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133 lines
3.4 KiB
Text
133 lines
3.4 KiB
Text
1) intro
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--------
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For efficient and accurate seeking, a plugin typically needs
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an index of possible seek positions (in time, bytes, samples, ...).
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The purpose of GstTimecache is to provide an infrastructure for
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plugins to maintain such an index.
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By creating a generic interface to this functionality, it is also
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possible for the application to make use of the generated index.
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possible use cases are: saving/loading of indexes, obtain an
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overview of the indexed stream (where is sample N, where is
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the keyframe...)
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This document describes a proposed API for this functionality.
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2) Requirements
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---------------
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Bare minimum:
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- store mappings between format1 and format2 (bytes<->time,
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samples<->tracks, ...)
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- update the mappings (add/remove/change entries)
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- query the index, locate entries.
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- store arbitrary extra metadata in the timecache (media file,
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time, author, ...)
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- set/get cache on elements.
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Nice to have:
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- cache groups with varying certainty that are merged.
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- API to control the indexing process, what gets indexed, how much,
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...)
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- Get notification when a new entry is added, together with an
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option to turn off in-memory caching, this could be used to
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do direct-to-disk indexing (for large files) insteda of to-memory.
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3) use cases
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------------
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a) indexing an mpeg systems stream
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- create an empty timecache object
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- create filesrc ! mpegparse
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- set timecache on mpegparse
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- run the pipeline to EOS
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- mpegparse will create a mapping from SCR to byte offset
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and add it to the cache.
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- read/save timecache
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b) running an mpeg with a previously created cache
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- create timecache object from saved index
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- create filesrc ! mpegparse
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- set the timecache on mpegparse
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- run the pipeline, seek, ...
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- mpegparse uses the timecache to do accurate seek to
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SCR timestamps.
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c) indexing an mpeg systems stream, SCR, video and audio elementary
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streams.
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- create an empty timecache object
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- create filesrc ! mpegdemux
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- set timecache on mpegdemux
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- run the pipeline to EOS
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- mpegparse will create a mappings for:
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* SCR to byte offset
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* PTS + streamid to byte offset
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and add it to the cache.
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- read/save timecache
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d) complete indexing of mpeg systems streams and audio/video
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streams.
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- create 3 empty timecaches
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- create filesrc ! mpegdemux video_%02d! mpeg2dec
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mpegdemux0 audio_%02d! mad
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- set timecaches on mpegdemux, mpeg2dec, mad
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- run pipeline to EOS
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- mpegdemux creates timecache for SCR, PTS + streamid
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- mpeg2dec creates timecache for frames + I/P/B id
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- mad creates timecache for mpeg audio headers
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- read/save timecaches
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seeking in a fully indexed stream:
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- seek on timestamp in mpeg2dec
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- mpeg2dec locates previous I frame in cache, does
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a timeseek on sinkpad
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- mpegparse uses the cache to convert the timestamp
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to an offset, then does a seek on offset
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- filesrc seeks to offset
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4) Entry types
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--------------
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- mapping between formats:
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<padid> <flags> <format> <position>, <format> <position>, ...
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- metadata, streaminfo, other objects:
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<padid> <format> <position>, <keyword> gpointer
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5) implementation
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-----------------
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We define an abstract base class GstTimeCache with following
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abstract methods:
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6) cache format example
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-----------------------
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We define a binary format for the cache for space/speed reasons.
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It is entirely possible to create alternative implementations.
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