[libcamera-devel] [PATCH 4/5] src: ipa: raspberrypi: Move initial frame drop decision to AGC

[David Plowman]

Hi again Laurent

Thanks for the comments, yes, I agree there's a little more to think about.

On Sat, 5 Dec 2020 at 19:50, Laurent Pinchart
<laurent.pinchart at ideasonboard.com> wrote:
>
> Hi Naush and David,
>
> On Fri, Dec 04, 2020 at 04:01:42PM +0000, Naushir Patuck wrote:
> > On Wed, 2 Dec 2020 at 11:53, David Plowman wrote:
> >
> > > Previously the CamHelper was returning the number of frames to drop
> > > (on account of AGC converging). This wasn't really appropriate, it's
> > > better for the AGC to do it, which now also knows when exposure and
> > > gain have been explicitly set and therefore fewer (or no) frame drops
> > > are necessary at all.
> > >
> > > The CamHelper::HideFramesStartup method should now just be returning
> > > the number of frames to hide because they're bad/invalid in some way,
> > > not worrying about the AGC. For many sensors, the correct value for
> > > this is zero.
> > >
> > > Signed-off-by: David Plowman <david.plowman at raspberrypi.com>
> > > ---
> > >  src/ipa/raspberrypi/cam_helper.cpp  | 6 +++---
> > >  src/ipa/raspberrypi/raspberrypi.cpp | 8 ++++++++
> > >  2 files changed, 11 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/src/ipa/raspberrypi/cam_helper.cpp
> > > b/src/ipa/raspberrypi/cam_helper.cpp
> > > index c8ac3232..6efa0d7f 100644
> > > --- a/src/ipa/raspberrypi/cam_helper.cpp
> > > +++ b/src/ipa/raspberrypi/cam_helper.cpp
> > > @@ -82,10 +82,10 @@ bool CamHelper::SensorEmbeddedDataPresent() const
> > >  unsigned int CamHelper::HideFramesStartup() const
> > >  {
> > >         /*
> > > -        * By default, hide 6 frames completely at start-up while AGC etc. sort
> > > -        * themselves out (converge).
> > > +        * The number of frames when a camera first starts that shouldn't be
> > > +        * displayed as they are invalid in some way.
> > >          */
> > > -       return 6;
> > > +       return 0;
> > >  }
> > >
> > >  unsigned int CamHelper::HideFramesModeSwitch() const
> > > diff --git a/src/ipa/raspberrypi/raspberrypi.cpp
> > > b/src/ipa/raspberrypi/raspberrypi.cpp
> > > index 0300b8d9..ddabdb31 100644
> > > --- a/src/ipa/raspberrypi/raspberrypi.cpp
> > > +++ b/src/ipa/raspberrypi/raspberrypi.cpp
> > > @@ -192,6 +192,14 @@ int IPARPi::start(const IPAOperationData &ipaConfig,
> > > IPAOperationData *result)
> > >         unsigned int dropFrame = 0;
> > >         if (firstStart_) {
> > >                 dropFrame = helper_->HideFramesStartup();
> > > +
> > > +               /* The AGC algorithm may want us to drop more frames. */
> > > +               RPiController::AgcAlgorithm *agc = dynamic_cast<RPiController::AgcAlgorithm *>(
> > > +                       controller_.GetAlgorithm("agc"));
> > > +               if (agc)
> > > +                       dropFrame = std::max(dropFrame, agc->GetDropFrames());
> > > +               LOG(IPARPI, Debug) << "Drop " << dropFrame << " frames on startup";
> > > +
> >
> > All looks good with this change, however, I have a possibly silly
> > question.  In the previous code, our startup frames would account for
> > convergence in AGC, AWB, and ALS.  Here we are explicitly accounting for
> > convergence only in AGC since helper_->HideFramesStartup()  will return 0
> > by default.  Does it matter? Should each derived CamHelper return a
> > non-zero number here?
>
> Unless I'm mistaken, HideFramesStartup() is meant to report how many
> incorrectly exposed (and "gained") frames are produced by the sensor at
> startup, even if exposure time and gain are programmed before starting
> the sensor. This will certainly impact AWB and ALS as they will have
> trouble operating if the frame is greatly underexposed, but the sensor
> is otherwise not involved in AWB and ALS. I thus don't think CamHelper
> should take AWB and ALS into account.
>
> With this new split of responsibilities, with CamHelper reporting the
> number of frames that are bad (for different reasons, underexposed,
> incorrect metadata, ...) and the algorithms then deciding how long they
> need before initially converging, Shouldn't agc->GetDropFrames() be
> given the HideFramesStartup() value as a parameter, and return a new
> number, instead of taking the maximum between the two ?

Yes, I was already planning to change this as it's not quite right,
but I think I need to change it again. I think the correct behaviour
is as follows

Firstly, I'll look into giving AWB and ALSC the same
GetConvergenceFrames method as the AGC.

Next, I think the GetConvergenceFrames methods should be given the
MistrustFramesStartup number. If they actually have no convergence to
do (e.g. exposure and gain fixed), they return 0. Otherwise they add
their number to the MistrustFramesStartup number that they were given
and return that.

Finally, the dropFrames number is set to the maximum of
HideFramesStartup and all the GetConvergenceFrames values that we
obtained.

This gets the two important use cases right:

1. When things aren't fixed, and we give the algorithms as long as
they say plus the MistrustFramesStartup number, and

2. When everything is fixed, all the algorithms should tell us "zero",
so that we only skip frames if HideFramesStartup tells us to.

>
> I also wonder if we then need the hide/mistrust split anymore,
> especially with the comment in CamHelperOv5647::MistrustFramesStartup()
> that mentions underexposed frames. Is there still a difference between
> the two concepts ?

So I think there is, as discussed above. MistrustFramesModeSwitch,
though not used in my description above, still has a use too. If you
were to stop and restart the camera, so the algorithms don't need to
converge at all, you might still need to tell them to ignore a
frame's-worth of statistics, if they're thought to be bad in some way
(otherwise they risk making the algorithm go haywire when it tries to
process them).

>
> Finally, do we actually need to report a number of frames to drop at
> startup to the pipeline handler, can't we rely on the algorithms status
> reported through AeLocked and AwbLocked ? Maybe we should report only
> the number of frames that are definitely bad based on the CamHelper, and
> use algorithm status to report initial convergence of the algorithms ?
>
> > >                 mistrustCount_ = helper_->MistrustFramesStartup();
> > >         } else {
> > >                 dropFrame = helper_->HideFramesModeSwitch();

It's a good point. In practice we've found that users (OEMs in
particular) can be very sensitive about getting something on the
display asap, even if the AE/AWB isn't fully locked. On the other
hand, it still needs to look "visually locked", avoiding any obviously
bad frames, so we've always ended up in this sort of compromise
position where we actually display frames a little before they're
"technically" locked.

I'll make up another patch set on Monday with both your and Naush's
feedback, addressed, so thanks for all the suggestions.

Best regards
David

>
> --
> Regards,
>
> Laurent Pinchart