[libcamera-devel] [PATCH v2 1/2] libcamera: controls: Update the ColourTemperature control to be writable

[David Plowman]

Hi Kieran

In our implementation setting a fixed colour temperature is just like
setting fixed colour gains. The AWB is "disabled" at that point, and
you don't get any feedback as to what it thinks the colour temperature
really is (as AWB is not doing a search any more). Setting a fixed
colour temperature overrides any fixed colour gains, and also vice
versa, because they basically do the same thing (i.e. set fixed colour
gains).

On the Pi, the AwbMode is a bit different because it still does a
search, but over a restricted range. I could imagine that other
platforms do it more like "setting a fixed colour temperature",
though.

Do you think some of these points need further explanation?

David

On Mon, 4 Dec 2023 at 23:21, Kieran Bingham
<kieran.bingham at ideasonboard.com> wrote:
>
> Hi David,
>
> Sorry - Coming back to this one as there are things that aren't clear
> enough to merge it yet.
>
> Quoting Kieran Bingham (2023-11-28 10:25:08)
> > Quoting David Plowman via libcamera-devel (2023-11-23 13:09:04)
> > > Implementations are allowed to expose this as a settable control so
> > > that applications can demand a specific colour temperature. The
> > > description is updated to this effect.
> > >
> > > Signed-off-by: David Plowman <david.plowman at raspberrypi.com>
> > > ---
> > >  src/libcamera/control_ids.yaml | 8 +++++---
> > >  1 file changed, 5 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/src/libcamera/control_ids.yaml b/src/libcamera/control_ids.yaml
> > > index c3232abf..8fbcc5f6 100644
> > > --- a/src/libcamera/control_ids.yaml
> > > +++ b/src/libcamera/control_ids.yaml
> > > @@ -312,9 +312,11 @@ controls:
> > >
> > >    - ColourTemperature:
> > >        type: int32_t
> > > -      description: Report the current estimate of the colour temperature, in
> > > -        kelvin, for this frame. The ColourTemperature control can only be
> > > -        returned in metadata.
> > > +      description: |
> > > +        Report the current estimate of the colour temperature, in kelvin, for
> > > +        this frame. An implementation may also allow this control to be set so
> > > +        as to disable the AWB algorithm whilst setting the colour gains to the
> > > +        correct fixed ones for this colour temperature.
> >
> > This certainly frames it as a mostly metadata control rather than a
> > control-control. So I think that's fine as that's possibly the
> > expectation for the most part.
> >
> > I have in mind to do some documentation udpates in the near future so I
> > wonder if I should look through all the controls and normalise how we
> > present the descriptions if they are read-only or settable or only
> > if supported by the pipeline handler...
> >
> > But I don't object to this for now so:
>
> How does this ColourTemperature relate to the AwbMode ? Which one takes
> precedence if set? (I assume based on the current text that this one
> overrides any setting of an AwbMode).
>
> What happens to ColourGains control if this is set? Does it override
> those too?  Or do they take precedence? Or is this just a helper to be
> able to set ColourGains based on the tuning?
>
> Does this control end up being an equivalent internal implementation
> detail of setting the AwbModes, i.e. perhaps is setting AwbCloudy the
> same as setting:
>
>   { ColourTemperature, 6000 },
>
> or perhaps setting AwbIncandescent would be:
>
>   { ColourTemperature, 2700 },
>
> And does such usage override the need/requirement to have an AwbCustom
> mode (which currently looks quite ill defined?
>
> If the user sets ColourTemperature, will there still be anything that
> can measure the ColourTemperature, and would return perhaps the
> 'measured' ColourTemperature back in the metadata? or would it always
> return what the user 'Set' it to ?
>
> --
> Kieran
>
>
> >
> > Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> >
> > >
> > >    - Saturation:
> > >        type: float
> > > --
> > > 2.39.2
> > >