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videodecoder: Extend docs and add comments

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Update the documentation block for the base class, and add a comment
block about the reverse-playback logic and implementation.
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Jan Schmidt 2012-06-19 23:16:12 +10:00
parent bdb4f7c101
commit ca09aaabc7
1 changed files with 182 additions and 32 deletions
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214
gst-libs/gst/video/gstvideodecoder.c
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@ -30,22 +30,25 @@
* This base class is for video decoders turning encoded data into raw video
* frames.
*
* GstVideoDecoder and subclass should cooperate as follows.
* The GstVideoDecoder base class and derived subclasses should cooperate as follows:
* <orderedlist>
* <listitem>
* <itemizedlist><title>Configuration</title>
* <listitem><para>
* Initially, GstVideoDecoder calls @start when the decoder element
* is activated, which allows subclass to perform any global setup.
* is activated, which allows the subclass to perform any global setup.
* </para></listitem>
* <listitem><para>
* GstVideoDecoder calls @set_format to inform subclass of caps
* GstVideoDecoder calls @set_format to inform the subclass of caps
* describing input video data that it is about to receive, including
* possibly configuration data.
* While unlikely, it might be called more than once, if changing input
* parameters require reconfiguration.
* </para></listitem>
* <listitem><para>
* Incoming data buffers are processed as needed, described in Data Processing below.
* </para></listitem>
* <listitem><para>
* GstVideoDecoder calls @stop at end of all processing.
* </para></listitem>
* </itemizedlist>
@ -54,72 +57,98 @@
* <itemizedlist>
* <title>Data processing</title>
* <listitem><para>
* Base class gathers input data, and optionally allows subclass
* The base class gathers input data, and optionally allows subclass
* to parse this into subsequently manageable chunks, typically
* corresponding to and referred to as 'frames'.
* </para></listitem>
* <listitem><para>
* Input frame is provided to subclass' @handle_frame. The ownership of
* the frame is given to @handle_frame.
* Each input frame is provided in turn to the subclass' @handle_frame callback.
* The ownership of the frame is given to the @handle_frame callback.
* </para></listitem>
* <listitem><para>
* If codec processing results in decoded data, subclass should call
* If codec processing results in decoded data, the subclass should call
* @gst_video_decoder_finish_frame to have decoded data pushed.
* downstream.
* downstream. Otherwise, the subclass must call @gst_video_decoder_drop_frame, to
* allow the base class to do timestamp and offset tracking, and possibly to
* requeue the frame for a later attempt in the case of reverse playback.
* </para></listitem>
* </itemizedlist>
* </listitem>
* <listitem>
* <itemizedlist><title>Shutdown phase</title>
* <listitem><para>
* GstVideoDecoder class calls @stop to inform the subclass that data
* The GstVideoDecoder class calls @stop to inform the subclass that data
* parsing will be stopped.
* </para></listitem>
* </itemizedlist>
* </listitem>
* <listitem>
* <itemizedlist><title>Additional Notes</title>
* <listitem>
* <itemizedlist><title>Seeking/Flushing</title>
* <listitem><para>
* When the pipeline is seeked or otherwise flushed, the subclass is informed via a call
* to its @reset callback, with the hard parameter set to true. This indicates the
* subclass should drop any internal data queues and timestamps and prepare for a fresh
* set of buffers to arrive for parsing and decoding.
* </para></listitem>
* </itemizedlist>
* </listitem>
* <listitem>
* <itemizedlist><title>End Of Stream</title>
* <listitem><para>
* At end-of-stream, the subclass @parse function may be called some final times with the
* at_eos parameter set to true, indicating that the element should not expect any more data
* to be arriving, and it should parse and remaining frames and call
* gst_video_decoder_have_frame() if possible.
* </para></listitem>
* </itemizedlist>
* </listitem>
* </itemizedlist>
* </listitem>
* </orderedlist>
*
* Subclass is responsible for providing pad template caps for
* The subclass is responsible for providing pad template caps for
* source and sink pads. The pads need to be named "sink" and "src". It also
* needs to set the fixed caps on srcpad, when the format is ensured. This
* is typically when base class calls subclass' @set_format function, though
* it might be delayed until calling @gst_video_decoder_finish_frame.
* needs to provide information about the ouptput caps, when they are known.
* This may be when the base class calls the subclass' @set_format function,
* though it might be during decoding, before calling
* @gst_video_decoder_finish_frame. This is done via
* @gst_video_decoder_set_output_state
*
* Subclass is also responsible for providing (presentation) timestamps
* The subclass is also responsible for providing (presentation) timestamps
* (likely based on corresponding input ones). If that is not applicable
* or possible, baseclass provides limited framerate based interpolation.
* or possible, the base class provides limited framerate based interpolation.
*
* Similarly, the baseclass provides some limited (legacy) seeking support
* (upon explicit subclass request), as full-fledged support
* Similarly, the base class provides some limited (legacy) seeking support
* if specifically requested by the subclass, as full-fledged support
* should rather be left to upstream demuxer, parser or alike. This simple
* approach caters for seeking and duration reporting using estimated input
* bitrates.
* bitrates. To enable it, a subclass should call
* @gst_video_decoder_set_estimate_rate to enable handling of incoming byte-streams.
*
* Baseclass provides some support for reverse playback, in particular
* The base class provides some support for reverse playback, in particular
* in case incoming data is not packetized or upstream does not provide
* fragments on keyframe boundaries. However, subclass should then be prepared
* for the parsing and frame processing stage to occur separately (rather
* than otherwise the latter immediately following the former),
* and should ensure the parsing stage properly marks keyframes or rely on
* upstream to do so properly for incoming data.
