On 6/7/24 1:29 PM, Hendrik Leppkes wrote:
On Fri, Jun 7, 2024 at 9:47 AM Hendrik Leppkes <h.lepp...@gmail.com> wrote:
On Mon, May 27, 2024 at 3:47 PM llyyr <llyyr.pub...@gmail.com> wrote:
instead of just resyncing and skipping a bunch of TS packets, leading to
a loss of frames.
Before this, a stray byte with the value of 0x47 in TP_extra_header
would throw off the detection of where TS packets start.
A typical file that could cause issues would look like this:
00000300: 238f 4780 4750 1110 0000 01e0 0000 84c0
^^ ^^
The first four bytes here are TP_extra_header and the actual TS packet
starts at offset 0x304
FFmpeg would try to read a packet at 0x300 but since nothing skips the
4 byte TP_extra_header, find that the first byte is not 0x47 and
immediately go into mpegts_resync, and incorrectly detect the stray 0x47
in the TP_extra_header at 0x302 as the new sync byte.
Fix this by correctly skipping the first 4 bytes if the source packet
is 192 bytes.
Signed-off-by: llyyr <llyyr.pub...@gmail.com>
---
libavformat/mpegts.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/libavformat/mpegts.c b/libavformat/mpegts.c
index c66a1ea6ede0..0d80ad80f1aa 100644
--- a/libavformat/mpegts.c
+++ b/libavformat/mpegts.c
@@ -2944,6 +2944,12 @@ static int read_packet(AVFormatContext *s, uint8_t *buf,
int raw_packet_size,
AVIOContext *pb = s->pb;
int len;
+ // 192 bytes source packet that start with a 4 bytes TP_extra_header
+ // followed by 188 bytes of TS packet. The sync byte is at offset 4, so
skip
+ // the first 4 bytes otherwise we'll end up syncing to the wrong packet.
+ if (raw_packet_size == TS_DVHS_PACKET_SIZE)
+ avio_skip(pb, 4);
+
I think this doesn't work with mpegts_resync, since it always resyncs
for the packet start to be on the 0x47 marker, not 4 bytes before it.
So if sync is lost, it would never recover, if I read this right.
Since we're dealing with a special case for 192 byte packets here
anyway, maybe a special case in mpegts_resync that just checks if raw
size = 192 && [4] = 0x47 would handle this with less potential
fallout?
I've looked at this again and I don't see a way to gracefully handle it
in mpegts_resync. It's much cleaner to just skip the header that's not
relevant to us so we line up with the first packet being the sync byte.
FWIW this can be reproduced with
`ffmpeg -i <file> -c copy out.h264`
Sample file: https://0x0.st/XVJi.m2ts
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