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Reduce latency by 1 frame

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To packetize the H.264 raw stream, av_parser_parse2() (called by
av_read_frame()) knows that it has received a full frame only after it
has received some data for the next frame. As a consequence, the client
always waited until the next frame before sending the current frame to
the decoder!

On the device side, we know packets boundaries. To reduce latency,
make the device always transmit the "frame meta" to packetize the stream
manually (it was already implemented to send PTS, but only enabled on
recording).

On the client side, replace av_read_frame() by manual packetizing and
parsing.

<https://stackoverflow.com/questions/50682518/replacing-av-read-frame-to-reduce-delay>
<https://trac.ffmpeg.org/ticket/3354>
This commit is contained in:
Romain Vimont
2019-07-31 01:55:32 +02:00
parent a90ccbdf3b
commit 63af7fbafe
6 changed files with 180 additions and 187 deletions
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335
app/src/stream.c
View File

@@ -22,54 +22,8 @@
#define HEADER_SIZE 12
#define NO_PTS UINT64_C(-1)
static struct frame_meta *
frame_meta_new(uint64_t pts) {
struct frame_meta *meta = SDL_malloc(sizeof(*meta));
if (!meta) {
return meta;
}
meta->pts = pts;
meta->next = NULL;
return meta;
}
static void
frame_meta_delete(struct frame_meta *frame_meta) {
SDL_free(frame_meta);
}
static bool
receiver_state_push_meta(struct receiver_state *state, uint64_t pts) {
struct frame_meta *frame_meta = frame_meta_new(pts);
if (!frame_meta) {
return false;
}
// append to the list
// (iterate to find the last item, in practice the list should be tiny)
struct frame_meta **p = &state->frame_meta_queue;
while (*p) {
p = &(*p)->next;
}
*p = frame_meta;
return true;
}
static uint64_t
receiver_state_take_meta(struct receiver_state *state) {
struct frame_meta *frame_meta = state->frame_meta_queue; // first item
SDL_assert(frame_meta); // must not be empty
uint64_t pts = frame_meta->pts;
state->frame_meta_queue = frame_meta->next; // remove the item
frame_meta_delete(frame_meta);
return pts;
}
static int
read_packet_with_meta(void *opaque, uint8_t *buf, int buf_size) {
struct stream *stream = opaque;
struct receiver_state *state = &stream->receiver_state;
stream_recv_packet(struct stream *stream, AVPacket *packet) {
// The video stream contains raw packets, without time information. When we
// record, we retrieve the timestamps separately, from a "meta" header
// added by the server before each raw packet.
@@ -82,60 +36,30 @@ read_packet_with_meta(void *opaque, uint8_t *buf, int buf_size) {
//
// It is followed by <packet_size> bytes containing the packet/frame.
if (!state->remaining) {
#define HEADER_SIZE 12
uint8_t header[HEADER_SIZE];
ssize_t r = net_recv_all(stream->socket, header, HEADER_SIZE);
if (r == -1) {
return AVERROR(errno);
}
if (r == 0) {
return AVERROR_EOF;
}
// no partial read (net_recv_all())
SDL_assert_release(r == HEADER_SIZE);
uint64_t pts = buffer_read64be(header);
state->remaining = buffer_read32be(&header[8]);
if (pts != NO_PTS && !receiver_state_push_meta(state, pts)) {
LOGE("Could not store PTS for recording");
// we could not save the PTS, the recording would be broken
return AVERROR(ENOMEM);
}
uint8_t header[HEADER_SIZE];
ssize_t r = net_recv_all(stream->socket, header, HEADER_SIZE);
if (r < HEADER_SIZE) {
return false;
}
SDL_assert(state->remaining);
uint64_t pts = buffer_read64be(header);
uint32_t len = buffer_read32be(&header[8]);
SDL_assert(len);
if (buf_size > state->remaining) {
buf_size = state->remaining;
if (av_new_packet(packet, len)) {
LOGE("Could not allocate packet");