* fragments on keyframe boundaries. However, the subclass should then be prepared
* for the parsing and frame processing stage to occur separately (in normal
* forward processing, the latter immediately follows the former),
* The subclass also needs to ensure the parsing stage properly marks keyframes,
* unless it knows the upstream elements will do so properly for incoming data.
*
* Things that subclass need to take care of:
* The bare minimum that a functional subclass needs to implement is:
* <itemizedlist>
* <listitem><para>Provide pad templates</para></listitem>
* <listitem><para>
* Set source pad caps when appropriate
* Inform the base class of output caps via @gst_video_decoder_set_output_state
* </para></listitem>
* <listitem><para>
* Configure some baseclass behaviour parameters.
* </para></listitem>
* <listitem><para>
* Optionally parse input data, if it is not considered packetized.
* Parse input data, if it is not considered packetized from upstream
* Data will be provided to @parse which should invoke @gst_video_decoder_add_to_frame and
* @gst_video_decoder_have_frame as appropriate.
* @gst_video_decoder_have_frame to separate the data belonging to each video frame.
* </para></listitem>
* <listitem><para>
* Accept data in @handle_frame and provide decoded results to
* @gst_video_decoder_finish_frame.
* @gst_video_decoder_finish_frame, or call @gst_video_decoder_drop_frame.
* </para></listitem>
* </itemizedlist>
*/
@ -143,6 +172,127 @@
*
*/
/* Implementation notes:
* The Video Decoder base class operates in 2 primary processing modes, depending
* on whether forward or reverse playback is requested.
*
* Forward playback:
* * Incoming buffer -> @parse() -> add_to_frame()/have_frame() -> handle_frame() ->
* push downstream
*
* Reverse playback is more complicated, since it involves gathering incoming data regions
* as we loop backwards through the upstream data. The processing concept (using incoming
* buffers as containing one frame each to simplify things) is:
*
* Upstream data we want to play:
* Buffer encoded order: 1 2 3 4 5 6 7 8 9 EOS
* Keyframe flag: K K
* Groupings: AAAAAAA BBBBBBB CCCCCCC
*
* Input:
* Buffer reception order: 7 8 9 4 5 6 1 2 3 EOS
* Keyframe flag: K K
* Discont flag: D D D
*
* - Each Discont marks a discont in the decoding order.
* - The keyframes mark where we can start decoding.
*
* Initially, we prepend incoming buffers to the gather queue. Whenever the
* discont flag is set on an incoming buffer, the gather queue is flushed out
* before the new buffer is collected.
*
* The above data will be accumulated in the gather queue like this:
*
* gather queue: 9 8 7
* D
*
* Whe buffer 4 is received (with a DISCONT), we flush the gather queue like
* this:
*
* while (gather)
* take head of queue and prepend to parse queue (this reverses the sequence,
* so parse queue is 7 -> 8 -> 9)
*
* Next, we process the parse queue, which now contains all un-parsed packets (including
* any leftover ones from the previous decode section)
*
* for each buffer now in the parse queue:
* Call the subclass parse function, prepending each resulting frame to
* the parse_gather queue. Buffers which precede the first one that
* produces a parsed frame are retained in the parse queue for re-processing on
* the next cycle of parsing.
*
* The parse_gather queue now contains frame objects ready for decoding, in reverse order.
* parse_gather: 9 -> 8 -> 7
*
* while (parse_gather)
* Take the head of the queue and prepend it to the decode queue
* If the frame was a keyframe, process the decode queue
* decode is now 7-8-9
*
* Processing the decode queue results in frames with attached output buffers
* stored in the 'output_queue' ready for outputting in reverse order.
*
* After we flushed the gather queue and parsed it, we add 4 to the (now empty) gather queue.
* We get the following situation:
*
* gather queue: 4
* decode queue: 7 8 9
*
* After we received 5 (Keyframe) and 6:
*
* gather queue: 6 5 4
* decode queue: 7 8 9
*
* When we receive 1 (DISCONT) which triggers a flush of the gather queue:
*
* Copy head of the gather queue (6) to decode queue:
*
* gather queue: 5 4
* decode queue: 6 7 8 9
*
* Copy head of the gather queue (5) to decode queue. This is a keyframe so we
* can start decoding.
*
* gather queue: 4
* decode queue: 5 6 7 8 9
*
* Decode frames in decode queue, store raw decoded data in output queue, we
* can take the head of the decode queue and prepend the decoded result in the
* output queue:
*
* gather queue: 4
* decode queue:
* output queue: 9 8 7 6 5
*
* Now output all the frames in the output queue, picking a frame from the
* head of the queue.
*
* Copy head of the gather queue (4) to decode queue, we flushed the gather
* queue and can now store input buffer in the gather queue:
*
* gather queue: 1
* decode queue: 4
*
* When we receive EOS, the queue looks like:
*
* gather queue: 3 2 1
* decode queue: 4
*
* Fill decode queue, first keyframe we copy is 2:
*
* gather queue: 1
* decode queue: 2 3 4
*
* Decoded output:
*
* gather queue: 1
* decode queue:
* output queue: 4 3 2
*
* Leftover buffer 1 cannot be decoded and must be discarded.
*/
#include "gstvideodecoder.h"
#include "gstvideoutils.h"