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Ogg Skeleton 4.0 with Keyframe Index
Chris Pearce, Mozilla Corporation
9 June 2010
Seeking in an Ogg file is typically implemented as a bisection search
over the pages in the file. The Ogg physical bitstream is bisected and
the next Ogg page's end-time is extracted. The bisection continues until
it reaches an Ogg page with an end-time close enough to the seek target
time. However in media containing streams which have keyframes and
interframes, such as Theora streams, your bisection search won't
necessarily terminate at a keyframe. Thus if you begin decoding after your
first bisection terminates, you're likely to only get partial incomplete
frames, with "visual artifacts", until you decode up to the next keyframe.
So to eliminate these visual artifacts, after the first bisection
terminates, you must extract the keyframe's timestamp from the last Theora
page's granulepos, and seek again back to the start of the keyframe and
decode forward until you reach the frame at the seek target.
This is further complicated by the fact that packets often span multiple
Ogg pages, and that Ogg pages from different streams can be interleaved
between spanning packets.
The bisection method above works fine for seeking in local files, but
for seeking in files served over the Internet via HTTP, each bisection
or non sequential read can trigger a new HTTP request, which can have
very high latency, making seeking very slow.
The Skeleton 4.0 bitstream attempts to alleviate this problem, by
providing an index of periodic keyframes for every content stream in an
Ogg segment. Note that the Skeleton 4.0 track only holds data for the
segment or "link" in which it resides. So if two Ogg files are concatenated
together ("chained"), the Skeleton 4.0's keyframe indexes in the first Ogg
segment (the first "link" in the "chain") do not contain information
about the keyframes in the second Ogg segment (the second link in the chain).
Each content track has a separate index, which is stored in its own
packet in the Skeleton 4.0 track. The index for streams without the
concept of a keyframe, such as Vorbis streams, can instead record the
time position at periodic intervals, which achieves the same result.
When this document refers to keyframes, it also implicitly refers to these
independent periodic samples from keyframe-less streams.
All the Skeleton 4.0 track's pages appear in the header pages of the Ogg
segment. This means the all the keyframe indexes are immediately
available once the header packets have been read when playing the media
over a network connection.
For every content stream in an Ogg segment, the Ogg index bitstream
provides seek algorithms with an ordered table of "key points". A key
point is intrinsically associated with exactly one stream, and stores the
offset, o, of the last page which lies before all data required to decode
the keyframe, as well as the presentation time of the keyframe t, as a
fraction of seconds.
The offset is relative from the beginning of the Ogg segment, and is exactly
the first byte of the a page in the indexed stream, so if you seek to a
keypoint's offset and don't find the beginning of a page there, or you find
a page from another stream, you can assume that the Ogg segment has been
modified since the index was constructed, and the index can be considered
invalid. The time t is the keyframe's presentation time corresponding to the
granulepos, and is represented as a fraction in seconds. Note that if a
stream requires any preroll, this will be accounted for in the time stored
in the keypoint.
The Skeleton 4.0 track contains one index for each content stream in the
file. To seek in an Ogg file which contains keyframe indexes, first
construct the set which contains every active streams' last keypoint which
has time less than or equal to the seek target time. This tells you a known
point on every stream which lies before the seek target. Then from that set
of key points, select the key point with the smallest byte offset. You then
verify that there's a page from the keypoint's stream found at exactly that
offset, and if so, you can begin decoding. You are guaranteed to pass
keyframes on all streams with time less than or equal to your seek target
time while decoding up to the seek target. However if you don't encounter
a keyframe with the same presentation time as is stored in the keypoint,
then the index is invalid (possibly the file has been changed without
updating the index) and you must either fallback to a bisection search, or
keep decoding if you've landed "close enough" to the seek target.
Be aware that you cannot assume that any or all Ogg files will contain
keyframe indexes, so when implementing Ogg seeking, you must gracefully
fall-back to a bisection search or other seek algorithm when the index
is not present, or when it is invalid.
The Skeleton 4.0 index packets also stores meta data about the segment in
which it resides. It stores the timestamps of the first and last samples
in its track. This also allows you to determine the duration of the
indexed Ogg media without having to decode the start and end of the
Ogg segment to calculate the difference (which is the duration). With the
index packets storing the start and end times of every track, you can
calculate the duration as the end time of the last active stream minus the
start time of first active stream.
The Skeleton 4.0 BOS packet contains the length of the indexed segment
in bytes. This is so that if the seek target is outside of the indexed range,
you can immediately move to the next/previous segment and either seek using
that segment's index, or narrow the bisection window if that segment has no
index. You can also use the segement length to verify if the index is valid.
If the contents of the segment have changed, it's highly likely that the
length of the segment has changed as well. When you load the segment's
header pages, you should check the length of the physical segment, and if it
doesn't match the length stored in the Skeleton header packet, you know that
either the index is out of date, or the file has been chained since indexing.
The Skeleton 4.0 BOS packet also contains the offset of the first non header
page in the Ogg segment. This means that if you wish to delay loading of an
index for whatever reason, you can skip forward to that offset, and start
decoding from that offset forwards.
When using the index to seek, you must verify that the index is still
correct. You can consider the index invalid if any of the following are true:
1. The segment doesn't end at the segment length offset stored in the
Skeleton BOS packet (note that a new "link" in a "chain" can start
at the end of the segment), or
2. after a seek to a keypoint's offset, you don't land exactly on a page
boundary, or
3. after a seek to a keypoint's offset, you don't land on a page which
belongs to that keypoint's stream.
