Re: [PATCH v1 16/19] v4l: Add encoding camera controls.

Hans Verkuil
2013-09-12T04:20:37Z

Hi Pawel,

On 09/12/2013 03:10 AM, Pawel Osciak wrote:
> On Tue, Sep 10, 2013 at 6:17 PM, Hans Verkuil <hverkuil@xs4all.nl> wrote:
>> On Mon 9 September 2013 11:09:57 Sylwester Nawrocki wrote:
>>> On 09/09/2013 11:00 AM, Kamil Debski wrote:
>>> [...]
>>>>>>> We have QP controls separately for H264, H263 and MPEG4. Why is that?
>>>>>>> Which one should I use for VP8? Shouldn't we unify them instead?
>>>>>>
>>>>>> I can't quite remember the details, so I've CCed Kamil since he added
>>>>> those controls.
>>>>>> At least the H264 QP controls are different from the others as they
>>>>>> have a different range. What's the range for VP8?
>>>>>
>>>>> Yes, it differs, 0-127.
>>>>> But I feel this is pretty unfortunate, is it a good idea to multiply
>>>>> controls to have one per format when they have different ranges
>>>>> depending on the selected format in general? Perhaps a custom handler
>>>>> would be better?
>>>>>
>>>>>> I'm not sure why the H263/MPEG4 controls weren't unified: it might be
>>>>>> that since the
>>>>>> H264 range was different we decided to split it up per codec. But I
>>>>>> seem to remember that there was another reason as well.
>>>>
>>>> We had a discussion about this on linux-media mailing list. It can be found
>>>> here:
>>>> http://comments.gmane.org/gmane.linux.drivers.video-input-infrastructure/32606
>>>> In short, it is a mix of two reasons: one - the valid range is different for
>>>> different formats and second - implementing controls which have different
>>>> min/max values depending on format was not easy.
>>>
>>> Hmm, these seem pretty vague reasons. And since some time we have support
>>> for dynamic control range update [1].
>>
>> I don't think we should change this. We chose to go with separate controls,
>> and we should stick with that. We might do it differently today, but it's
>> not a big deal.
>
> I guess I can't reuse MPEG controls as you suggested then.

Sure you can. Just call it V4L2_CID_MPEG_VIDEO_VPX_MIN_QP etc. The name 'MPEG'
was an unfortunate choice, it should have been 'CODEC'. And in fact in the spec
it's now known as the Codec Control Class. The QP settings are most definitely
codec controls, not camera or vpx controls.

> But I could
> either create a new VPX class, or keep these ones in camera class and
> create separate VPX class later for codecs. It's technically possible
> for UVC to have different constraints on some controls though, even if
> the codec is the same, potentially creating more confusion...

That's nothing new for uvc and there isn't anything you can do about it.
I don't think this will cause problems in practice.

Regards,

Hans

>
>
>>
>> Regards,
>>
>> Hans
>>
>>>
>>>> On the one hand I am thinking that now, when we have more codecs, it would
>>>> be better
>>>> to have a single control, on the other hand what about backward
>>>> compatibility?
>>>> Is there a graceful way to merge H263 and H264 QP controls?
>>>
>>> [1] https://patchwork.linuxtv.org/patch/16436/
>>>
>>> --
>>> Regards,
>>> Sylwester
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