Hi Emeric,
On 15.03.2012 15:13, Emeric Grange wrote:
I actually made some code that does exactly the opposite, for H.264
and VP8, removing the start code from the first buffer containing it
(so the first buffer alltoghether or just the 3 first bytes of the
first buffer). I think the start codes, at least for H..264 are just
here so the decoding application (in this case the state tracker) can
make sure the buffers are valid video buffers, and are not used in the
decoding process.
At least for H264 and AMD hardware that isn't such a good idea. The
startcodes are the indicators for the hardware decoder where slices
actually starts and ends (hence the name), so if the decoder doesn't
sees a startcode at the beginning of the buffer it will just skip
forward until it finds something that looks valid. And if I understood
Maarten Lankhorst correctly the nvidia hardware is doing this in a very
similar way.
In my VP8 implementation I require a 0x9D012A start code, similar to
H.264 that requires a 0x000001. I asked on the VDPAU mailing list if
it was a good idea, and what was the purpose of these start code in
the first place but I did not get an answer.
The idea behind startcodes is to have something that doesn't normally
occur in the encoded bitstream, so you can do network/transport
synchronization with it. In most of the modern codecs (MPEG2, MPEG4,
H264, VC-1) there seems to be some kind of consensus to use the bytes
0x00 0x00 0x01 + some code for that, but it could actually be any unique
combination of bytes.
So when a network error occur (for example cause it is a satellite, DVB,
whatever wireless broadcast transmission) the decoder can always
fallback to search for the next start code and so resynchronize the stream.
However I have no idea if a VC1 decoder requires the presence of start
codes, and your implementation is fine by me, I can still hook a
vlVdpDecoderRemoveVP8Startcode() function similar to
vlVdpDecoderFixVC1Startcode().
Well, the problem with VC-1 is that certain container formats (IIRC WMV)
doesn't use start codes, so they are not suited for streaming/broadcast
transmission, but on the other hand safes 4 bytes of space for each
frame (yeah another great Microsoft invention). So since the hardware
decoders seem to need the start codes we must add them manually.
Honestly I have no idea what to do in the VP8 case, so do whatever you
think is right there.
Christian.
_______________________________________________
mesa-dev mailing list
mesa-dev@lists.freedesktop.org
http://lists.freedesktop.org/mailman/listinfo/mesa-dev
