Firstly, I'm loving the live555 libraries, if you're reading Ross - a big thank
you for making this open source! I've learnt a lot about the technologies from
running up and debugging through the source code :o)
I've been using the RTSP client of live555 to receive a 'full-hd' stream of
H264 encoded video over RTSP, sent by VLC (2.0.5). I have been using the NAL
units sent from live555 (via H264VideoRTPSource) and passing them directly (no
packetizer or grouper e.g, just prefixing the start code) to a Media Foundation
H264 decoder. This worked perfectly, so then I changed decoder to avcodec
(ffmpeg libraries) and the same video
source has corrupted decoder output (sort of grey smudges with bits of the
actual frames in).
I've been reading through the live555 code and searching through posts on
forums to try and see if this is
a issue with live555 or the av decoder. FYI, the same RTSP stream works in
VLC as client, but *only* with the use of a H264 packetizer in between the RTSP
and decoder modules. What I see if I WireShark the RTP packets is what looks
like several NAL units per timestamp, and when I debug VLC I notice these are
"collected" by the packetizer before being presented to avcodec, but my code
just passes these one by one to avcodec.
Then I found this post, which seemed to indicate the person asking the question
was seeing H264 over RTP with a similar nature:
http://stackoverflow.com/questions/6756770/libavcodec-how-to-tell-end-of-access-unit-when-decoding-h-264-stream
So as an experiment I have put some extra code in my RTPSink which collects
these NAL units together based on the RTP timestamp and then I pass this
collection to the decoder. This appears to work perfectly with both avcodec
and the media foundation h264
decoder.
I've sent a lot of other H264 streams fine from VLC, but only the full-hd ones
seem to need this "collection" method in my live555-based client. These other
streams play just fine using avcodec just passing the NAL units directly to
the decoder.
I'm not asking VLC to re-encode the footage just read the .mov file (H264/AAC)
and serve it up as RTSP - which makes me wonder if the source file was encoded
in a non-standard way, somehow fragmenting the NAL units or maybe splitting
them down because the units are so large being for full hd?
I'm happy enough leaving the collection method in my RTSP client, maybe perhaps
just for H264, but I was wondering if this is masking a problem with something
else, or maybe collecting by timestamp isn't a good idea for some reason?
Thanks in advance for any suggestions,
Mark.
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