How do I properly unwrap FLV video into raw and valid h264 segments for gstreamer buffers? - gstreamer

Question

I have written an RTMP server in rust that successfully allows RTMP publishers to connect, push a video stream, and RTMP clients can connect and watch those video streams successfully.
When a video RTMP packet comes in, I attempt to unwrap the video from the FLV container via:
// TODO: The FLV spec has the AVCPacketType and composition time as the first parts of the
// AVCPACKETTYPE. It's unclear if these two fields are part of h264 or FLV specific.
let flv_tag = data.split_to(1);
let is_sequence_header;
let codec = if flv_tag[0] & 0x07 == 0x07 {
is_sequence_header = data[0] == 0x00;
VideoCodec::H264
} else {
is_sequence_header = false;
VideoCodec::Unknown
};
let is_keyframe = flv_tag[0] & 0x10 == 0x10;
After this runs data contains the AVCVIDEOPACKET with the flv tag removed. When I send this video to other RTMP clients i just prepend the correct flv tag to it and send it off.
Now I am trying to pass the video packets to gstreamer in order to do in process transcoding. To do this I set up an appsrc | avdec_264 pipeline, and gave the appsrc component the following caps:
video_source.set_caps(Some(
&Caps::builder("video/x-h264")
.field("alignment", "nal")
.field("stream-format", "byte-stream")
.build()
));
Now when an RTMP publisher sends a video packet, I take the (attempted) unwrapped video packet and pass it to my appsrc via
pub fn push_video(&self, data: Bytes, timestamp: RtmpTimestamp) {
let mut buffer = Buffer::with_size(data.len()).unwrap();
{
let buffer = buffer.get_mut().unwrap();
buffer.set_pts(ClockTime::MSECOND * timestamp.value as u64);
let mut samples = buffer.map_writable().unwrap();
{
let samples = samples.as_mut_slice();
for index in 0..data.len() {
samples[index] = data[index];
}
}
}
self.video_source.push_buffer(buffer).unwrap();
}
When this occurs the following gstreamer debug output appears
2022-02-09T18:25:15Z INFO gstreamer_mmids_scratchpad] Pushing packet #0 (is_sequence_header:true, is_keyframe=true)
[2022-02-09T18:25:15Z INFO gstreamer_mmids_scratchpad] Connection 63397d56-16fb-4b54-a622-d991b5ad2d8e sent audio data
0:00:05.531722000 7516 000001C0C04011C0 INFO GST_EVENT gstevent.c:973:gst_event_new_segment: creating segment event bytes segment start=0, offset=0, stop=-1, rate=1.000000, applied_rate=1.000000, flags=0x00, time=0, base=0, position 0, duration -1
0:00:05.533525000 7516 000001C0C04011C0 INFO basesrc gstbasesrc.c:3018:gst_base_src_loop:<video_source> marking pending DISCONT
0:00:05.535385000 7516 000001C0C04011C0 WARN videodecoder gstvideodecoder.c:2818:gst_video_decoder_chain:<video_decode> Received buffer without a new-segment. Assuming timestamps start from 0.
0:00:05.537381000 7516 000001C0C04011C0 INFO GST_EVENT gstevent.c:973:gst_event_new_segment: creating segment event time segment start=0:00:00.000000000, offset=0:00:00.000000000, stop=99:99:99.999999999, rate=1.000000, applied_rate=1.000000, flags=0x00, time=0:00:00.000000000, base=0:00:00.000000000, position 0:00:00.000000000, duration 99:99:99.999999999
[2022-02-09T18:25:15Z INFO gstreamer_mmids_scratchpad] Pushing packet #1 (is_sequence_header:false, is_keyframe=true)
0:00:05.563445000 7516 000001C0C04011C0 INFO libav :0:: Invalid NAL unit 0, skipping.
[2022-02-09T18:25:15Z INFO gstreamer_mmids_scratchpad] Pushing packet #2 (is_sequence_header:false, is_keyframe=false)
0:00:05.579274000 7516 000001C0C04011C0 ERROR libav :0:: No start code is found.
0:00:05.581338000 7516 000001C0C04011C0 ERROR libav :0:: Error splitting the input into NAL units.
0:00:05.583337000 7516 000001C0C04011C0 WARN libav gstavviddec.c:2068:gst_ffmpegviddec_handle_frame:<video_decode> Failed to send data for decoding
[2022-02-09T18:25:15Z INFO gstreamer_mmids_scratchpad] Pushing packet #3 (is_sequence_header:false, is_keyframe=false)
0:00:05.595253000 7516 000001C0C04011C0 ERROR libav :0:: No start code is found.
0:00:05.597204000 7516 000001C0C04011C0 ERROR libav :0:: Error splitting the input into NAL units.
0:00:05.599262000 7516 000001C0C04011C0 WARN libav gstavviddec.c:2068:gst_ffmpegviddec_handle_frame:<video_decode> Failed to send data for decoding
Based on this I figured this might be caused by the non-data portions of the AVCVIDEOPACKET not being part of the h264 flow, but an FLV specific flow. So I tried ignoring the first 4 bytes (AVCPacketType and CompositionTime fields) of each packet I wrote to the buffer:
pub fn push_video(&self, data: Bytes, timestamp: RtmpTimestamp) {
let mut buffer = Buffer::with_size(data.len() - 4).unwrap();
{
let buffer = buffer.get_mut().unwrap();
buffer.set_pts(ClockTime::MSECOND * timestamp.value as u64);
let mut samples = buffer.map_writable().unwrap();
{
let samples = samples.as_mut_slice();
for index in 4..data.len() {
samples[index - 4] = data[index];
}
}
}
self.video_source.push_buffer(buffer).unwrap();
}
This essentially gave me the same logging output and errors. This is reproducible with the h264parse plugin as well.
What am I missing in the unwrapping process to pass raw h264 video to gstreamer?
Edit:
Realizing I misread the pad template I tried the following caps instead
video_source.set_caps(Some(
&Caps::builder("video/x-h264")
.field("alignment", "au")
.field("stream-format", "avc")
.build()
));
This also failed with pretty simmilar output.

Answer 1

I think I finally figured this out.
The first thing is that I need to include removing the AVCVIDEOPACKET headers (packet type and composition time fields). These are not part of the h264 format and thus cause parsing errors.
The second thing I needed to do was to not pass the sequence header as a buffer to the source. Instead the sequence header bytes need to be set as the codec_data field for the appsrc's caps. This now allows for no parsing errors when passing the video data to h264parse, and even gives me a correctly sized window.
The third thing I was missing is the correct dts and pts values. It turns out the RTMP timestamp I'm given is the dts, and pts = AVCVIDEOPACKET.CompositionTime + dts.