[vlc-devel] Problems with using FFMPEG's "h264.c" to decode H264/RTP streams
finlayson at live.com
Tue Jul 19 04:24:29 CEST 2005
Michael (cc. "vlc-devel"),
I have recently been trying to update VLC (and the appropriate LIVE.COM
RTSP/RTP client code) to receive and render incoming H.264 RTP streams,
using your FFMPEG H.264 decoder ("libavcodec/h264.c"). Unfortunately I
have run into a snag that - to be solved cleanly - may require a minor
change to your code.
The problem is that your function "decode_nal_units()" does not like to be
passed a buffer that contains just a single NAL unit; instead, it seems to
want a buffer that contains all NAL units that make up an 'access unit'
(i.e., frame). If "decode_nal_units()" is fed just a single NAL unit, then
- at the time that "ff_er_frame_end()" is called, "error_count" has not yet
been decremented to zero, and so we get errors about 'concealing' data.
However, the H.264/RTP client code (in the LIVE.COM libraries and VLC)
wants to feed the decoder one NAL unit at a time. There are two reasons
for this. First, it improves resiliency to packet loss. (If a RTP packet
is lost, then this will cause the loss only of one NAL - not of the entire
'access unit' (frame).) Second, your code (quite reasonably) expects a
"nal_size" field to precede each NAL unit in the buffer. The cleanest
place to prepend this size field is in VLC's RTP interface code
("modules/demux/livedotcom.cpp"), but unfortunately this can be done only
if this code is delivered one NAL unit at a time - not an entire 'access
unit' at a time.
Given this, would it be possible for you to update your code so that
"decode_nal_units()" can properly handle a buffer that contains just a
single NAL unit?
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