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91 lines
3.8 KiB
Text
91 lines
3.8 KiB
Text
Forcing keyframes
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-----------------
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Consider the following use case:
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We have a pipeline that performs video and audio capture from a live source,
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compresses and muxes the streams and writes the resulting data into a file.
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Inside the uncompressed video data we have a specific pattern inserted at
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specific moments that should trigger a switch to a new file, meaning, we close
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the existing file we are writing to and start writing to a new file.
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We want the new file to start with a keyframe so that one can start decoding
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the file immediately.
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Components:
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1) We need an element that is able to detect the pattern in the video stream.
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2) We need to inform the video encoder that it should start encoding a keyframe
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starting from exactly the frame with the pattern.
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3) We need to inform the demuxer that it should flush out any pending data and
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start creating the start of a new file with the keyframe as a first video
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frame.
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4) We need to inform the sink element that it should start writing to the next
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file. This requires application interaction to instruct the sink of the new
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filename. The application should also be free to ignore the boundary and
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continue to write to the existing file. The application will typically use
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an event pad probe to detect the custom event.
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Implementation:
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The implementation would consist of generating a GST_EVENT_CUSTOM_DOWNSTREAM
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event that marks the keyframe boundary. This event is inserted into the
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pipeline by the application upon a certain trigger. In the above use case this
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trigger would be given by the element that detects the pattern, in the form of
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an element message.
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The custom event would travel further downstream to instruct encoder, muxer and
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sink about the possible switch.
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The information passed in the event consists of:
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name: GstForceKeyUnit
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(G_TYPE_UINT64)"timestamp" : the timestamp of the buffer that
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triggered the event.
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(G_TYPE_UINT64)"stream-time" : the stream position that triggered the
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event.
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(G_TYPE_UINT64)"running-time" : the running time of the stream when the
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event was triggered.
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(G_TYPE_BOOLEAN)"all-headers" : Send all headers, including those in
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the caps or those sent at the start of
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the stream.
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.... : optional other data fields.
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Note that this event is purely informational, no element is required to
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perform an action but it should forward the event downstream, just like any
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other event it does not handle.
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Elements understanding the event should behave as follows:
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1) The video encoder receives the event before the next frame. Upon reception
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of the event it schedules to encode the next frame as a keyframe.
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Before pushing out the encoded keyframe it must push the GstForceKeyUnit
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event downstream.
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2) The muxer receives the GstForceKeyUnit event and flushes out its current state,
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preparing to produce data that can be used as a keyunit. Before pushing out
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the new data it pushes the GstForceKeyUnit event downstream.
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3) The application receives the GstForceKeyUnit on a sink padprobe of the sink
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and reconfigures the sink to make it perform new actions after receiving
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the next buffer.
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Upstream
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--------
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When using RTP packets can get lost or receivers can be added at any time,
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they may request a new key frame.
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An downstream element sends an upstream "GstForceKeyUnit" event up the
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pipeline.
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When an element produces some kind of key unit in output, but has
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no such concept in its input (like an encoder that takes raw frames),
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it consumes the event (doesn't pass it upstream), and instead sends
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a downstream GstForceKeyUnit event and a new keyframe.
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