[libcamera-devel] [PATCH] libcamera: controls: Add controls for AEC/AGC flicker avoidance

[David Plowman]

Hi Kieran

Thanks for the reply.

On Wed, 25 Jan 2023 at 15:47, Kieran Bingham
<kieran.bingham at ideasonboard.com> wrote:
>
> Hi David,
>
> Quoting David Plowman via libcamera-devel (2023-01-25 13:20:18)
> > Flicker is the term used to describe brightness banding or oscillation
> > of images caused typically by artificial lighting driven by a 50 or
> > 60Hz mains supply. We add two controls intended to be used by AEC/AGC
> > algorithms:
> >
> > AeFlickerMode to determine whether flicker avoidance is active or not.
> >
> > AeFlickerPeriod to specify a custom flicker period when the period is
> > other than 50 or 60Hz.
>
> Oh excellent, this had been flickering in the back of my mind for some
> time.
>
> I expect you're going to be providing an implementation for
> pipeline/raspberrypi, but this should be easy to add to
> pipeline/uvcvideo too with appropriate mapping to:
>
> #define V4L2_CID_POWER_LINE_FREQUENCY   (V4L2_CID_BASE+24)
> enum v4l2_power_line_frequency {
>         V4L2_CID_POWER_LINE_FREQUENCY_DISABLED  = 0,
>         V4L2_CID_POWER_LINE_FREQUENCY_50HZ      = 1,
>         V4L2_CID_POWER_LINE_FREQUENCY_60HZ      = 2,
>         V4L2_CID_POWER_LINE_FREQUENCY_AUTO      = 3,
> };

I wonder what "AUTO" is intended to do. Perhaps it selects based on
location, perhaps it does something cleverer. I guess it's up to the
driver.

>
>
> > Signed-off-by: David Plowman <david.plowman at raspberrypi.com>
> > ---
> >  src/libcamera/control_ids.yaml | 52 ++++++++++++++++++++++++++++++++++
> >  1 file changed, 52 insertions(+)
> >
> > diff --git a/src/libcamera/control_ids.yaml b/src/libcamera/control_ids.yaml
> > index adea5f90..5e5428ea 100644
> > --- a/src/libcamera/control_ids.yaml
> > +++ b/src/libcamera/control_ids.yaml
> > @@ -14,6 +14,58 @@ controls:
> >
> >          \sa ExposureTime AnalogueGain
> >
> > +  - AeFlickerMode:
> > +      type: int32_t
> > +      description: |
> > +        Set the flicker mode, which determines whether, and how, the AGC/AEC
> > +        algorithm attempts to hide flicker effects caused by the duty cycle of
> > +        artificial lighting.
> > +
> > +        Although implementation dependent, many algorithms for "flicker
> > +        avoidance" work by restricting the exposure time to integer multiples
> > +        of this cycle period, wherever possible.
> > +
> > +        Implementations may not support all of the flicker modes listed below.
>
> Hrm... this makes me wonder how an application can determine what is
> possible, and what we would expect if the application tries to set a
> mode that isn't possible on that device.
>
> I don't think we have anything yet at the framework level to be able to
> expose which subset of a mode are valid for a given camera...
>
> For example, the UVC pipeline wouldn't support FlickerCustom.
>
> (I don't think that itself blocks the definition of these options though)

You could set a max control value that excludes the "top" choices, I
think I've seen that done before. Maybe one might swap the "custom"
and "auto" values. But a better solution for unhandled control values
would be nice!

David

>
> > +
> > +      enum:
> > +        - name: FlickerOff
> > +          value: 0
> > +          description: No flicker avoidance is performed.
> > +        - name: Flicker50Hz
> > +          value: 1
> > +          description: 50Hz flicker avoidance.
> > +            Suppress flicker effects caused by lighting runing with a
> > +            100Hz period (such as produced by 50Hz mains electricity).
> > +        - name: Flicker60Hz
> > +          value: 2
> > +          description: 60Hz flicker avoidance.
> > +            Suppress flicker effects caused by lighting running with a
> > +            120Hz period (such as produced by 60Hz mains electricity).
> > +        - name: FlickerCustom
> > +          value: 3
> > +          description: Custom flicker avoidance.
> > +            Suppress flicker effects caused by lighting running with a
> > +            period specified by AeFlickerPeriod.
> > +
> > +            \sa AeFlickerPeriod
> > +        - name: FlickerAuto
> > +          value: 4
> > +          description: Automatic flicker period detection and avoidance.
> > +            The system will automatically determine the most likely value
> > +            of the flicker period, and avoid flicker of this frequency.
> > +
> > +  - AeFlickerPeriod:
> > +      type: int32_t
> > +      description: Custom flicker period in microseconds.
> > +        This value is taken as the current flicker period to avoid. It
> > +        is used when the AeFlickerMode is set to FlickerCustom.
> > +
> > +        For example, to avoid 50Hz mains flicker, you could set the period
> > +        to 10000, corresponding to 10ms (twice the mains frequency), and
> > +        AeFlickerMode to FlickerCustom.
> > +
> > +        \sa AeFlickerMode
>
> I like that applications would be able to control this so precisely,
> rather than just the standard 50/60Hz modes.
>
> --
> Kieran
>
>
> > +
> >    - AeLocked:
> >        type: bool
> >        description: |
> > --
> > 2.30.2
> >