Implementing GstToc support in GStreamer elements

1. General info about GstToc structure

GstToc introduces a general way to handle chapters within multimedia formats.
GstToc can be represented as tree structure with arbitrary hierarchy. Tree item
can be either of two types: sequence or alternative. Sequence types acts like a
part of the media data, for example audio track in CUE sheet, or part of the
movie. Alternative types acts like some kind of selection to process a different
version of the media content, for example DVD angles.
GstToc has one constraint on the tree structure: it does not allow different
entry types on the same level of the hierarchy, i.e. you shouldn't
have editions and chapters mixed together. Here is an example of right TOC:

  -------  TOC  -------
           /  \
   edition1    edition2
   |           |
   -chapter1   -chapter3
   -chapter2       

Here are two editions (alternatives), the first contains two chapters (sequence
type), and the second has only one chapter. And here is an example of invalid
TOC:

  -------  TOC  -------
           /  \
   edition1    chapter1
   |                 
   -chapter1
   -chapter2    
 
Here you have edition1 and chapter1 mixed on the same level of hierarchy,
and such TOC will be considered broken.

GstToc has 'entries' field of GList type which consists of children items.
Each item is of type GstTocEntry. Also GstToc has list of tags and
GstStructure called 'info'. Please, use GstToc.info and GstTocEntry.info
fields this way: create a GstStructure, put all info related to your element
there and put this structure into the 'info' field under the name of your
element. Some fields in the 'info' structure can be used for internal
purposes, so you should use it in the way described above to not to
overwrite already existent fields.

Let's look at GstTocEntry a bit closer. One of the most important fields
is 'uid', which must be unique for each item within the TOC. This is used
to identify each item inside TOC, especially when element receives TOC
select event with UID to seek on. Field 'subentries' of type GList contains
children items of type GstTocEntry. Thus you can achieve arbitrary hierarchy
level. Field 'type' can be either GST_TOC_ENTRY_TYPE_CHAPTER or
GST_TOC_ENTRY_TYPE_EDITION which corresponds to chapter or edition type of
item respectively. Field 'tags' is a list of tags related to the item. And field
'info' is similar to GstToc.info described above.

So, a little more about managing GstToc. Use gst_toc_new() and gst_toc_unref()
to create/free it. GstTocEntry can be created using gst_toc_entry_new().
While building GstToc you can set start and stop timestamps for each item using
gst_toc_entry_set_start_stop() and loop_type and repeat_count using
gst_toc_entry_set_loop().
The best way to process already created GstToc is to recursively go through
the 'entries' and 'subentries' fields.

Applications and plugins should not rely on TOCs having a certain kind of
structure, but should allow for different alternatives. For example, a
simple CUE sheet embedded in a file may be presented as a flat list of
track entries, or could have a top-level edition node (or some other
alternative type entry) with track entries underneath that node; or even
multiple top-level edition nodes (or some other alternative type entries)
each with track entries underneath, in case the source file has extracted
a track listing from different sources).


2. TOC scope: global and current

There are two main consumers for TOC information: applications and elements
in the pipeline that are TOC writers (such as e.g. matroskamux).

Applications typically want to know the entire table of contents (TOC) with
all entries that can possibly be selected.

TOC writers in the pipeline, however, would not want to write a TOC for all
possible/available streams, but only for the current stream.

When transcoding a title from a DVD, for example, the application would still
want to know the entire TOC, with all titles, the chapters for each title,
and the available angles. When transcoding to a file, we only want the TOC
information that is relevant to the transcoded stream to be written into
the file structure, e.g. the chapters of the title being transcoded (or
possibly only chapters 5-7 if only those have been selected for playback/
transcoding).

This is why we may need to create two different TOCs for those two types
of consumers.

Elements that extract TOC information should send TOC events downstream.

Like with tags, sinks will post a TOC message on the bus for the application
with the global TOC, once a global TOC event reaches the sink.


3. Working with GstMessage

If a table of contents is available, applications will receive a TOC message
on the pipeline's GstBus.

A TOC message will be posted on the bus by sinks when the receive a TOC event
containing a TOC with global scope. Elements extracting TOCs should not post
a TOC message themselves, but send a TOC event downstream.

The reason for this is that there may be cascades of TOCs (e.g. a zip archive
containing multiple matroska files, each with a TOC).

