[libcamera-devel] [PATCH 1/1] libcamera: controls: Add StartupFrame control

[David Plowman]

Hi again

Thanks for the discussion on this! Mostly I probably want to reiterate
what Naush has said.

On Mon, 5 Jun 2023 at 10:32, Naushir Patuck <naush at raspberrypi.com> wrote:
>
> Hi Laurent,
>
> David is away this week so his reply will be delayed.
>
> On Mon, 5 Jun 2023 at 09:17, Laurent Pinchart
> <laurent.pinchart at ideasonboard.com> wrote:
> >
> > Hello,
> >
> > On Wed, May 31, 2023 at 01:57:16PM +0100, Naushir Patuck via libcamera-devel wrote:
> > > Hi David,
> > >
> > > Thank you for this patch.  Indeed removing the drop frame logic the pipeline
> > > handler would simplify things!
> >
> > That's a change I would welcome :-)
> >
> > > On Wed, 31 May 2023 at 13:50, David Plowman via libcamera-devel wrote:
> > > >
> > > > This control is passed back in a frame as metadata to indicate whether
> > > > the camera system is still in a startup phase, and the application is
> > > > advised to avoid using the frame.
> > > >
> > > > Signed-off-by: David Plowman <david.plowman at raspberrypi.com>
> > > > ---
> > > >  src/libcamera/control_ids.yaml | 15 +++++++++++++++
> > > >  1 file changed, 15 insertions(+)
> > > >
> > > > diff --git a/src/libcamera/control_ids.yaml b/src/libcamera/control_ids.yaml
> > > > index adea5f90..4742d907 100644
> > > > --- a/src/libcamera/control_ids.yaml
> > > > +++ b/src/libcamera/control_ids.yaml
> > > > @@ -694,6 +694,21 @@ controls:
> > > >              Continuous AF is paused. No further state changes or lens movements
> > > >              will occur until the AfPauseResume control is sent.
> > > >
> > > > +  - StartupFrame:
> > > > +      type: bool
> > > > +      description: |
> > > > +        The value true indicates that the camera system is still in a startup
> > > > +        phase where the output images may not be reliable, or that certain of
> > > > +        the control algorithms (such as AEC/AGC, AWB, and possibly others) may
> > > > +        still be changing quite rapidly.
> >
> > I think we need to decide with a bit more details what constitute a
> > "startup frame" and what doesn't, or we will have all kinds of
> > inconsistent behaviour.
> >
> > I read it that we have multiple criteria on which we base the decision:
> >
> > - output images not being "reliable"
> > - 3A algorithms convergence
> > - "possibly others"
> >
> > The second criteria is fairly clear, but I'm thinking we should possibly
> > exclude it from the startup frames and report convergence of the 3A
> > algorithms instead.
>
> The 3A "startup" convergence is included as a criteria here because during
> startup, we drive the AE/AWB/ALSC as aggressively (i.e. no filtering) as
> possible to achieve convergence as fast as we possibly can.  This means the
> image output can oscillate quite badly - hence applications should avoid
> displaying or consuming them.
>
> I feel that this startup phase needs to be treated differently compared to a
> normal "converging" phase where the algorithms are filtering the outputs and the
> transitions our smooth, and conversely the application can (and probably should)
> display/consume these frames.

Basically, yes. The startup phase is different. For some algorithms we
indicate "converged" already, but that doesn't mean you shouldn't
display "unconverged" frames - of course we do. But during startup we
can expect algorithms possibly to overshoot and the recommendation is
simply not to use them. I suppose it doesn't mean an application
couldn't do something different - but we want to make the recommended
behaviour easy.

>
> >
> > The first and third criteria are quite vague. If I recall correctly, the
> > first one includes bad frames from the sensor (as in completely
> > corrupted frames, such as frames that are all black or made of random
> > data). Those are completely unusable for applications, is there a value
> > in still making them available instead of dropping them correctly ?
>
> This is what we do now (plus dropping the startup frames that may be good from
> the sensor but still in the 3A startup phase), but I think a key goals of this
> change is be consistent (i.e. let the application handle all frames always), and
> also help with avoiding some framebuffer allocations to help with startup
> convergence performance.
>
> Regards,
> Naush
>
> >
> > What are the other reasons a frame would be a "startup frame" ?

There is a genuine discussion to have here, I think. There are some
very clear reasons for telling an application not to use a frame - if
it's complete garbage, for example. And there are some more "advisory"
ones, which is what we can have for several frames. Categorising the
"usability" of a frame is certainly an idea, though I don't know if we
want to do that without a clear reason.

But it's also very hard to predict the behaviour of all pipeline
handlers in this respect. Perhaps someone has a pipeline handler where
the first frame after every mode switch comes out badly, perhaps it
needs to consume a frame first for its IPAs to sort themselves out.
But maybe another PH doesn't. So the idea is to have a portable way to
indicate this kind of thing so that applications don't have to start
guessing the behaviour underneath.

Note that these "behaviours" can be quite complex. For us, it's
completely different if you request fixed exposure and gain, and
colour gains. But only slightly different, IIRC, if you don't give the
colour gains. Forcing this kind of stuff into applications,
particularly ones that expect to use different PHs, feels quite
undesirable.

Don't know if I've really added anything, but I hope it makes a bit of sense!

Thanks
David

> >
> > > > +
> > > > +        Applications are advised to avoid using these frames. Mostly, they will
> > > > +        occur when the camera system starts for the first time, although,
> > > > +        depending on the sensor and the implementation, they could occur at
> > > > +        other times.
> > > > +
> > > > +        The value false indicates that this is a normal frame.
> > >
> > > Just throwing it out there, but would it be useful if this control was an
> > > integer with the count of startup frames left to handle? A value of 0, or the
> > > absence of the control would indicate this is a "valid" frame.
> > >
> > > > +
> > > >    # ----------------------------------------------------------------------------
> > > >    # Draft controls section
> > > >
> >
> > --
> > Regards,
> >
> > Laurent Pinchart