return false;
}
ssize_t r = net_recv(stream->socket, buf, buf_size);
if (r == -1) {
return errno ? AVERROR(errno) : AVERROR_EOF;
}
if (r == 0) {
return AVERROR_EOF;
r = net_recv_all(stream->socket, packet->data, len);
if (r < len) {
av_packet_unref(packet);
return false;
}
SDL_assert(state->remaining >= r);
state->remaining -= r;
packet->pts = pts != NO_PTS ? pts : AV_NOPTS_VALUE;
return r;
}
static int
read_raw_packet(void *opaque, uint8_t *buf, int buf_size) {
struct stream *stream = opaque;
ssize_t r = net_recv(stream->socket, buf, buf_size);
if (r == -1) {
return errno ? AVERROR(errno) : AVERROR_EOF;
}
if (r == 0) {
return AVERROR_EOF;
}
return r;
return true;
}
static void
@@ -145,55 +69,136 @@ notify_stopped(void) {
SDL_PushEvent(&stop_event);
}
static bool
process_config_packet(struct stream *stream, AVPacket *packet) {
if (stream->recorder && !recorder_write(stream->recorder, packet)) {
LOGE("Could not send config packet to recorder");
return false;
}
return true;
}
static bool
process_frame(struct stream *stream, AVPacket *packet) {
if (stream->decoder && !decoder_push(stream->decoder, packet)) {
return false;
}
if (stream->recorder) {
packet->dts = packet->pts;
if (!recorder_write(stream->recorder, packet)) {
LOGE("Could not write frame to output file");
return false;
}
}
return true;
}
static bool
stream_parse(struct stream *stream, AVPacket *packet) {
uint8_t *in_data = packet->data;
int in_len = packet->size;
uint8_t *out_data = NULL;
int out_len = 0;
int r = av_parser_parse2(stream->parser, stream->codec_ctx,
&out_data, &out_len, in_data, in_len,
AV_NOPTS_VALUE, AV_NOPTS_VALUE, -1);
// PARSER_FLAG_COMPLETE_FRAMES is set
SDL_assert(r == in_len);
SDL_assert(out_len == in_len);
if (stream->parser->key_frame == 1) {
packet->flags |= AV_PKT_FLAG_KEY;
}
bool ok = process_frame(stream, packet);
if (!ok) {
LOGE("Could not process frame");
return false;
}
return true;
}
static bool
stream_push_packet(struct stream *stream, AVPacket *packet) {
bool is_config = packet->pts == AV_NOPTS_VALUE;
// A config packet must not be decoded immetiately (it contains no
// frame); instead, it must be concatenated with the future data packet.
if (stream->has_pending || is_config) {
size_t offset;
if (stream->has_pending) {
offset = stream->pending.size;
if (av_grow_packet(&stream->pending, packet->size)) {
LOGE("Could not grow packet");
return false;
}
} else {
offset = 0;
if (av_new_packet(&stream->pending, packet->size)) {
LOGE("Could not create packet");
return false;
}
stream->has_pending = true;
}
memcpy(stream->pending.data + offset, packet->data, packet->size);
if (!is_config) {
// prepare the concat packet to send to the decoder
stream->pending.pts = packet->pts;
stream->pending.dts = packet->dts;
stream->pending.flags = packet->flags;
packet = &stream->pending;
}
}
if (is_config) {
// config packet
bool ok = process_config_packet(stream, packet);
if (!ok) {
return false;
}
} else {
// data packet
bool ok = stream_parse(stream, packet);
if (stream->has_pending) {
// the pending packet must be discarded (consumed or error)
stream->has_pending = false;
av_packet_unref(&stream->pending);
}
if (!ok) {
return false;
}
}
return true;
}
static int
run_stream(void *data) {
struct stream *stream = data;
AVFormatContext *format_ctx = avformat_alloc_context();
if (!format_ctx) {
LOGC("Could not allocate format context");
goto end;
}
unsigned char *buffer = av_malloc(BUFSIZE);
if (!buffer) {
LOGC("Could not allocate buffer");
goto finally_free_format_ctx;
}
// initialize the receiver state
stream->receiver_state.frame_meta_queue = NULL;
stream->receiver_state.remaining = 0;
// if recording is enabled, a "header" is sent between raw packets
