>Ogg Vorbis Documentation</title> + +<style type="text/css"> +body { + margin: 0 18px 0 18px; + padding-bottom: 30px; + font-family: Verdana, Arial, Helvetica, sans-serif; + color: #333333; + font-size: .8em; +} + +a { + color: #3366cc; +} + +img { + border: 0; +} + +#xiphlogo { + margin: 30px 0 16px 0; +} + +#content p { + line-height: 1.4; +} + +h1, h1 a, h2, h2 a, h3, h3 a { + font-weight: bold; + color: #ff9900; + margin: 1.3em 0 8px 0; +} + +h1 { + font-size: 1.3em; +} + +h2 { + font-size: 1.2em; +} + +h3 { + font-size: 1.1em; +} + +li { + line-height: 1.4; +} + +#copyright { + margin-top: 30px; + line-height: 1.5em; + text-align: center; + font-size: .8em; + color: #888888; + clear: both; +} +</style> + +</head> + +<body> + +<div id="xiphlogo"> + <a href="http://www.xiph.org/"><img src="fish_xiph_org.png" alt="Fish Logo and Xiph.Org"/></a> +</div> + +<h1>Programming with Xiph.Org <tt>libvorbis</tt></h1> + +<h2>Description</h2> + +<p>Libvorbis is the Xiph.Org Foundation's portable Ogg Vorbis CODEC +implemented as a programmatic library. Libvorbis provides primitives +to handle framing and manipulation of Ogg bitstreams (used by the +Vorbis for streaming), a full analysis (encoding) interface as well as +packet decoding and synthesis for playback.</p> + +<p>The libvorbis library does not provide any system interface; a +full-featured demonstration player included with the library +distribtion provides example code for a variety of system interfaces +as well as a working example of using libvorbis in production code.</p> + +<h2>Encoding Overview</h2> + +<h2>Decoding Overview</h2> + +<p>Decoding a bitstream with libvorbis follows roughly the following +steps:</p> + +<ol> +<li>Frame the incoming bitstream into pages</li> +<li>Sort the pages by logical bitstream and buffer then into logical streams</li> +<li>Decompose the logical streams into raw packets</li> +<li>Reconstruct segments of the original data from each packet</li> +<li>Glue the reconstructed segments back into a decoded stream</li> +</ol> + +<h3>Framing</h3> + +<p>An Ogg bitstream is logically arranged into pages, but to decode +the pages, we have to find them first. The raw bitstream is first fed +into an <tt>ogg_sync_state</tt> buffer using <tt>ogg_sync_buffer()</tt> +and <tt>ogg_sync_wrote()</tt>. After each block we submit to the sync +buffer, we should check to see if we can frame and extract a complete +page or pages using <tt>ogg_sync_pageout()</tt>. Extra pages are +buffered; allowing them to build up in the <tt>ogg_sync_state</tt> +buffer will eventually exhaust memory.</p> + +<p>The Ogg pages returned from <tt>ogg_sync_pageout</tt> need not be +decoded further to be used as landmarks in seeking; seeking can be +either a rough process of simply jumping to approximately intuited +portions of the bitstream, or it can be a precise bisection process +that captures pages and inspects data position. When seeking, +however, sequential multiplexing (chaining) must be accounted for; +beginning play in a new logical bitstream requires initializing a +synthesis engine with the headers from that bitstream. Vorbis +bitstreams do not make use of concurent multiplexing (grouping).</p> + +<h3>Sorting</h3> + +<p>The pages produced by <tt>ogg_sync_pageout</tt> are then sorted by +serial number to seperate logical bitstreams. Initialize logical +bitstream buffers (<tt>og_stream_state</tt>) using +<tt>ogg_stream_init()</tt>. Pages are submitted to the matching +logical bitstream buffer using <tt>ogg_stream_pagein</tt>; the serial +number of the page and the stream buffer must match, or the page will +be rejected. A page submitted out of sequence will simply be noted, +and in the course of outputting packets, the hole will be flagged +(<tt>ogg_sync_pageout</tt> and <tt>ogg_stream_packetout</tt> will +return a negative value at positions where they had to recapture the +stream).</p> + +<h3>Extracting packets</h3> + +<p>After submitting page[s] to a logical stream, read available packets +using <tt>ogg_stream_packetout</tt>.</p> + +<h3>Decoding packets</h3> + +<h3>Reassembling data segments</h3> + +<h2>Ogg Bitstream Manipulation Structures</h2> + +<p>Two of the Ogg bitstream data structures are intended to be +transparent to the developer; the fields should be used directly.