[FFmpeg-devel] [PATCH v3] avformat/mpegts: reduce buffering during initialization

[Andriy Gelman]

From: Andriy Gelman <andriy.gelman at gmail.com>

Reduces buffering latency with low bitrate streams, where
8192 bytes can mean several seconds.
---
 libavformat/mpegts.c | 60 +++++++++++++++++++++++++++-----------------
 1 file changed, 37 insertions(+), 23 deletions(-)

diff --git a/libavformat/mpegts.c b/libavformat/mpegts.c
index b04fd7b4f4..7d03500043 100644
--- a/libavformat/mpegts.c
+++ b/libavformat/mpegts.c
@@ -53,6 +53,10 @@
         (prev_dividend) = (dividend);                                          \
     } while (0)
 
+#define PROBE_PACKET_MAX_BUF 8192
+#define PROBE_PACKET_STEP 512
+#define PROBE_PACKET_MARGIN 5
+
 enum MpegTSFilterType {
     MPEGTS_PES,
     MPEGTS_SECTION,
@@ -591,28 +595,42 @@ static int analyze(const uint8_t *buf, int size, int packet_size,
     return best_score - FFMAX(stat_all - 10*best_score, 0)/10;
 }
 
-/* autodetect fec presence. Must have at least 1024 bytes  */
-static int get_packet_size(const uint8_t *buf, int size)
+/* autodetect fec presence */
+static int get_packet_size(AVIOContext* pb)
 {
     int score, fec_score, dvhs_score;
+    int margin;
+    int ret;
 
-    if (size < (TS_FEC_PACKET_SIZE * 5 + 1))
-        return AVERROR_INVALIDDATA;
+    /*init buffer to store stream for probing */
+    uint8_t buf[PROBE_PACKET_MAX_BUF] = {0};
+    int buf_size = 0;
 
-    score      = analyze(buf, size, TS_PACKET_SIZE,      0);
-    dvhs_score = analyze(buf, size, TS_DVHS_PACKET_SIZE, 0);
-    fec_score  = analyze(buf, size, TS_FEC_PACKET_SIZE,  0);
-    av_log(NULL, AV_LOG_TRACE, "score: %d, dvhs_score: %d, fec_score: %d \n",
-            score, dvhs_score, fec_score);
-
-    if (score > fec_score && score > dvhs_score)
-        return TS_PACKET_SIZE;
-    else if (dvhs_score > score && dvhs_score > fec_score)
-        return TS_DVHS_PACKET_SIZE;
-    else if (score < fec_score && dvhs_score < fec_score)
-        return TS_FEC_PACKET_SIZE;
-    else
-        return AVERROR_INVALIDDATA;
+    while (buf_size + PROBE_PACKET_STEP <= PROBE_PACKET_MAX_BUF) {
+        ret = avio_read(pb, buf + buf_size, PROBE_PACKET_STEP);
+        if (ret != PROBE_PACKET_STEP)
+            return AVERROR_INVALIDDATA;
+        buf_size += ret;
+
+        score      = analyze(buf, buf_size, TS_PACKET_SIZE,      0);
+        dvhs_score = analyze(buf, buf_size, TS_DVHS_PACKET_SIZE, 0);
+        fec_score  = analyze(buf, buf_size, TS_FEC_PACKET_SIZE,  0);
+        av_log(NULL, AV_LOG_TRACE, "Probe: %d, score: %d, dvhs_score: %d, fec_score: %d \n",
+            buf_size, score, dvhs_score, fec_score);
+
+        if (buf_size < PROBE_PACKET_MAX_BUF)
+            margin = PROBE_PACKET_MARGIN; /*if buffer not filled */
+        else
+            margin = 0;
+
+        if (score > FFMAX(fec_score, dvhs_score) + margin)
+            return TS_PACKET_SIZE;
+        else if (dvhs_score > FFMAX(score, fec_score) + margin)
+            return TS_DVHS_PACKET_SIZE;
+        else if (fec_score > FFMAX(score, dvhs_score) + margin)
+            return TS_FEC_PACKET_SIZE;
+    }
+    return AVERROR_INVALIDDATA;
 }
 
 typedef struct SectionHeader {
@@ -2841,8 +2859,6 @@ static int mpegts_read_header(AVFormatContext *s)
 {
     MpegTSContext *ts = s->priv_data;
     AVIOContext *pb   = s->pb;
-    uint8_t buf[8 * 1024] = {0};
-    int len;
     int64_t pos, probesize = s->probesize;
 
     s->internal->prefer_codec_framerate = 1;
@@ -2850,10 +2866,8 @@ static int mpegts_read_header(AVFormatContext *s)
     if (ffio_ensure_seekback(pb, probesize) < 0)
         av_log(s, AV_LOG_WARNING, "Failed to allocate buffers for seekback\n");
 
-    /* read the first 8192 bytes to get packet size */
     pos = avio_tell(pb);
-    len = avio_read(pb, buf, sizeof(buf));
-    ts->raw_packet_size = get_packet_size(buf, len);
+    ts->raw_packet_size = get_packet_size(pb);
     if (ts->raw_packet_size <= 0) {
         av_log(s, AV_LOG_WARNING, "Could not detect TS packet size, defaulting to non-FEC/DVHS\n");
         ts->raw_packet_size = TS_PACKET_SIZE;
-- 
2.20.1