int (*read_packet)(void *, uint8_t *, int) =
stream->recorder ? read_packet_with_meta : read_raw_packet;
AVIOContext *avio_ctx = avio_alloc_context(buffer, BUFSIZE, 0, stream,
read_packet, NULL, NULL);
if (!avio_ctx) {
LOGC("Could not allocate avio context");
// avformat_open_input takes ownership of 'buffer'
// so only free the buffer before avformat_open_input()
av_free(buffer);
goto finally_free_format_ctx;
}
format_ctx->pb = avio_ctx;
if (avformat_open_input(&format_ctx, NULL, NULL, NULL) < 0) {
LOGE("Could not open video stream");
goto finally_free_avio_ctx;
}
AVCodec *codec = avcodec_find_decoder(AV_CODEC_ID_H264);
if (!codec) {
LOGE("H.264 decoder not found");
goto end;
}
stream->codec_ctx = avcodec_alloc_context3(codec);
if (!stream->codec_ctx) {
LOGC("Could not allocate codec context");
goto end;
}
if (stream->decoder && !decoder_open(stream->decoder, codec)) {
LOGE("Could not open decoder");
goto finally_close_input;
goto finally_free_codec_ctx;
}
if (stream->recorder && !recorder_open(stream->recorder, codec)) {
@@ -201,50 +206,40 @@ run_stream(void *data) {
goto finally_close_decoder;
}
AVPacket packet;
av_init_packet(&packet);
packet.data = NULL;
packet.size = 0;
stream->parser = av_parser_init(AV_CODEC_ID_H264);
if (!stream->parser) {
LOGE("Could not initialize parser");
goto finally_close_recorder;
}
while (!av_read_frame(format_ctx, &packet)) {
if (SDL_AtomicGet(&stream->stopped)) {
// if the stream is stopped, the socket had been shutdown, so the
// last packet is probably corrupted (but not detected as such by
// FFmpeg) and will not be decoded correctly
av_packet_unref(&packet);
goto quit;
}
if (stream->decoder && !decoder_push(stream->decoder, &packet)) {
av_packet_unref(&packet);
goto quit;
}
if (stream->recorder) {
// we retrieve the PTS in order they were received, so they will
// be assigned to the correct frame
uint64_t pts = receiver_state_take_meta(&stream->receiver_state);
packet.pts = pts;
packet.dts = pts;
// no need to rescale with av_packet_rescale_ts(), the timestamps
// are in microseconds both in input and output
if (!recorder_write(stream->recorder, &packet)) {
LOGE("Could not write frame to output file");
av_packet_unref(&packet);
goto quit;
}
// We must only pass complete frames to av_parser_parse2()!
// It's more complicated, but this allows to reduce the latency by 1 frame!
stream->parser->flags |= PARSER_FLAG_COMPLETE_FRAMES;
for (;;) {
AVPacket packet;
bool ok = stream_recv_packet(stream, &packet);
if (!ok) {
// end of stream
break;
}
ok = stream_push_packet(stream, &packet);
av_packet_unref(&packet);
if (avio_ctx->eof_reached) {
if (!ok) {
// cannot process packet (error already logged)
break;
}
}
LOGD("End of frames");
quit:
if (stream->has_pending) {
av_packet_unref(&stream->pending);
}
av_parser_close(stream->parser);
finally_close_recorder:
if (stream->recorder) {
recorder_close(stream->recorder);
}
@@ -252,13 +247,8 @@ finally_close_decoder:
if (stream->decoder) {
decoder_close(stream->decoder);
}
finally_close_input:
avformat_close_input(&format_ctx);
finally_free_avio_ctx:
av_free(avio_ctx->buffer);
av_free(avio_ctx);
finally_free_format_ctx:
avformat_free_context(format_ctx);
finally_free_codec_ctx:
avcodec_free_context(&stream->codec_ctx);
end:
notify_stopped();
return 0;
@@ -270,7 +260,7 @@ stream_init(struct stream *stream, socket_t socket,
stream->socket = socket;
stream->decoder = decoder,
stream->recorder = recorder;
SDL_AtomicSet(&stream->stopped, 0);
stream->has_pending = false;
}
bool
@@ -287,7 +277,6 @@ stream_start(struct stream *stream) {
void
stream_stop(struct stream *stream) {
SDL_AtomicSet(&stream->stopped, 1);
if (stream->decoder) {
decoder_interrupt(stream->decoder);
}