</p> + +<h3>ogg_packet</h3> + +<pre> +typedef struct { + unsigned char *packet; + long bytes; + long b_o_s; + long e_o_s; + + size64 granulepos; + +} ogg_packet; +</pre> + +<dl> +<dt>packet:</dt> +<dd>a pointer to the byte data of the raw packet</dd> +<dt>bytes:</dt> +<dd>the size of the packet' raw data</dd> +<dt>b_o_s:</dt> +<dd>beginning of stream; nonzero if this is the first packet of + the logical bitstream</dd> +<dt>e_o_s:</dt> +<dd>end of stream; nonzero if this is the last packet of the + logical bitstream</dd> +<dt>granulepos:</dt> +<dd>the absolute position of this packet in the original + uncompressed data stream.</dd> +</dl> + +<h4>encoding notes</h4> + +<p>The encoder is responsible for setting all of +the fields of the packet to appropriate values before submission to +<tt>ogg_stream_packetin()</tt>; however, it is noted that the value in +<tt>b_o_s</tt> is ignored; the first page produced from a given +<tt>ogg_stream_state</tt> structure will be stamped as the initial +page. <tt>e_o_s</tt>, however, must be set; this is the means by +which the stream encoding primitives handle end of stream and cleanup.</p> + +<h4>decoding notes</h4> + +<p><tt>ogg_stream_packetout()</tt> sets the fields +to appropriate values. Note that granulepos will be >= 0 only in the +case that the given packet actually represents that position (ie, only +the last packet completed on any page will have a meaningful +<tt>granulepos</tt>). Intervening frames will see <tt>granulepos</tt> set +to -1.</p> + +<h3>ogg_page</h3> + +<pre> +typedef struct { + unsigned char *header; + long header_len; + unsigned char *body; + long body_len; +} ogg_page; +</pre> + +<dl> +<dt>header:</dt> +<dd>pointer to the page header data</dd> +<dt>header_len:</dt> +<dd>length of the page header in bytes</dd> +<dt>body:</dt> +<dd>pointer to the page body</dd> +<dt>body_len:</dt> +<dd>length of the page body</dd> +</dl> + +<p>Note that although the <tt>header</tt> and <tt>body</tt> pointers do +not necessarily point into a single contiguous page vector, the page +body must immediately follow the header in the bitstream.</p> + +<h2>Ogg Bitstream Manipulation Functions</h2> + +<h3> +int ogg_page_bos(ogg_page *og); +</h3> + +<p>Returns the 'beginning of stream' flag for the given Ogg page. The +beginning of stream flag is set on the initial page of a logical +bitstream.</p> + +<p>Zero indicates the flag is cleared (this is not the initial page of a +logical bitstream). Nonzero indicates the flag is set (this is the +initial page of a logical bitstream).</p> + +<h3> +int ogg_page_continued(ogg_page *og); +</h3> + +<p>Returns the 'packet continued' flag for the given Ogg page. The packet +continued flag indicates whether or not the body data of this page +begins with packet continued from a preceeding page.</p> + +<p>Zero (unset) indicates that the body data begins with a new packet. +Nonzero (set) indicates that the first packet data on the page is a +continuation from the preceeding page.</p> + +<h3> +int ogg_page_eos(ogg_page *og); +</h3> + +<p>Returns the 'end of stream' flag for a give Ogg page. The end of page +flag is set on the last (terminal) page of a logical bitstream.</p> + +<p>Zero (unset) indicates that this is not the last page of a logical +bitstream. Nonzero (set) indicates that this is the last page of a +logical bitstream and that no addiitonal pages belonging to this +bitstream may follow.</p> + +<h3> +size64 ogg_page_granulepos(ogg_page *og); +</h3> + +<p>Returns the position of this page as an absolute position within the +original uncompressed data. The position, as returned, is 'frames +encoded to date up to and including the last whole packet on this +page'. Partial packets begun on this page but continued to the +following page are not included. If no packet ends on this page, the +frame position value will be equal to the frame position value of the +preceeding page. If none of the original uncompressed data is yet +represented in the logical bitstream (for example, the first page of a +bitstream consists only of a header packet; this packet encodes only +metadata), the value shall be zero.</p> + +<p>The units of the framenumber are determined by media mapping. A +vorbis audio bitstream, for example, defines one frame to be the +channel values from a single sampling period (eg, a 16 bit stereo +bitstream consists of two samples of two bytes for a total of four +bytes, thus a frame would be four bytes). A video stream defines one +frame to be a single frame of video.</p> + +<h3> +int ogg_page_pageno(ogg_page *og); +</h3> + +<p>Returns the sequential page number of the given Ogg page. The first +page in a logical bitstream is numbered zero; following pages are +numbered in increasing monotonic order.