Re: multi-rates vic

["Lawrence A. Rowe" <Rowe@bmrc.berkeley.edu>]

Aurelien Amacker wrote:
> 
> Hi,
> 
> does anyone knows if it's possible to hack OM Vic or AG Vic in a way
> that it can send the same video streams at two different bit rates (one
> low and one high) to two multicast groups?
> 
> I've modified AG Vic in order to do that but the problem is that there
> is two open() on the same video driver, and the X11 driver is the only
> one to allow that.
> 
> Thanks for any idea
> 
> Aurélien
---

yes, it will take some effort but you should be able to do it.  you will
need to modify the VideoAgent and the VideoPipe classes written in otcl
and the video-device*.cc abstractions written in c++ to support a
command to split a captured stream and call more than one encoder
class.  if you look in a specific video grabber (e.g., video-v4l.cc:834)
you will see that the grab method copies the image to a YuvFrame and
then calls the encoder using code like:
	target_->recv (&f);
where target_ points at the encoder object and recv is the method called
when an object wants to pass along data to another object.  in the
current code, the encoder is setup by the VideoPipe class when vic does
a transmit operation.

what you need to do is modify the target_ slot to be a list of encoders
to which the captured image should be passed.  and, you will likely need
to replicate the YuvFrame being passed.  just clone the existing
instance.

you will need to setup some protocol/code to allocate a second encoder
and link it into the VideoCapture device abstraction.  I suspect this
should be done in VideoAgent but I know it will take changes to
VideoPipeline as that is where the encoder object is created.  you can
configure that encoder with whatever parameters you want (e.g.,
different format, quality, bitrate, etc.).  if you want a different
frame size or a different frame rate, you will need to implement that in
the grab routine.

lastly, you will need to setup a different network session to which the
encoder should send the encoded packets.  i am pretty sure the network
code can support multiple open multicast sessions (right andrew?), but i
don't know what the abstractions are for openning sessions and binding
them to the encoder.  although, the binding is done using the same
mechanism as the binding between capture objects and encoders.  if you
look at the h261 encoder (i.e., encoder-h261.cc:675) you will see code
like:
    target = target_list_->first_;
    while (target != NULL) {
        pb->attach();
        target->recv(pb);
        target = target->next();
    }
    pb->release();
this sends the packet (pb) to several sessions -- see the call on
target->recv which is similar to the call above.

so, this is do-able, but it will take some work.  and, you should try to
generalize it so it is possible to support this function with many
different capture devices.
	Larry
-- 
Professor Lawrence A. Rowe          Internet:  Rowe@BMRC.Berkeley.EDU
Computer Science Division - EECS       Phone: 510-642-5117
University of California, Berkeley       Fax: 510-642-5615
Berkeley, CA 94720-1776            URL: http://bmrc.berkeley.edu/~larry