Hi Sylwester,
On Wednesday 21 September 2011 14:28:25 Sylwester Nawrocki wrote:
> On 09/21/2011 12:17 AM, Sakari Ailus wrote:
> > On Tue, Sep 20, 2011 at 11:25:31PM +0200, Sylwester Nawrocki wrote:
> >> On 09/20/2011 10:57 PM, Sakari Ailus wrote:
> >>> On Tue, Sep 20, 2011 at 01:58:58PM +0200, Sylwester Nawrocki wrote:
> >>>> V4L2_CID_POWER_LINE_FREQUENCY control allows applications to instruct
> >>>> a driver what is the power line frequency so an appropriate filter
> >>>> can be used by the device to cancel flicker by compensating the light
> >>>> intensity ripple and thus. Currently in the menu we have entries for
> >>>> 50 and 60 Hz and for entirely disabling the anti-flicker filter.
> >>>> However some devices are capable of automatically detecting the
> >>>> frequency, so add V4L2_CID_POWER_LINE_FREQUENCY_AUTO entry for them.
> >>>>
> >>>> Signed-off-by: Sylwester Nawrocki<s.nawrocki@xxxxxxxxxxx>
> >>>> Signed-off-by: Kyungmin Park<kyungmin.park@xxxxxxxxxxx>
> >>>> Acked-by: Laurent Pinchart<laurent.pinchart@xxxxxxxxxxxxxxxx>
> >>>> ---
> >>>>
> >>>> Documentation/DocBook/media/v4l/controls.xml | 5 +++--
> >>>> drivers/media/video/v4l2-ctrls.c | 1 +
> >>>> include/linux/videodev2.h | 1 +
> >>>> 3 files changed, 5 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/Documentation/DocBook/media/v4l/controls.xml
> >>>> b/Documentation/DocBook/media/v4l/controls.xml index 2420e4a..c6b3c46
> >>>> 100644
> >>>> --- a/Documentation/DocBook/media/v4l/controls.xml
> >>>> +++ b/Documentation/DocBook/media/v4l/controls.xml
> >>>> @@ -232,8 +232,9 @@ control is deprecated. New drivers and
> >>>> applications should use the
> >>>>
> >>>> <entry>Enables a power line frequency filter to avoid
> >>>>
> >>>> flicker. Possible values for<constant>enum
> >>>> v4l2_power_line_frequency</constant> are:
> >>>> <constant>V4L2_CID_POWER_LINE_FREQUENCY_DISABLED</constant> (0),
> >>>>
> >>>> -<constant>V4L2_CID_POWER_LINE_FREQUENCY_50HZ</constant> (1) and
> >>>> -<constant>V4L2_CID_POWER_LINE_FREQUENCY_60HZ</constant> (2).</entry>
> >>>> +<constant>V4L2_CID_POWER_LINE_FREQUENCY_50HZ</constant> (1),
> >>>> +<constant>V4L2_CID_POWER_LINE_FREQUENCY_60HZ</constant> (2) and
> >>>> +<constant>V4L2_CID_POWER_LINE_FREQUENCY_AUTO</constant> (3).</entry>
> >>>
> >>> A stupid question: wouldn't this be a case for a new control for
> >>> automatic power line frequency, in other words enabling or disabling
> >>> it?
> >>
> >> IMO this would complicate things in kernel and user land, without any
> >> reasonable positive effects. AUTO seems to fit well here, it's just
> >> another mode of operation of a power line noise filter. Why make things
> >> more complicated than they need to be ?
> >
> > The advantage would be to be able to get the power line frquency if
> > that's supported by the hardware. This implementation excludes that.
> > Such information might be interesting to add e.g. to the image's exif
> > data.
>
> AFAIU, the power line frequency filter just modifies frame exposure time to
> be multiple of half of the mains frequency period. So it's the exposure
> time that gets finally affected. Maybe there is some hardware that
> supports retrieving of the detected frequency, however I'm not aware of
> it. And it doesn't seem useful unless you want to use camera as some
> non-standard measurement tool. It also takes some time until the detection
> algorithm locks, during this time an undefined frequency value would be
> read.
>
> I believe the filter settings do not really apply to still capture as it
> involves periodic operation, like preview. Even if we had this as meta
> data tag, there are more direct raw image parameters than the PL noise
> filter frequency.
>
> I feel uncomfortable with having 2 controls, where one can disable the
> filter and the other enable it with AUTO setting.
> Let's say the sensor supports 4 distinct settings of the filter: OFF, 50HZ,
> 60HZ, AUTO. (there is already one sensor driver in mainline that support
> it - ov519). How do we map this onto 2 controls ?
>
> What do we return from the menu control that covers { OFF, 50HZ, 60HZ }
> when AUTO mode is enabled through the other control and H/W doesn't allow
> to read the detected frequency ?
>
> I think, for the 2 controls we would need the DISABLED entry not to belong
> to V4L2_CID_POWER_LINE_FREQUENCY at first place.
>
> > Not sure if that's important, though.
>
> I would say no, but someone can prove me wrong. And who knows what kind of
> strange H/W future brings.
I think it all boils down to whether V4L2_CID_POWER_LINE_FREQUENCY is the
power line frequency filter control or the power line frequency control. In
the first case it doesn't make sense to use two separate controls. In the
second case it could.
I don't think we need two controls for this, but that's just a personal
opinion of the "I don't think we need to bother" type :-)
--
Regards,
Laurent Pinchart
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