[FFmpeg-devel] [PATCH] Use v4l2 input format automatically if filename starts with "/dev/video"

[Reimar Döffinger]

On Sat, Oct 25, 2014 at 01:40:11PM +0200, wm4 wrote:
> On Sat, 25 Oct 2014 13:14:26 +0200
> Reimar Döffinger <Reimar.Doeffinger at gmx.de> wrote:
> > wm4, if you have a simple way of doing this check, please explain it.
> > Otherwise I think EXTENSION score is good enough to signal "well, we
> > guess that's what it is".
> 
> There are two solutions:
> - actually open the video device, and try a v4l-only ioctl() to test
>   whether it's really a device (the artificial separation between
>   avio/probing/actual opening becomes rather annoying here)

That would end up trying a v4l ioctl on each and every file that
FFmpeg tries to open (unless you mean only after the /dev/video check,
but that would not solve the issue you mention later).
That feels a bit heavy-weight.
I guess it's not unreasonable if done in addition, but on the other
hand, is this likely to ever increase user experience in reality?

> - add a specific protocol prefix that forces the input format, and which
>   lets the user invoke video capture (think MPlayer's "tv://")
> 
> All in all, there's much you can do about raw devices. A video device
> probably doesn't even need to be named /dev/video..., nor does that
> filename need to be a reliable indicator that the device understand
> V4L. Whatever you do, it can't be perfect, because you're working on a
> too low level and system specific layer. "tv://" could solve that
> actually rather elegantly, including abstracting platform differences
> from the user.

I admittedly just assumed that v4l2:///dev/video/... would work
currently. If not, that sounds like something that can and should
be fixed.
However as a convenience feature, I think it is good that /dev/video
just ends up picking the v4l2 input, because in almost all cases
that would be the desired behaviour.
It should be possible to override the other way via file:///dev/video/... I believe?
If I am wrong about any of that, that would probably change my opinion.

And I probably should leave this discussion to someone with a clue about
this part of the code.

Regards,
Reimar