liboggz: Ogg basics

# Ogg basics

## Scope

This section provides a minimal introduction to Ogg concepts, covering only that which is required to use liboggz.

For more detailed information, see the Ogg homepage or IETF RFC 3533 The Ogg File Format version 0.

## Terminology

The monospace text below is quoted directly from RFC 3533. For each concept introduced, tips related to liboggz are provided in bullet points.

### Physical and Logical Bitstreams

The raw data of an Ogg stream, as read directly from a file or network socket, is called a physical bitstream.

       The result of an Ogg encapsulation is called the "Physical (Ogg)
       Bitstream".  It encapsulates one or several encoder-created
       bitstreams, which are called "Logical Bitstreams".  A logical
       bitstream, provided to the Ogg encapsulation process, has a
       structure, i.e., it is split up into a sequence of so-called
       "Packets".  The packets are created by the encoder of that logical
       bitstream and represent meaningful entities for that encoder only
       (e.g., an uncompressed stream may use video frames as packets).

### Packets and Pages

Within the Ogg format, packets are written into pages. You can think of pages like pages in a book, and packets as items of the actual text. Consider, for example, individual poems or short stories as the packets. Pages are of course all the same size, and a few very short packets could be written into a single page. On the other hand, a very long packet will use many pages.

- liboggz handles the details of writing packets into pages, and of reading packets from pages; your application deals only with `ogg_packet` structures.
- Each `ogg_packet` structure contains a block of data and its length in bytes, plus other information related to the stream structure as explained below.

### serialno

Each logical bitstream is uniquely identified by a serial number or serialno.

- Packets are always associated with a serialno. This is not actually part of the `ogg_packet` structure, so wherever you see an `ogg_packet` in the liboggz API, you will see an accompanying serialno.

       This unique serial number is created randomly and does not have any
       connection to the content or encoder of the logical bitstream it
       represents.

- Use oggz_serialno_new() to generate a new serial number for each logical bitstream you write.
- Use an OggzTable , keyed by serialno, to store and retrieve data related to each logical bitstream.

### b_o_s and e_o_s

       bos page: The initial page (beginning of stream) of a logical
          bitstream which contains information to identify the codec type
          and other decoding-relevant information.

       eos page: The final page (end of stream) of a logical bitstream.

- Every `ogg_packet` contains b_o_s and e_o_s flags. Of course each of these will be set only once per logical bitstream. See the Structuring section below for rules on setting b_o_s and e_o_s when interleaving logical bitstreams.
- This documentation will refer to bos and eos packets (not pages) as that is more closely represented by the API. The bos packet is the only packet on the bos page, and the eos packet is the last packet on the eos page.

### granulepos

       granule position: An increasing position number for a specific
          logical bitstream stored in the page header.  Its meaning is
          dependent on the codec for that logical bitstream

- Every `ogg_packet` contains a granulepos. The granulepos of each packet is used mostly for seeking.

## Structuring

The general structure of an Ogg stream is governed by various rules.

### Secondary header packets

Some data sources require initial setup information such as comments and codebooks to be present near the beginning of the stream (directly following the b_o_s packets.

       Ogg also allows but does not require secondary header packets after
       the bos page for logical bitstreams and these must also precede any
       data packets in any logical bitstream.  These subsequent header
       packets are framed into an integral number of pages, which will not
       contain any data packets.  So, a physical bitstream begins with the
       bos pages of all logical bitstreams containing one initial header
       packet per page, followed by the subsidiary header packets of all
       streams, followed by pages containing data packets.

- liboggz handles the framing of packets into low-level pages. To ensure that the pages used by secondary headers contain no data packets, set the flush parameter of oggz_write_feed() to OGGZ_FLUSH_AFTER when queueing the last of the secondary headers.
- or, equivalently, set flush to OGGZ_FLUSH_BEFORE when queueing the first of the data packets.

### Sequencing b_o_s and e_o_s packets

The following rules apply for sequencing bos and eos packets in a physical bitstream:

       ... All bos pages of all logical bitstreams MUST appear together at
       the beginning of the Ogg bitstream.

       ... eos pages for the logical bitstreams need not all occur
       contiguously.  Eos pages may be 'nil' pages, that is, pages
       containing no content but simply a page header with position
       information and the eos flag set in the page header.

- oggz_write_feed() will fail with a return value of OGGZ_ERR_BOS if an attempt is made to queue a late bos packet

### Interleaving logical bitstreams

       It is possible to consecutively chain groups of concurrently
       multiplexed bitstreams.  The groups, when unchained, MUST stand on
       their own as a valid concurrently multiplexed bitstream.  The
       following diagram shows a schematic example of such a physical
       bitstream that obeys all the rules of both grouped and chained
       multiplexed bitstreams.

                   physical bitstream with pages of
              different logical bitstreams grouped and chained
          -------------------------------------------------------------
          |*A*|*B*|*C*|A|A|C|B|A|B|A#|C|...|B|C|B#|C#|*D*|D|...|D#|
          -------------------------------------------------------------
           bos bos bos             eos           eos eos bos       eos

       In this example, there are two chained physical bitstreams, the first
       of which is a grouped stream of three logical bitstreams A, B, and C.
       The second physical bitstream is chained after the end of the grouped
       bitstream, which ends after the last eos page of all its grouped
       logical bitstreams.  As can be seen, grouped bitstreams begin
       together - all of the bos pages MUST appear before any data pages.
       It can also be seen that pages of concurrently multiplexed bitstreams
       need not conform to a regular order.  And it can be seen that a
       grouped bitstream can end long before the other bitstreams in the
       group end.

- oggz_write_feed() will fail, returning an explicit error value, if an attempt is made to queue a packet in violation of these rules.

## References

This introduction to the Ogg format is derived from IETF RFC 3533 The Ogg File Format version 0 in accordance with the following copyright statement pertaining to the text of RFC 3533:

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