mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-14 05:12:09 +00:00
97 lines
3.6 KiB
Markdown
97 lines
3.6 KiB
Markdown
# Forcing keyframes
|
|
|
|
Consider the following use case:
|
|
|
|
We have a pipeline that performs video and audio capture from a live
|
|
source, compresses and muxes the streams and writes the resulting data
|
|
into a file.
|
|
|
|
Inside the uncompressed video data we have a specific pattern inserted
|
|
at specific moments that should trigger a switch to a new file, meaning,
|
|
we close the existing file we are writing to and start writing to a new
|
|
file.
|
|
|
|
We want the new file to start with a keyframe so that one can start
|
|
decoding the file immediately.
|
|
|
|
## Components
|
|
|
|
1) We need an element that is able to detect the pattern in the video
|
|
stream.
|
|
|
|
2) We need to inform the video encoder that it should start encoding a
|
|
keyframe starting from exactly the frame with the pattern.
|
|
|
|
3) We need to inform the demuxer that it should flush out any pending
|
|
data and start creating the start of a new file with the keyframe as
|
|
a first video frame.
|
|
|
|
4) We need to inform the sink element that it should start writing to
|
|
the next file. This requires application interaction to instruct the
|
|
sink of the new filename. The application should also be free to
|
|
ignore the boundary and continue to write to the existing file. The
|
|
application will typically use an event pad probe to detect the
|
|
custom event.
|
|
|
|
## Implementation
|
|
|
|
### Downstream
|
|
|
|
The implementation would consist of generating a `GST_EVENT_CUSTOM_DOWNSTREAM`
|
|
event that marks the keyframe boundary. This event is inserted into the
|
|
pipeline by the application upon a certain trigger. In the above use case
|
|
this trigger would be given by the element that detects the pattern, in the
|
|
form of an element message.
|
|
|
|
The custom event would travel further downstream to instruct encoder,
|
|
muxer and sink about the possible switch.
|
|
|
|
The information passed in the event consists of:
|
|
|
|
**GstForceKeyUnit**
|
|
|
|
- **"timestamp"** (`G_TYPE_UINT64`): the timestamp of the buffer that
|
|
triggered the event.
|
|
|
|
- **"stream-time"** (`G_TYPE_UINT64`): the stream position that triggered the event.
|
|
|
|
- **"running-time"** (`G_TYPE_UINT64`): the running time of the stream when
|
|
the event was triggered.
|
|
|
|
- **"all-headers"** (`G_TYPE_BOOLEAN`): Send all headers, including
|
|
those in the caps or those sent at the start of the stream.
|
|
|
|
- **...**: optional other data fields.
|
|
|
|
Note that this event is purely informational, no element is required to
|
|
perform an action but it should forward the event downstream, just like
|
|
any other event it does not handle.
|
|
|
|
Elements understanding the event should behave as follows:
|
|
|
|
1) The video encoder receives the event before the next frame. Upon
|
|
reception of the event it schedules to encode the next frame as a
|
|
keyframe. Before pushing out the encoded keyframe it must push the
|
|
`GstForceKeyUnit` event downstream.
|
|
|
|
2) The muxer receives the `GstForceKeyUnit` event and flushes out its
|
|
current state, preparing to produce data that can be used as a
|
|
keyunit. Before pushing out the new data it pushes the
|
|
`GstForceKeyUnit` event downstream.
|
|
|
|
3) The application receives the `GstForceKeyUnit` on a sink padprobe of
|
|
the sink and reconfigures the sink to make it perform new actions
|
|
after receiving the next buffer.
|
|
|
|
### Upstream
|
|
|
|
When using RTP packets can get lost or receivers can be added at any
|
|
time, they may request a new key frame.
|
|
|
|
An downstream element sends an upstream `GstForceKeyUnit` event up the
|
|
pipeline.
|
|
|
|
When an element produces some kind of key unit in output, but has no
|
|
such concept in its input (like an encoder that takes raw frames), it
|
|
consumes the event (doesn't pass it upstream), and instead sends a
|
|
downstream `GstForceKeyUnit` event and a new keyframe.
|