Re: video4linux loopback device

[vasaka@xxxxxxxxx]

On Fri, Jul 24, 2009 at 6:48 PM, Mauro Carvalho
Chehab<mchehab@xxxxxxxxxxxxx> wrote:
> Em Fri, 24 Jul 2009 17:07:29 +0200
> Folkert van Heusden <folkert@xxxxxxxxxxxxxx> escreveu:
>
>> > >> > Any chance that the video4linux loopback device will be integrated in
>> > >> > the main video4linux distribution and included in the kernel?
>> > >> > http://www.lavrsen.dk/foswiki/bin/view/Motion/VideoFourLinuxLoopbackDevice
>> > >> > or
>> > >> > http://sourceforge.net/projects/v4l2vd/
>> > >> > or
>> > >> > http://code.google.com/p/v4l2loopback/
>> > >> > This enables userspace postprocessors to feed the altered stream to
>> > >> > applications like amsn and skype for videochats.
>> > >> If the postprocessor application is LGPL, the better is to add it at libv4l.
>> > >
>> > > I don't agree as the postprocessor implements fun-effects: warhol,
>> > > puzzle, circles, wobble, etc.
>> >
>> > and how one can make skype to get pictue from libv4l or alter the
>> > picture which application is getting through libv4l?
>> > again some proxy is needed here.
>>
>> Correct so we definately need some external video loopback device.
>> It gives as a much greater flexibility.
>
> libv4l intercepts V4L2 calls by using LD_PRELOAD. Without it, skype doesn't
> work with most of the cams, since it doesn't support several of the output
> formats common on most cameras. I have lots of different webcam models here. I
> think only one or two are directly supported by skype.
>
> libv4l already have some postprocessor effects (currently limited to image
> enhancements like auto-gain/auto-bright and similar stuff), but I don't see why
> not adding there other effects including the fun ones.
>
> Hans has several plans of improvements for it, including more post-processing
> effects and the ability of duplicating a video to more than one application at
> the same time.
>
> The big problem with a v4l2 loopback is that you're adding something that it is
> not proper to kernel, since it were not designed to be a proxy between two
> userspace applications. This is the space where libraries were designed. Due to
> that, you can affect the machine performance, due to the usage of kernel memory
> barriers.
>
> Also, you'll likely add extra buffering costs of copying control data and
> stream mapping in order to access it  from/to kernel where you could just pass
> a buffer between the effect processors and do a simple function call.
>
>
>
> Cheers,
> Mauro
>

Sounds interesting. will libv4l have ability to provide video from
sources other than v4l2 devices?
One of the things I want to achieve is to provide talking head to
video chatting programs instead of usual video stream.

vasaka

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