v4l2 api: supported resolution negotiation

v4l2 api: supported resolution negotiation

[Matwey V. Kornilov]

Hello,

I learned from V2L2 API how to detect all supported formats using
VIDIOC_ENUM_FMT.
When I perform VIDIOC_S_FMT I don't know how to fill fmt.pix.width and
fmt.pix.height, since I know only format.
How should I negotiate device resolution? Could you point me?

Re: v4l2 api: supported resolution negotiation

[Sakari Ailus]

On Sun, Oct 04, 2015 at 12:23:08PM +0300, Matwey V. Kornilov wrote:
> Hello,
> 
> I learned from V2L2 API how to detect all supported formats using
> VIDIOC_ENUM_FMT.
> When I perform VIDIOC_S_FMT I don't know how to fill fmt.pix.width and
> fmt.pix.height, since I know only format.
> How should I negotiate device resolution? Could you point me?

VIDIOC_ENUM_FRAMESIZES may give you hints, but it's optional. You can use
values you prefer to try if drivers support them; I think the GStreamer
v4lsrc tries very small and very large values. The driver will clamp them to
a supported values which are passed to the application from the IOCTL.

<URL:http://hverkuil.home.xs4all.nl/spec/media.html#vidioc-enum-framesizes>

-- 
Regards,

Sakari Ailus
e-mail: sakari.ailus@iki.fi	XMPP: sailus@retiisi.org.uk

Re: v4l2 api: supported resolution negotiation

[Nicolas Dufresne]

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Le dimanche 04 octobre 2015 à 21:49 +0300, Sakari Ailus a écrit :
> I think the GStreamer
> v4lsrc tries very small and very large values. The driver will clamp
> them to
> a supported values which are passed to the application from the
> IOCTL.

In GStreamer we try ENUM_FRAMESIZE, and when no supported, we fallback
to try_fmt() with 1x1 (driver will raise it to the minimum) and then
MAX,MAX (driver will lower it to the maximum). We assume that the
driver supports a range, as iterating over all possibilities takes too
much time.

Nicolas

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Re: v4l2 api: supported resolution negotiation

[Hans Verkuil]

On 10/04/2015 08:49 PM, Sakari Ailus wrote:
> On Sun, Oct 04, 2015 at 12:23:08PM +0300, Matwey V. Kornilov wrote:
>> Hello,
>>
>> I learned from V2L2 API how to detect all supported formats using
>> VIDIOC_ENUM_FMT.
>> When I perform VIDIOC_S_FMT I don't know how to fill fmt.pix.width and
>> fmt.pix.height, since I know only format.
>> How should I negotiate device resolution? Could you point me?
> 
> VIDIOC_ENUM_FRAMESIZES may give you hints, but it's optional. You can use
> values you prefer to try if drivers support them; I think the GStreamer
> v4lsrc tries very small and very large values. The driver will clamp them to
> a supported values which are passed to the application from the IOCTL.
> 
> <URL:http://hverkuil.home.xs4all.nl/spec/media.html#vidioc-enum-framesizes>
> 

You can also call G_FMT first, then change the pixelformat and call S_FMT.
This will only change the pixelformat and leave the resolution the same (unless
the driver has some restrictions for the new pixelformat, but that's unlikely).

Regards,

	Hans