GstMessage with GstToc can be created using gst_message_new_toc() and parsed
with gst_message_parse_toc(). The 'updated' parameter in these methods indicates
whether the TOC was just discovered (set to false) or TOC was already found and
have been updated (set to true). This message will typically be posted by sinks
to pipeline in case you have discovered TOC data within your element.


4. Working with GstEvent

There are two types of TOC-related events:

 - downstream TOC events that contain TOC information and
   travel downstream

 - toc-select events that travel upstream and can be used to select
   a certain TOC entry for playback (similar to seek events)

GstToc supports select event through GstEvent infrastructure. The idea is the
following: when you receive TOC select event, parse it with
gst_event_parse_toc_select() and seek stream (if it is not streamable) for
specified TOC UID (you can use gst_toc_find_entry() to find entry in TOC by UID).
To create TOC select event use gst_event_new_toc_select(). The common action on
such event is to seek to specified UID within your element. 


5. Implementation coverage, Specifications, ...

Below is a list of container formats, links to documentation and a summary of
toc related features. Each section title also indicates whether reading/writing
a toc is implemented. Below hollow bullet point 'o' indicate no support and
filled bullets '*' indicate that this feature is handled.


AIFC: -/-
http://www-mmsp.ece.mcgill.ca/Documents/AudioFormats/AIFF/Docs/AIFF-1.3.pdf
o 'MARK'
o 'INST'

The 'MARK' chunk defines a list of (cue-id, position_in_samples, label).

The 'INST' chunk contains a sustainLoop and releaseLoop, each consisting of 
(loop-type, cue-begin, cue-end)


FLAC: read/write
http://xiph.org/flac/format.html#metadata_block_cuesheet
* METADATA_BLOCK_CUESHEET
  * CUESHEET_TRACK
    o CUESHEET_TRACK_INDEX
    
Both CUESHEET_TRACK and CUESHEET_TRACK_INDEX have a (relative) offset in
samples. CUESHEET_TRACK has ISRC metadata.


MKV: read/write
http://matroska.org/technical/specs/chapters/index.html
* Chapters and Editions each having a uid
* Chapter have start/end time and metadata:
  ChapString, ChapLanguage, ChapCountry

  
MP4:
* elst

The 'elst' atom contains a list of edits. Each edit consists of (length, start,
play-back speed).


OGG: -/-
https://wiki.xiph.org/Chapter_Extension
o VorbisComment fields called CHAPTERxxx and CHAPTERxxxNAME with xxx being a
number between 000 and 999.


WAV: read/write
http://www.sonicspot.com/guide/wavefiles.html
* 'cue '
o 'plst'
* 'adtl'
  * 'labl'
  * 'note'
  o 'ltxt'
  o 'smpl'

The 'cue ' chunk defines a list of markers in the stream with 'cue-id's. The
'smpl' chunk defines a list of regions in the stream with 'cue-id's in the same
namespace (?).

The various 'adtl' chunks: 'labl', 'note' and 'ltxt' refer to the 'cue-id's.

A 'plst' chunk defines a sequence of segments (cue-id, length_samples, repeats).
The 'smpl' chunk defines a list of loops (cue-id, beg, end, loop-type, repeats).


6. Conclusion/Ideas/Future work

Based on the data of chapter 5, a few thoughts and observations that can be used
to extend and refine our API. These things below are not reflecting the current
implementation.

All formats have table of [cue-id, cue-start, (cue-end), (extra tags)]
- cue-id is commonly represented as and unsigned int 32bit
- cue-end is optional
- extra tags could be represented as a structure/taglist

Many formats have metadata that references the cue-table.
- loops in instruments in wav, aifc
- edit lists in wav, mp4

For mp4.edtl, wav.plst we could expose two editions.
1) the edit list is flattened: default, for playback
2) the stream has the raw data and the edit list is there as chapter markers:
   useful for editing software

We might want to introduce a new GST_TOC_ENTRY_TYPE_MARKER or _CUE. This would
be a sequence entry-type and it would not be used for navigational purposes, but
to attach data to a point in time (envelopes, loops, ...).

API wise there is some overlap between:
- exposing multiple audio/video tracks as pads or as ToC editions. For ToC
  editions, we have the TocSelect event.
- exposing subtitles as a sparse stream or as as ToC sequence of markers with
  labels