While loading the Skeleton BOS header, you should always check the Skeleton
version field to ensure your decoder correctly knows how to parse the Skeleton
Be aware that a keyframe index may not index all keyframes in the Ogg segment,
it may only index periodic keyframes instead.
Unless otherwise specified, all integers and fields in the bitstream are
encoded with the least significant bit coming first in each byte.
Integers and fields comprising of more than one byte are encoded least
significant byte first (i.e. little endian byte order).
The Skeleton 4.0 track is intended to be backwards compatible with the
Skeleton 3.0 specification, available at . Unless specified
differently here, it is safe to assume that anything specified for a
Skeleton 3.0 track holds for a Skeleton 4.0 track.
As per the Skeleton 3.0 track, an Ogg segment containing a Skeleton 4.0 track
must begin with a "fishead" BOS packet on a page by itself, with the
following format:
1. Identifier: 8 bytes, "fishead\0".
2. Version major: 2 Byte unsigned integer denoting the major version (4)
3. Version minor: 2 Byte unsigned integer denoting the minor version (0)
4. Presentationtime numerator: 8 Byte signed integer
5. Presentationtime denominator: 8 Byte signed integer
6. Basetime numerator: 8 Byte signed integer
7. Basetime denominator: 8 Byte signed integer
8. UTC [ISO8601]: a 20 Byte string containing a UTC time
13. [NEW] The length of the segment, in bytes: 8 byte unsigned integer,
0 if unknown.
14. [NEW] The offset of the first non-header page in bytes: 8 byte unsigned
integer, 0 if unknown.
In Skeleton 4.0 the "fisbone" packets contain two new compulsory
message-header fields "Role" and "Name". The fisbone packets still follow
after the other streams' BOS pages and secondary header pages.
Before the Skeleton EOS page in the segment header pages come the
Skeleton 4.0 keyframe index packets. There should be one index packet for
each content track in the Ogg segment, but index packets are not required
for a Skeleton 4.0 track to be considered valid. Each keypoint in the index
is stored in a "keypoint", which in turn stores an offset, and timestamp.
In order to save space, the offsets and timestamps are stored as
deltas, and then variable byte-encoded. The offset and timestamp deltas
store the difference between the keypoint's offset and timestamp from the
previous keypoint's offset and timestamp. So to calculate the page offset
of a keypoint you must sum the offset deltas of up to and including the
keypoint in the index.
The variable byte encoded integers are encoded using 7 bits per byte to
store the integer's bits, and the high bit is set in the last byte used
to encode the integer. The bits and bytes are in little endian byte order.
For example, the integer 7843, or 0001 1110 1010 0011 in binary, would be
stored as two bytes: 0xBD 0x23, or 1011 1101 0010 0011 in binary.
Each index packet contains the following:
1. Identifier 6 bytes: "index\0". Bytes [0...5].
2. The serialno of the stream this index applies to, as a 4 byte field.
Bytes [6...9].
3. The number of keypoints in this index packet, 'n' as a 8 byte
unsigned integer. This can be 0. Bytes [10...17].
4. The timestamp denominator for this stream, as an 8 byte signed
integer. All timestamps, including keypoint timestamps, first and
last sample timestamps are fractions of seconds over this denominator.
This must not be 0. Bytes [18...25].
5. First-sample-time numerator: 8 byte signed integer representing
the numerator for the presentation time of the first sample in the track.
Divide this by the timestamp denominator to determine the presentation
time of the first sample in seconds. Bytes [26..33].
6. Last-sample-time numerator: 8 byte signed integer representing the end
time of the last sample in the track. Divide this by the timestamp
denominator to determine the end time of the last sample in seconds.
Bytes [34..41].
7. 'n' key points, each of which contain, in the following order:
- the keyframe's page's byte offset delta, as a variable byte encoded
integer. This is the number of bytes that this keypoint is after the
preceeding keypoint's offset, or from the start of the segment if this
is the first keypoint. The keypoint's page start is therefore the sum
of the byte-offset-deltas of all the keypoints which come before it.
- the presentation time numerator delta, of a key frame which starts on
or shortly after the page at the keypoint's offset, as a variable byte
encoded integer. This is the difference from the previous keypoint's
timestamp numerator. The keypoint's timestamp numerator is therefore
the sum of all the timestamp numerator deltas up to and including the
keypoint's. Divide the timestamp numerator sum by the timestamp
denominator stored earlier in the index packet to determine the
presentation time of the keyframe in seconds.
The key points are stored in increasing order by offset (and thus by
presentation time as well).
The byte offsets stored in keypoints are relative to the start of the Ogg
bitstream segment. So if you have a physical Ogg bitstream made up of two
chained Oggs, the offsets in the second Ogg segment's bitstream's index
are relative to the beginning of the second Ogg in the chain, not the first.
Also note that if a physical Ogg bitstream is made up of chained Oggs, the
presence of an index in one segment does not imply that there will be an
index in any other segment.
The exact number of keyframes used to construct key points in the index
is up to the indexer, but to limit the index size, we recommend
including at most one key point per every 64KB of data, or every 2000ms,
whichever is least frequent.
As per the Skeleton 3.0 track, the last packet in the Skeleton 4.0 track
is an empty EOS packet.