mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-27 10:40:34 +00:00
85 lines
3.1 KiB
Text
85 lines
3.1 KiB
Text
ogg and the granulepos
|
|
----------------------
|
|
|
|
an ogg streams contains pages with a serial number and a granule pos. The granulepos
|
|
is a number that is codec specific and denotes the 'position' of the last packet
|
|
in that page.
|
|
|
|
ogg has therefore no notion about time, it only knows about bytes and granule positions.
|
|
|
|
The granule position is just a number, it can contain gaps or can just be any random
|
|
number.
|
|
|
|
|
|
theora and the granulepos
|
|
-------------------------
|
|
|
|
the granulepos in theora consists of the framenumber of the last keyframe shifted some
|
|
amount of bits plus the number of p/b-frames.
|
|
|
|
This means that given a framenumber or a timestamp one cannot generate the granulepos
|
|
for that frame. eg frame 10 could have several valid granulepos values depending on if
|
|
the last keyframe was on frame 5 or 0. Given a granulepos we can, however, create a
|
|
unique correct timestamp and a framenumber.
|
|
|
|
in a raw theroa stream we use the granulepos as the offset field.
|
|
|
|
|
|
vorbis and granulepos
|
|
---------------------
|
|
|
|
the granulepos in vorbis happens to be the same as the sample counter. conversion to and
|
|
from granulepos is therefore easy.
|
|
|
|
in a raw vorbis stream we use the granulepos as the offset field.
|
|
|
|
|
|
What can ogg do?
|
|
----------------
|
|
|
|
An ogg demuxer can read pages and get the granuleposition from it. It can ask the decoder
|
|
elements to convert a granulepos to time.
|
|
|
|
An ogg demuxer can also get the granulepos of the first and the last page of a stream to
|
|
get the start and end timestamp of that stream. It can also get the length in bytes of
|
|
the stream (when the peer is seekable, that is).
|
|
|
|
An ogg demuxer is therefore basically able to seek to any byte position and timestamp.
|
|
When asked to seek to a given granulepos, the ogg demuxer should always convert the
|
|
value to a timestamp using the peer decoder element conversion function. It can then
|
|
binary search the file to eventually end up on the page with the given granule pos or
|
|
a granulepos with the same timestamp.
|
|
|
|
Seeking in ogg currently
|
|
------------------------
|
|
|
|
When seeking in an ogg, the decoders can choose to forward the seek event as a
|
|
granulepos or a timestamp to the ogg demuxer.
|
|
|
|
In the case of a granulepos, the ogg demuxer will seek back to the beginning of
|
|
the stream and skip pages until it finds one with the requested timestamp.
|
|
|
|
In the case of a timestamp, the ogg demuxer also seeks back to the beginning of
|
|
the stream. For each page it reads, it asks the decoder element to convert the
|
|
granulepos back to a timestamp. The ogg demuxer keeps on skipping pages until the
|
|
page has a timestamp bigger or equal to the requested one.
|
|
|
|
It is therefore important that the decoder elements in vorbis can convert a granulepos
|
|
into a timestamp or never seek on timestamp on the oggdemuxer.
|
|
|
|
The default format on the oggdemuxer source pads is currently defined as a the
|
|
granulepos of the packets, it is also the value of the OFFSET field in the GstBuffer.
|
|
|
|
|
|
Oggmux
|
|
------
|
|
|
|
The oggmuxer uses the offset fields to fill in the granulepos in the pages.
|
|
|
|
|
|
TODO
|
|
----
|
|
|
|
- use the OFFSET field in the GstBuffer to store/read the granulepos as
|
|
opposed to the OFFSET_END field.
|
|
- Seeking should be implemented with a binary search.
|