</p> + +<h3> +int ogg_page_serialno(ogg_page *og); +</h3> + +<p>Returns the serial number of the given Ogg page. The serial number is +used as a handle to distinguish various logical bitstreams in a +physical Ogg bitstresm. Every logical bitstream within a +physical bitstream must use a unique (within the scope of the physical +bitstream) serial number, which is stamped on all bitstream pages.</p> + +<h3> +int ogg_page_version(ogg_page *og); +</h3> + +<p>Returns the revision of the Ogg bitstream structure of the given page. +Currently, the only permitted number is zero. Later revisions of the +bitstream spec will increment this version should any changes be +incompatable.</p> + +<h3> +int ogg_stream_clear(ogg_stream_state *os); +</h3> + +<p>Clears and deallocates the internal storage of the given Ogg stream. +After clearing, the stream structure is not initialized for use; +<tt>ogg_stream_init</tt> must be called to reinitialize for use. +Use <tt>ogg_stream_reset</tt> to reset the stream state +to a fresh, intiialized state.</p> + +<p><tt>ogg_stream_clear</tt> does not call <tt>free()</tt> on the pointer +<tt>os</tt>, allowing use of this call on stream structures in static +or automatic storage. <tt>ogg_stream_destroy</tt>is a complimentary +function that frees the pointer as well.</p> + +<p>Returns zero on success and non-zero on failure. This function always +succeeds.</p> + +<h3> +int ogg_stream_destroy(ogg_stream_state *os); +</h3> + +<p>Clears and deallocates the internal storage of the given Ogg stream, +then frees the storage associated with the pointer <tt>os</tt>.</p> + +<p><tt>ogg_stream_clear</tt> does not call <tt>free()</tt> on the pointer +<tt>os</tt>, allowing use of that call on stream structures in static +or automatic storage.</p> + +<p>Returns zero on success and non-zero on failure. This function always +succeeds.</p> + +<h3> +int ogg_stream_init(ogg_stream_state *os,int serialno); +</h3> + +<p>Initialize the storage associated with <tt>os</tt> for use as an Ogg +stream. This call is used to initialize a stream for both encode and +decode. The given serial number is the serial number that will be +stamped on pages of the produced bitstream (during encode), or used as +a check that pages match (during decode).</p> + +<p>Returns zero on success, nonzero on failure.</p> + +<h3> +int ogg_stream_packetin(ogg_stream_state *os, ogg_packet *op); +</h3> + +<p>Used during encoding to add the given raw packet to the given Ogg +bitstream. The contents of <tt>op</tt> are copied; +<tt>ogg_stream_packetin</tt> does not retain any pointers into +<tt>op</tt>'s storage. The encoding proccess buffers incoming packets +until enough packets have been assembled to form an entire page; +<tt>ogg_stream_pageout</tt> is used to read complete pages.</p> + +<p>Returns zero on success, nonzero on failure.</p> + +<h3> +int ogg_stream_packetout(ogg_stream_state *os,ogg_packet *op); +</h3> + +<p>Used during decoding to read raw packets from the given logical +bitstream. <tt>ogg_stream_packetout</tt> will only return complete +packets for which checksumming indicates no corruption. The size and +contents of the packet exactly match those given in the encoding +process. </p> + +<p>Returns zero if the next packet is not ready to be read (not buffered +or incomplete), positive if it returned a complete packet in +<tt>op</tt> and negative if there is a gap, extra bytes or corruption +at this position in the bitstream (essentially that the bitstream had +to be recaptured). A negative value is not necessarily an error. It +would be a common occurence when seeking, for example, which requires +recapture of the bitstream at the position decoding continued.</p> + +<p>If the return value is positive, <tt>ogg_stream_packetout</tt> placed +a packet in <tt>op</tt>. The data in <tt>op</tt> points to static +storage that is valid until the next call to +<tt>ogg_stream_pagein</tt>, <tt>ogg_stream_clear</tt>, +<tt>ogg_stream_reset</tt>, or <tt>ogg_stream_destroy</tt>. The +pointers are not invalidated by more calls to +<tt>ogg_stream_packetout</tt>.</p> + +<h3> +int ogg_stream_pagein(ogg_stream_state *os, ogg_page *og); +</h3> + +<p>Used during decoding to buffer the given complete, pre-verified page +for decoding into raw Ogg packets. The given page must be framed, +normally produced by <tt>ogg_sync_pageout</tt>, and from the logical +bitstream associated with <tt>os</tt> (the serial numbers must match). +The contents of the given page are copied; <tt>ogg_stream_pagein</tt> +retains no pointers into <tt>og</tt> storage.