Enforcing color profile

Enforcing color profile

[Stefan Witzel]

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?

Best regards,
Stefan


[1] https://assortedhackery.com/patching-cam-link-to-play-nicer-on-linux/

Re: Enforcing color profile

[Hans Verkuil]

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/
> 

Re: Enforcing color profile

[Stefan Witzel]

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/
>> 

Re: Enforcing color profile

[Stefan Witzel]

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/
>>>

Re: Enforcing color profile

[Mauro Carvalho Chehab]

Em Tue, 1 Sep 2020 15:37:23 +0200
Stefan Witzel <s.witzel@gmx.de> escreveu:

> 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.

Hmm... let me try to understand something. I don't have this device.
I'm just curious about what's happening here ;-)

After looking at the patches and at:
	https://assortedhackery.com/patching-cam-link-to-play-nicer-on-linux/

it sounded to me that the UVC driver reports support for 3 different
fourccs:

	Pixel Format: 'YUYV' # V4L2_PIX_FMT_YUYV
	Pixel Format: 'NV12' # V4L2_PIX_FMT_NV12
	Pixel Format: 'YU12' # V4L2_PIX_FMT_YUV420

all of them at 1080 p resolution. 

It is very common that devices would support multiple video
formats. So, the above looks fine on my eyes, provided that the
device has support for those 3 YUV format variants.

-

If I understood well, YUYV works fine (with is a variant of 4:2:2). 

The problem is with 4:2:0 formats (NV12 and YU12):

	https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/pixfmt-nv12.html
	https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/pixfmt-yuv420.html

that don't work properly with Discord. Is that right?
Btw, are both NV12 and YU12 broken there?

Did you try using those formats with a different application,
like qv4l2 or qvidcap?

> 
> Best,
> Stefan
> 
> On 17.08.20 16:50, Stefan Witzel wrote:
> > Hi Hans,

Please don't top-post!

> >
> > 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/  
> >>>  



Thanks,
Mauro

Re: Enforcing color profile

[Stefan Witzel]

Hello,

Sorry for replying so late. All you say is correct. I tried qv4l2 now
and it confirms what you are suggesting: YUYV works fine, NV12 and YU12
show strangely distorted variants of the actual image. I would have sent
you a picture but qv4l2 crashed when I tried (even for YUYV).

Best,
Stefan

On 01.09.20 16:33, Mauro Carvalho Chehab wrote:
> Em Tue, 1 Sep 2020 15:37:23 +0200
> Stefan Witzel <s.witzel@gmx.de> escreveu:
>
>> 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.
>
> Hmm... let me try to understand something. I don't have this device.
> I'm just curious about what's happening here ;-)
>
> After looking at the patches and at:
> 	https://assortedhackery.com/patching-cam-link-to-play-nicer-on-linux/
>
> it sounded to me that the UVC driver reports support for 3 different
> fourccs:
>
> 	Pixel Format: 'YUYV' # V4L2_PIX_FMT_YUYV
> 	Pixel Format: 'NV12' # V4L2_PIX_FMT_NV12
> 	Pixel Format: 'YU12' # V4L2_PIX_FMT_YUV420
>
> all of them at 1080 p resolution.
>
> It is very common that devices would support multiple video
> formats. So, the above looks fine on my eyes, provided that the
> device has support for those 3 YUV format variants.
>
> -
>
> If I understood well, YUYV works fine (with is a variant of 4:2:2).
>
> The problem is with 4:2:0 formats (NV12 and YU12):
>
> 	https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/pixfmt-nv12.html
> 	https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/pixfmt-yuv420.html
>
> that don't work properly with Discord. Is that right?
> Btw, are both NV12 and YU12 broken there?
>
> Did you try using those formats with a different application,
> like qv4l2 or qvidcap?
>
>>
>> Best,
>> Stefan
>>
>> On 17.08.20 16:50, Stefan Witzel wrote:
>>> Hi Hans,
>
> Please don't top-post!
>
>>>
>>> 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/
>>>>>
>
>
>
> Thanks,
> Mauro
>