[Libav-user] (no subject)

[Luke Clemens]

The magic is in avcodec_encode_video() and avcodec_decode_video2(). The
documentation for it is in the header file where it's defined. There are
some examples online. You setup a codec context, then pass an uncompressed
image to avcodec_encode_video(), and it will spit out a compressed buffer.

There are different options for sending over the network.

1) You can use the current network method you use now, and just send those
compressed packets instead of uncompressed frames. The only catch is that
you may have to send some setup information to your client as well so it
knows how to setup the decoder. If you always use the same format, then you
won't have to worry about that. It's slightly more complicated if you use P
frames. for 16ms or less, you won't want to use B frames. The simplest is
mjpeg, but it will use 3x more bandwidth than mpeg4 or H.264.

2) The other option is to use libavformat to send over the network. You
setup an ouput context and then write those frames to it with
av_write_frame(), and it will send the packets via RTP or another format
that you specify.

At the client end you also have two choices, which depend on the option you
chose for the server.

If you chose option 1, then you'll need to setup a decode context, receive
the packets you sent, and feed them to avcodec_decode_video2(). You might
need extra information about the codec so you might have to receive that
also and use it to determine how to setup the decode context.

If you chose option 2, then you'll setup an input format context and
repeatedly call av_read_frame() and feed the resulting frames
into avcodec_decode_video2().

There are a few examples and tutorials on the web if you google around.
Especially for the compression/decompression parts. Unfortunately, those few
tutorials are all that's out there. One of the popular ones for decoding is
here: http://dranger.com/ffmpeg/. There is one for encoding in
libavcodec/api-example.c . And of course the source code for ffplay,
mplayer, vlc, and other opensource players is available, but if you're a C++
guy like me, you'll probably find it pretty ugly.

Also, make sure you have enough space in your buffers for padding at the end
- I didn't realize it and it took a long time to figure out why I was
getting errors. The details are in the documentation for the encode/decode
functions.

--luke

On Thu, Aug 25, 2011 at 1:49 PM, <david.weber at l-3com.com> wrote:

>  Hey all, I have searched quite a bit, but haven't found a good answer.
> So I thought I'd post it here.****
>
> ** **
>
> I have a custom video device which gives me a simple framebuffer, (24-bit
> BGR).****
>
> ** **
>
> I am trying to stream it elsewhere, but would like to encode/compress it in
> realtime.****
>
> ** **
>
> On the client side, I would decode it, and display it in an OpenGL window.
> ****
>
> ** **
>
> I have it working today, with uncompressed images, but it is killing the
> network (as one would expect).****
>
> ** **
>
> ** **
>
> ** **
>
> Are there any examples for making something like this work?****
>
> ** **
>
> In short:****
>
> ** **
>
> [Custom Video Framebuffer] -> [encode] -> [network] -> [decode] ->
> [display]****
>
> ** **
>
> ** **
>
> Considerations that I have to deal with:****
>
> ** **
>
> **1.)    **Relatively realtime.  I have <16ms to encode a 640x480x3 frame
> buffer, and ship it off.****
>
> **2.)    **I need a streaming-type on the wire format.  The clients may be
> turned on/off at any point, so i need to support this.****
>
> ** **
>
> ** **
>
> The naive implementation, is simply running the video frames though zlib,
> which is shrinking the data nicely, but it's rather expensive, and makes me
> feel dirty.  All of the examples I can find, are reading off an existing
> encoded file, but not quite what I need.****
>
> ** **
>
> ** **
>
> Thanks for your input****
>
> ** **
>
> --dw****
>
> _______________________________________________
> Libav-user mailing list
> Libav-user at ffmpeg.org
> http://ffmpeg.org/mailman/listinfo/libav-user
>
>


-- 
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Luke Clemens
http://clemens.bytehammer.com
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