How to set pixelaspect in struct v4l2_cropcap returned by VIDIOC_CROPCAP?

How to set pixelaspect in struct v4l2_cropcap returned by VIDIOC_CROPCAP?

[Georgi Chorbadzhiyski]

Guys I'm adding v4l2 output device support for VLC/ffmpeg/libav (I'm using
v4l2loopback [1] driver for testing) but I have a problem which I can't seem
to find a solution.

VLC [2] uses VIDIOC_CROPCAP [3] to detect the pixelaspect ratio of the input
it receives from v4l2 device. But I can't seem to find a way to set struct
v4l2_cropcap.pixelaspect when I'm outputting data to the device and the
result is that VLC assumes pixelaspect is 1:1.

I was hoping that VIDIOC_S_CROP [4] would allow setting pixelaspect but
according to docs that is not case. What am I missing?

How to set pixelaspect values returned by VIDIOC_CROPCAP?

[1]: https://github.com/umlaeute/v4l2loopback
[2]: http://git.videolan.org/?p=vlc.git;a=blob;f=modules/access/v4l2/demux.c;hb=HEAD#l248
[3]: http://www.linuxtv.org/downloads/v4l-dvb-apis/vidioc-cropcap.html
[4]: http://www.kernel.org/doc/htmldocs/media/vidioc-g-crop.html

-- 
Georgi Chorbadzhiyski
http://georgi.unixsol.org/

Re: How to set pixelaspect in struct v4l2_cropcap returned by VIDIOC_CROPCAP?

[Hans Verkuil]

On Sun September 16 2012 17:32:52 Georgi Chorbadzhiyski wrote:
> Guys I'm adding v4l2 output device support for VLC/ffmpeg/libav (I'm using
> v4l2loopback [1] driver for testing) but I have a problem which I can't seem
> to find a solution.
> 
> VLC [2] uses VIDIOC_CROPCAP [3] to detect the pixelaspect ratio of the input
> it receives from v4l2 device. But I can't seem to find a way to set struct
> v4l2_cropcap.pixelaspect when I'm outputting data to the device and the
> result is that VLC assumes pixelaspect is 1:1.
> 
> I was hoping that VIDIOC_S_CROP [4] would allow setting pixelaspect but
> according to docs that is not case. What am I missing?

The pixelaspect ratio returned by CROPCAP depends on the current video standard
of the video receiver or transmitter.

So for video capture the pixelaspect depends on the standard (50 vs 60 Hz) and
the horizontal sampling frequency of the video receiver (hardware specific).

For video output the pixelaspect depends also on the standard and on how the
transmitter goes from digital to analog pixels (the reverse of what a receiver
does).

It is *not* the pixelaspect of the video data itself. For output it is the
pixel aspect that the transmitter expects. Any difference between the two will
need to be resolved somehow, typically by software or hardware scaling.

Regards,

	Hans

> 
> How to set pixelaspect values returned by VIDIOC_CROPCAP?
> 
> [1]: https://github.com/umlaeute/v4l2loopback
> [2]: http://git.videolan.org/?p=vlc.git;a=blob;f=modules/access/v4l2/demux.c;hb=HEAD#l248
> [3]: http://www.linuxtv.org/downloads/v4l-dvb-apis/vidioc-cropcap.html
> [4]: http://www.kernel.org/doc/htmldocs/media/vidioc-g-crop.html
> 
> 

Re: How to set pixelaspect in struct v4l2_cropcap returned by VIDIOC_CROPCAP?