</p> + +<p>Returns zero on success and non-zero on failure.</p> + +<h3> +int ogg_stream_pageout(ogg_stream_state *os, ogg_page *og); +</h3> + +<p>Used during encode to read complete pages from the stream buffer. The +returned page is ready for sending out to the real world.</p> + +<p>Returns zero if there is no complete page ready for reading. Returns +nonzero when it has placed data for a complete page into +<tt>og</tt>. Note that the storage returned in og points into internal +storage; the pointers in <tt>og</tt> are valid until the next call to +<tt>ogg_stream_pageout</tt>, <tt>ogg_stream_packetin</tt>, +<tt>ogg_stream_reset</tt>, <tt>ogg_stream_clear</tt> or +<tt>ogg_stream_destroy</tt>.</p> + +<h3> +int ogg_stream_reset(ogg_stream_state *os); +</h3> + +<p>Resets the given stream's state to that of a blank, unused stream; +this may be used during encode or decode.</p> + +<p>Note that if used during encode, it does not alter the stream's serial +number. In addition, the next page produced during encoding will be +marked as the 'initial' page of the logical bitstream.</p> + +<p>When used during decode, this simply clears the data buffer of any +pending pages. Beginning and end of stream cues are read from the +bitstream and are unaffected by reset.</p> + +<p>Returns zero on success and non-zero on failure. This function always +succeeds.</p> + +<h3> +char *ogg_sync_buffer(ogg_sync_state *oy, long size); +</h3> + +<p>This call is used to buffer a raw bitstream for framing and +verification. <tt>ogg_sync_buffer</tt> handles stream capture and +recapture, checksumming, and division into Ogg pages (as required by +<tt>ogg_stream_pagein</tt>).</p> + +<p><tt>ogg_sync_buffer</tt> exposes a buffer area into which the decoder +copies the next (up to) <tt>size</tt> bytes. We expose the buffer +(rather than taking a buffer) in order to avoid an extra copy many +uses; this way, for example, <tt>read()</tt> can transfer data +directly into the stream buffer without first needing to place it in +temporary storage.</p> + +<p>Returns a pointer into <tt>oy</tt>'s internal bitstream sync buffer; +the remaining space in the sync buffer is at least <tt>size</tt> +bytes. The decoder need not write all of <tt>size</tt> bytes; +<tt>ogg_sync_wrote</tt> is used to inform the engine how many bytes +were actually written. Use of <tt>ogg_sync_wrote</tt> after writing +into the exposed buffer is mandantory.</p> + +<h3> +int ogg_sync_clear(ogg_sync_state *oy); +</h3> + +<p><tt>ogg_sync_clear</tt> +clears and deallocates the internal storage of the given Ogg sync +buffer. After clearing, the sync structure is not initialized for +use; <tt>ogg_sync_init</tt> must be called to reinitialize for use. +Use <tt>ogg_sync_reset</tt> to reset the sync state and buffer to a +fresh, intiialized state.</p> + +<p><tt>ogg_sync_clear</tt> does not call <tt>free()</tt> on the pointer +<tt>oy</tt>, allowing use of this call on sync structures in static +or automatic storage. <tt>ogg_sync_destroy</tt>is a complimentary +function that frees the pointer as well.</p> + +<p>Returns zero on success and non-zero on failure. This function always +succeeds.</p> + +<h3> +int ogg_sync_destroy(ogg_sync_state *oy); +</h3> + +<p>Clears and deallocates the internal storage of the given Ogg sync +buffer, then frees the storage associated with the pointer +<tt>oy</tt>.</p> + +<p>An alternative function,<tt>ogg_sync_clear</tt>, does not call +<tt>free()</tt> on the pointer <tt>oy</tt>, allowing use of that call on +stream structures in static or automatic storage.</p> + +<p>Returns zero on success and non-zero on failure. This function always +succeeds.</p> + +<h3> +int ogg_sync_init(ogg_sync_state *oy); +</h3> + +<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> +int ogg_sync_pageout(ogg_sync_state *oy, ogg_page *og); +</h3> + +<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>, +<tt>ogg_sync_destroy</tt> or <tt>ogg_sync_reset</tt>.</p> + +<h3> +int ogg_sync_reset(ogg_sync_state *oy); +</h3> + +<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> + +<p>Returns zero on success, nonzero on failure.</p> + +<h3> +int ogg_sync_wrote(ogg_sync_state *oy, long bytes); +</h3> + +<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> + +<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> + +<div id="copyright"> + The Xiph Fish Logo is a + trademark (™) of Xiph.Org.<br/> + + These pages © 1994 - 2005 Xiph.Org. All rights reserved. +</div> + +</body> +</html> |