From e509b30d95228ef8a94fc407d10c057d117c04d4 Mon Sep 17 00:00:00 2001 From: "Reynaldo H. Verdejo Pinochet" Date: Thu, 29 Dec 2016 22:40:20 -0800 Subject: [PATCH] design: seeking: section breakdown, markup, readability and grammar fixes --- markdown/design/seeking.md | 86 +++++++++++++++++++++++----------------------- 1 file changed, 43 insertions(+), 43 deletions(-) diff --git a/markdown/design/seeking.md b/markdown/design/seeking.md index dbd7d0c..65d0b0f 100644 --- a/markdown/design/seeking.md +++ b/markdown/design/seeking.md @@ -1,11 +1,12 @@ # Seeking + Seeking in GStreamer means configuring the pipeline for playback of the media between a certain start and stop time, called the playback segment. By default a pipeline will play from position 0 to the total duration of the media at a rate of 1.0. -A seek is performed by sending a seek event to the sink elements of a -pipeline. Sending the seek event to a bin will by default forward the +A seek is performed by sending a `SEEK` event to the sink elements of a +pipeline. Sending the `SEEK` event to a bin will by default forward the event to all sinks in the bin. When performing a seek, the start and stop values of the segment can be @@ -20,7 +21,7 @@ Feedback of the seek operation can be immediately using the pipeline is discarded and playback starts from the new position immediately. -When the FLUSH flag is not set, the seek will be queued and executed as +When the `FLUSH` flag is not set, the seek will be queued and executed as soon as possible, which might be after all queues are emptied. Seeking can be performed in different formats such as time, frames or @@ -39,7 +40,7 @@ Non segment seeking will make the pipeline emit EOS when the configured segment has been played. Segment seeking (using the `GST_SEEK_FLAG_SEGMENT`) will not emit an -EOS at the end of the playback segment but will post a SEGMENT_DONE +EOS at the end of the playback segment but will post a `SEGMENT_DONE` message on the bus. This message is posted by the element driving the playback in the pipeline, typically a demuxer. After receiving the message, the application can reconnect the pipeline or issue other seek @@ -63,7 +64,7 @@ consumption is more important than accurately producing all frames. ## Generating seeking events -A seek event is created with `gst_event_new_seek ()`. +A seek event is created with `gst_event_new_seek()`. ## Seeking variants @@ -81,8 +82,8 @@ media is immediately played after the seek call returns. This seek type is typically performed after issuing segment seeks to finish the playback of the pipeline. -Performing a non-flushing seek in a PAUSED pipeline blocks until the -pipeline is set to playing again since all data passing is blocked in +Performing a non-flushing seek in a `PAUSED` pipeline blocks until the +pipeline is set to playing again, since all data passing is blocked in the prerolled sinks. ### segment seeking with FLUSH @@ -93,14 +94,13 @@ This seek is typically performed when starting seamless looping. This seek is typically performed when continuing seamless looping. -Demuxer/parser behaviour and `SEEK_FLAG_KEY_UNIT` and -`SEEK_FLAG_ACCURATE` +## `KEY_UNIT` and `ACCURATE` flags This section aims to explain the behaviour expected by an element with -regard to the KEY_UNIT and ACCURATE seek flags using the example of a -parser or demuxer. +regard to the `KEY_UNIT` and `ACCURATE` seek flags, using a parser +or demuxer as an example. -#### DEFAULT BEHAVIOUR: +### DEFAULT BEHAVIOUR: When a seek to a certain position is requested, the demuxer/parser will do two things (ignoring flushing and segment seeks, and simplified for @@ -114,17 +114,17 @@ To ensure that the data corresponding to the requested seek position can actually be decoded, a demuxer or parser needs to start pushing data from a keyframe/keyunit at or before the requested seek position. -Unless requested differently (via the KEY_UNIT flag), the start of the +Unless requested differently (via the `KEY_UNIT` flag), the start of the segment event should be the requested seek position. So by default a demuxer/parser will then start pushing data from -position DATA and send a segment event with start position SEG_START, -and DATA ⇐ SEG_START. +position DATA and send a segment event with start position `SEG_START`, +and `DATA ⇐ SEG_START`. -If DATA < SEG_START, a well-behaved video decoder will start decoding +If `DATA < SEG_START`, a well-behaved video decoder will start decoding frames from DATA, but take into account the segment configured by the demuxer via the segment event, and only actually output decoded video -frames from SEG_START onwards, dropping all decoded frames that are +frames from `SEG_START` onwards, dropping all decoded frames that are before the segment start and adjusting the timestamp/duration of the buffer that overlaps the segment start ("clipping"). A not-so-well-behaved video decoder will start decoding frames from DATA @@ -132,52 +132,52 @@ and push decoded video frames out starting from position DATA, in which case the frames that are before the configured segment start will usually be dropped/clipped downstream (e.g. by the video sink). -#### GST_SEEK_FLAG_KEY_UNIT: +### `GST_SEEK_FLAG_KEY_UNIT` -If the KEY_UNIT flag is specified, the demuxer/parser should adjust the +If the `KEY_UNIT` flag is specified, the demuxer/parser should adjust the segment start to the position of the key frame closest to the requested seek position and then start pushing out data from there. The nearest key frame may be before or after the requested seek position, but many implementations will only look for the closest keyframe