[libcamera-devel] reporting of gain and analogue gain

[David Plowman]

Hi Kieran

On Wed, 7 Oct 2020 at 15:58, Kieran Bingham
<kieran.bingham at ideasonboard.com> wrote:
>
> Hi David,
>
> On 07/10/2020 15:41, David Plowman wrote:
> > Hi everyone
> >
> > Whilst doing some maintenance on the Raspberry Pi AGC I noticed a bit
> > of an oversight - we have no way to report the total gain being
> > applied to an image. We have controls to report:
> >
> > * the exposure time of the sensor
> > * the analogue gain applied in the sensor
> > * red and blue colour gains applied by the AWB
> >
> > but nothing for any (global) digital gain demanded by the AGC. The
> > consequence is that when I capture a jpeg I always report an ISO value
> > based purely on the analogue gain which may be incorrect (when a
> > higher gain is required than the sensor allows then we make up the
> > difference with digital gain).
> >
> > Any thoughts on this? Obviously there could be a digital gain control...
>
> I assume the IPA would be then responsible for setting this? Is it
> read-only?

Certainly in the Raspberry Pi implementation it would be a read-only
control which the IPA sets for you. You could imagine some
implementations would allow you to set it (the old proprietary stack
of ours does, which I think is a source of confusion) but we're trying
to keep the behaviour simple.

>
> Would we expect an application to be able to control both the analogue
> gain and the digital gain manually? (When AGC is disabled I guess?)
>
> Or would they only see an overall 'gain' control?

The AGC algorithm we've implemented actually treats "gain" as a single
thing. If the analogue gain goes high enough that's what you'll get.
If you want to go higher, the analogue gain will max out and the
remainder will be digital gain.

>
> I'm suspecting that indeed, it would be exposed as a separate control,
> because it will expose a feature that the ISP is capable of delivering.

Yes, probably. I think maybe I feel another patch set coming on.
Perhaps I'll get the other AGC ones out of the way first while we wait
to see if anyone else has any opinions...

Thanks!
David

>
> >
> > Best regards
> > David
> > _______________________________________________
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> > libcamera-devel at lists.libcamera.org
> > https://lists.libcamera.org/listinfo/libcamera-devel
> >
>
> --
> Regards
> --
> Kieran