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Original commit message from CVS: * ext/ogg/README: * ext/ogg/gstoggmux.c: (gst_ogg_mux_set_header_on_caps): Buffer on caps is not boxed anymore.
217 lines
7.2 KiB
Text
217 lines
7.2 KiB
Text
ogg demuxer
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-----------
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This ogg demuxer has two modes of operation, which both share a significant
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amount of code. The first mode is the streaming mode which is automatically
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selected when the demuxer is connected to a non-getrange based element. When
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connected to a getrange based element the ogg demuxer can do full seeking
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with great efficiency.
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1) the streaming mode.
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In this mode, the ogg demuxer receives buffers in the _chain() function which
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are then simply submited to the ogg sync layer. Pages are then processed when the
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sync layer detects them, pads are created for new chains and packets are sent to
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the peer elements of the pads.
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In this mode, no seeking is possible. This is the typical case when the stream is
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read from a network source.
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In this mode, no setup is done at startup, the pages are just read and decoded.
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A new logical chain is detected when one of the pages has the BOS flag set. At this
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point the existing pads are removed and new pads are created for all the logical
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streams in this new chain.
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2) the random access mode.
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In this mode, the ogg file is first scanned to detect the position and length of
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all chains. This scanning is performed using a recursive binary search algorithm
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that is explained below.
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find_chains(start, end)
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{
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ret1 = read_next_pages (start);
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ret2 = read_prev_page (end);
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if (WAS_HEADER (ret1)) {
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}
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else {
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}
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}
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a) read first and last pages
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start end
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V V
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+-----------------------+-------------+--------------------+
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| 111 | 222 | 333 |
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BOS BOS BOS EOS
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after reading start, serial 111, BOS, chain[0] = 111
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after reading end, serial 333, EOS
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start serialno != end serialno, binary search start, (end-start)/2
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start bisect end
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V V V
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+-----------------------+-------------+--------------------+
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| 111 | 222 | 333 |
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after reading start, serial 111, BOS, chain[0] = 111
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after reading end, serial 222, EOS
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while (
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testcases
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---------
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a) stream without BOS
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+----------------------------------------------------------+
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111 |
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EOS
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b) chained stream, first chain without BOS
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+-------------------+--------------------------------------+
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111 | 222 |
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BOS EOS
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c) chained stream
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+-------------------+--------------------------------------+
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| 111 | 222 |
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BOS BOS EOS
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d) chained stream, second without BOS
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+-------------------+--------------------------------------+
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| 111 | 222 |
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BOS EOS
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ogg and the granulepos
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----------------------
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an ogg streams contains pages with a serial number and a granule pos. The granulepos
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is a number that is codec specific and denotes the 'position' of the last sample in
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the last packet in that page.
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ogg has therefore no notion about time, it only knows about bytes and granule positions.
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The granule position is just a number, it can contain gaps or can just be any random
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number.
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theora and the granulepos
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-------------------------
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the granulepos in theora consists of the framenumber of the last keyframe shifted some
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amount of bits plus the number of p/b-frames.
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This means that given a framenumber or a timestamp one cannot generate the granulepos
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for that frame. eg frame 10 could have several valid granulepos values depending on if
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the last keyframe was on frame 5 or 0. Given a granulepos we can, however, create a
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unique correct timestamp and a framenumber.
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in a raw theroa stream we use the granulepos as the offset field.
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The granulepos of an ogg page is the framenumber of the last frame in the page.
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vorbis and granulepos
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---------------------
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the granulepos in vorbis happens to be the same as the sample counter. conversion to and
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from granulepos is therefore easy.
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in a raw vorbis stream we use the granulepos as the offset field.
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The granulepos of an ogg page is the sample number of the next page in the ogg stream.
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What can ogg do?
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----------------
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An ogg demuxer can read pages and get the granuleposition from it. It can ask the decoder
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elements to convert a granulepos to time.
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An ogg demuxer can also get the granulepos of the first and the last page of a stream to
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get the start and end timestamp of that stream. It can also get the length in bytes of
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the stream (when the peer is seekable, that is).
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An ogg demuxer is therefore basically able to seek to any byte position and timestamp.
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When asked to seek to a given granulepos, the ogg demuxer should always convert the
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value to a timestamp using the peer decoder element conversion function. It can then
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binary search the file to eventually end up on the page with the given granule pos or
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a granulepos with the same timestamp.
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Seeking in ogg currently
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------------------------
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When seeking in an ogg, the decoders can choose to forward the seek event as a
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granulepos or a timestamp to the ogg demuxer.
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In the case of a granulepos, the ogg demuxer will seek back to the beginning of
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the stream and skip pages until it finds one with the requested timestamp.
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In the case of a timestamp, the ogg demuxer also seeks back to the beginning of
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the stream. For each page it reads, it asks the decoder element to convert the
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granulepos back to a timestamp. The ogg demuxer keeps on skipping pages until the
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page has a timestamp bigger or equal to the requested one.
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It is therefore important that the decoder elements in vorbis can convert a granulepos
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into a timestamp or never seek on timestamp on the oggdemuxer.
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The default format on the oggdemuxer source pads is currently defined as a the
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granulepos of the packets, it is also the value of the OFFSET field in the GstBuffer.
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Oggmux
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------
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The oggmuxer uses the offset fields to fill in the granulepos in the pages.
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TODO
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----
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- use the OFFSET field in the GstBuffer to store/read the granulepos as
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opposed to the OFFSET_END field.
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Ogg media mapping
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-----------------
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Ogg defines a mapping for each media type that it embeds.
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For Vorbis:
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- 3 header pages, with granulepos 0.
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- 1 page with 1 packet header identification
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- N pages with 2 packets comments and codebooks
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- granulepos is samplenumber of next page
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- one packet can contain a variable number of samples but one frame
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that should be handed to the vorbis decoder.
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For Theora
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- 3 header pages, with granulepos 0.
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- 1 page with 1 packet header identification
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- N pages with 2 packets comments and codebooks
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- granulepos is framenumber of last packet in page, where framenumber
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is a combination of keyframe number and p frames since keyframe.
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- one packet contains 1 frame
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