before the requested position. -Most media players and thumbnailers do (and should be doing) KEY_UNIT +Most media players and thumbnailers do (and should be doing) `KEY_UNIT` seeks by default, for performance reasons, to ensure almost-instant -responsiveness when scrubbing (dragging the seek slider in PAUSED or -PLAYING mode). This works well for most media, but results in suboptimal +responsiveness when scrubbing (dragging the seek slider in `PAUSED` or +`PLAYING` mode). This works well for most media, but results in suboptimal behaviour for a small number of *odd* files (e.g. files that only have one keyframe at the very beginning, or only a few keyframes throughout the entire stream). At the time of writing, a solution for this still needs to be found, but could be implemented demuxer/parser-side, e.g. -make demuxers/parsers ignore the KEY_UNIT flag if the position +make demuxers/parsers ignore the `KEY_UNIT` flag if the position adjustment would be larger than 1/10th of the duration or somesuch. Flags can be used to influence snapping direction for those cases where -it matters. SNAP_BEFORE will select the preceding position to the seek -target, and SNAP_AFTER will select the following one. If both flags are +it matters. `SNAP_BEFORE` will select the preceding position to the seek +target, and `SNAP_AFTER` will select the following one. If both flags are set, the nearest one to the seek target will be used. If none of these flags are set, the seeking implemention is free to select whichever it wants. #### Summary: - - if the KEY_UNIT flag is **not** specified, the demuxer/parser + - if the `KEY_UNIT` flag is **not** specified, the demuxer/parser should start pushing data from a key unit preceding the seek position (or from the seek position if that falls on a key unit), and the start of the new segment should be the requested seek position. - - if the KEY_UNIT flag is specified, the demuxer/parser should start + - if the `KEY_UNIT` flag is specified, the demuxer/parser should start pushing data from the key unit nearest the seek position (or from the seek position if that falls on a key unit), and the start of the new segment should be adjusted to the position of that key unit which was nearest the requested seek position (ie. the new segment start should be the position from which data is pushed). -### GST_SEEK_FLAG_ACCURATE: +### `GST_SEEK_FLAG_ACCURATE` -If the ACCURATE flag is specified in a seek request, the demuxer/parser -is asked to do whatever it takes (!) to make sure that the position +If the `ACCURATE` flag is specified in a seek request, the demuxer/parser +is asked to do whatever it takes (!) to make sure the position seeked to is accurate in relation to the beginning of the stream. This means that it is not acceptable to just approximate the position (e.g. using an average bitrate). The achieved position must be exact. In the @@ -185,30 +185,30 @@ worst case, the demuxer or parser needs to push data from the beginning of the file and let downstream clip everything before the requested segment start. -The ACCURATE flag does not affect what the segment start should be in -relation to the requested seek position. Only the KEY_UNIT flag (or its +The `ACCURATE` flag does not affect what the segment start should be in +relation to the requested seek position. Only the `KEY_UNIT` flag (or its absence) has any effect on that. -Video editors and frame-stepping applications usually use the ACCURATE +Video editors and frame-stepping applications usually use the `ACCURATE` flag. #### Summary: - - if the ACCURATE flag is **not** specified, it is up to the - demuxer/parser to decide how exact the seek should be. If the flag - is not specified, the expectation is that the demuxer/parser does a + - if the `ACCURATE` flag is **not** specified, it is up to the + demuxer/parser to decide how exact the seek should be. In this case, + the expectation is that the demuxer/parser does a resonable best effort attempt, trading speed for accuracy. In the absence of an index, the seek position may be approximated. - - if the ACCURATE flag is specified, absolute accuracy is required, + - if the `ACCURATE` flag **is** specified, absolute accuracy is required, and speed is of no concern. It is not acceptable to just approximate the seek position in that case. - - the ACCURATE flag does not imply that the segment starts at the + - the `ACCURATE` flag does not imply that the segment starts at the requested seek position or should be adjusted to the nearest - keyframe, only the KEY_UNIT flag determines that. + keyframe, only the `KEY_UNIT` flag determines that. -### ACCURATE and KEY_UNIT combinations: +### `ACCURATE` and `KEY_UNIT` combinations: All combinations of these two flags are valid: @@ -216,14 +216,14 @@ All combinations of these two flags are valid: from preceding key frame (or earlier), feel free to approximate the seek position - - only KEY_UNIT specified: segment starts from position of nearest + - only `KEY_UNIT` specified: segment starts from position of nearest keyframe, send data from nearest keyframe, feel free to approximate the seek position - - only ACCURATE specified: segment starts at seek position, send data + - only `ACCURATE` specified: segment starts at seek position, send data from preceding key frame (or earlier), do not approximate the seek position under any circumstances - - ACCURATE | KEY_UNIT specified: segment starts from position of + - `ACCURATE | KEY_UNIT` specified: segment starts from position of nearest keyframe, send data from nearest key frame, do not approximate the seek position under any circumstances -- 2.7.4