Re: Enforcing color profile

[Stefan Witzel <s.witzel@gmx.de>]

Hello,

Just in case someone should stumble across this thread: An appropriate
solution is available here:

  https://github.com/xkahn/camlink

As pointed out by Hans before the described problem is technically an
application bug, so one might say it should be fixed on application
level, which is what the solution above does.

Best,
Stefan

On 17.08.20 16:50, Stefan Witzel wrote:
> Hi Hans,
>
> I suppose it is ultimately an application bug. I was just hoping that the v4l driver cold somehow artificially restrict the device's feature list so as to make the buggy application (which seems to be pretty much any existing web conference app) work. That would be like the hardware workaround but on driver level.
>
> I do not yet have the device myself, so I'm relying on information from the website I quoted. I very much appreciate that you took the time to reply! I might come back with more concrete questions once I have the device.
>
> Thanks a lot! Best,
> Stefan
>
> Am 17. August 2020 15:34:00 MESZ schrieb Hans Verkuil <hverkuil@xs4all.nl>:
>> On 17/08/2020 14:09, Stefan Witzel wrote:
>>> Hello,
>>>
>>> I am not a V4L developer so if this is not the right place to bring
>> up
>>> the issue please tell me who to contact instead.
>>> The page [1] documents a problem with a particular device "Elgato Cam
>>> Link 4K" which provides various format options among which most
>> software
>>> does not consciously pick the one which it implicitly expects leading
>> to
>>> problems. There is a software workaround, transcoding and looping
>>> through a virtual device and a hardware fix flashing the problematic
>>> profiles away.
>>> However it appears to me that the most satisfactory solution is
>> hinted
>>> at in the sentence
>>>
>>>> I couldn't find any option in V4L to force selection of 'YUYV
>> 4:2:2', so instead
>>>
>>> So my question is: what changes would be necessary to allow v4l2-ctl
>> to
>>> enforce a particular format and who would be the right person to ask
>> to
>>> make such changes?
>>
>> It's there already:
>>
>> v4l2-ctl -v pixelformat=YUYV
>>
>> That will do the trick.
>>
>> If you plug in the device and run 'v4l2-ctl -V', which pixelformat does
>> it
>> report? I would expect it to be YUYV since that's first in the list.
>>
>> If apps like Discord pick YU12 instead, then that's weird. That's
>> something that
>> the application does, and v4l2-ctl won't help with that. I.e., it
>> appears to be
>> an application bug.
>>
>> One other alternative is that the device has broken 4:2:0 support, i.e.
>> it reports
>> these formats, but they don't actually work. You can use qv4l2 or
>> qvidcap to test
>> if these 4:2:0 formats are working.
>>
>> Regards,
>>
>> 	Hans
>>
>>
>>>
>>> Best regards,
>>> Stefan
>>>
>>>
>>> [1]
>> https://assortedhackery.com/patching-cam-link-to-play-nicer-on-linux/
>>>