[Georgi Chorbadzhiyski]

On 9/16/12 7:28 PM, Hans Verkuil wrote:
> On Sun September 16 2012 17:32:52 Georgi Chorbadzhiyski wrote:
>> Guys I'm adding v4l2 output device support for VLC/ffmpeg/libav (I'm using
>> v4l2loopback [1] driver for testing) but I have a problem which I can't seem
>> to find a solution.
>>
>> VLC [2] uses VIDIOC_CROPCAP [3] to detect the pixelaspect ratio of the input
>> it receives from v4l2 device. But I can't seem to find a way to set struct
>> v4l2_cropcap.pixelaspect when I'm outputting data to the device and the
>> result is that VLC assumes pixelaspect is 1:1.
>>
>> I was hoping that VIDIOC_S_CROP [4] would allow setting pixelaspect but
>> according to docs that is not case. What am I missing?
>
> The pixelaspect ratio returned by CROPCAP depends on the current video standard
> of the video receiver or transmitter.
>
> So for video capture the pixelaspect depends on the standard (50 vs 60 Hz) and
> the horizontal sampling frequency of the video receiver (hardware specific).
>
> For video output the pixelaspect depends also on the standard and on how the
> transmitter goes from digital to analog pixels (the reverse of what a receiver
> does).
>
> It is *not* the pixelaspect of the video data itself. For output it is the
> pixel aspect that the transmitter expects. Any difference between the two will
> need to be resolved somehow, typically by software or hardware scaling.

Since I'm using virtual output v4l2 loopback device this means I have to set the
standard somehow, right?

-- 
Georgi Chorbadzhiyski
http://georgi.unixsol.org/

Re: How to set pixelaspect in struct v4l2_cropcap returned by VIDIOC_CROPCAP?

[Georgi Chorbadzhiyski]

Around 09/17/2012 11:22 AM, Hans Verkuil scribbled:
> On Sun September 16 2012 18:49:00 Georgi Chorbadzhiyski wrote:
>> On 9/16/12 7:28 PM, Hans Verkuil wrote:
>>> On Sun September 16 2012 17:32:52 Georgi Chorbadzhiyski wrote:
>>>> Guys I'm adding v4l2 output device support for VLC/ffmpeg/libav (I'm using
>>>> v4l2loopback [1] driver for testing) but I have a problem which I can't seem
>>>> to find a solution.
>>>>
>>>> VLC [2] uses VIDIOC_CROPCAP [3] to detect the pixelaspect ratio of the input
>>>> it receives from v4l2 device. But I can't seem to find a way to set struct
>>>> v4l2_cropcap.pixelaspect when I'm outputting data to the device and the
>>>> result is that VLC assumes pixelaspect is 1:1.
>>>>
>>>> I was hoping that VIDIOC_S_CROP [4] would allow setting pixelaspect but
>>>> according to docs that is not case. What am I missing?
>>>
>>> The pixelaspect ratio returned by CROPCAP depends on the current video standard
>>> of the video receiver or transmitter.
>>>
>>> So for video capture the pixelaspect depends on the standard (50 vs 60 Hz) and
>>> the horizontal sampling frequency of the video receiver (hardware specific).
>>>
>>> For video output the pixelaspect depends also on the standard and on how the
>>> transmitter goes from digital to analog pixels (the reverse of what a receiver
>>> does).
>>>
>>> It is *not* the pixelaspect of the video data itself. For output it is the
>>> pixel aspect that the transmitter expects. Any difference between the two will
>>> need to be resolved somehow, typically by software or hardware scaling.
>>
>> Since I'm using virtual output v4l2 loopback device this means I have to set the
>> standard somehow, right?
> 
> Yes, just call VIDIOC_S_STD. But the loopback device driver needs to be modified to
> have cropcap return the aspect ratio belonging to the given standard (or just 1x1
> for non-PAL/NTSC resolutions).
> 
> I wish personally that this driver was being upstreamed to the kernel. I know that
> Mauro (subsystem maintainer) isn't too keen on it, but I think we can convince him
> that it is really a useful driver to have. And if it is part of distros anyway,
> then we should just accept it (after cleanup, of course).

The drivers is very useful I can vote for that with two hands. Along with snd-aloop
I'm able to create a stable video/audio source which was not possible before.

I'll try to add support for S_FMT to v4l2loopback, lets see where this would lead me
(for now I just hacked VLC to set the aspect from the command line instead of relying
on CROPCAP).

I don't know if the author(s) have any plans of upstreaming it (I've added them to cc,
guys any opinion on that?).

-- 
Georgi Chorbadzhiyski
http://georgi.unixsol.org/