design: seeking: section breakdown, markup, readability and grammar fixes

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