[libcamera-devel] [PATCH v2 1/3] libcamera: controls: Add frame duration control

[Jacopo Mondi]

Hi Naush,

    I won't go into details here as I would like Laurent to chime in
and express his view on this topic, but I would have a question in the
meantime.

On Tue, Jun 02, 2020 at 10:38:41AM +0100, Naushir Patuck wrote:
> Hi Jacopo,
>
>
> On Mon, 1 Jun 2020 at 16:40, Jacopo Mondi <jacopo at jmondi.org> wrote:
> >
> > Hi Naush,
> >
> > On Thu, May 28, 2020 at 09:52:51AM +0100, Naushir Patuck wrote:

[snip]

> > > > > On "FrameDuration" itself: would it make sense to only considered it
> > > > > when running with AE off ? In that case it would be applications
> > > > > responsibility to opportunely calculate the exposure time and the frame
> > > > > duration, but I think it's expected if you run in manual mode. And for
> > > > > reference, that's what Android does as well :)
> > > > >
> > > > > It would also rule out the need to specify how the frameDurationLimits
> > > > > would be used when running in manual mode and wanting to configure a
> > > > > precise value there, as I guess you would have to give to min and max
> > > > > the same value to express that.
> > >
> > > FrameDurationLimits should only really apply with AE enabled.  AE
> > > varies the shutter speed all over the place, that could cause
> > > framerate changes that we need to clip.  If AE is disabled and we have
> > > a manual shutter speed set, that dictates the frame duration, and any
> > > limits will not apply.  Perhaps I should add this in the description?
> > >
>
> I have to correct myself here.  FrameDurationLimits will also apply
> with manual shutter speeds when AE is disabled.  We could, say, have a
> manual shutter speed request of 25 ms, but still want to run at 30fps.
> There is a question on what happens if they clash, e.g. request a
> manual shutter speed of 66ms, and a FrameDuration of 30fps?  What gets
> prioritised?  Currently, in our implementation, FrameDuration will
> always be prioritised.

Is there a rationle behind this decision ? I'm asking as I see Android
going in the other direction, with the exposure time always overriding
the requested frame duration.

It is specified in their documentation that the maximum frame duration
a camera reports (lower frame rate) shall be at least the maximum
allowed exposure time, so that all possible exposure values are known
to be in the requested FPS range. I guess this should be enforced even
if we give frame duration priority, or should we cap the maximum frame
duration to the maximum available exposure time ?

Just trying to get more data points to make an informed decision, as
your question on what has to be prioritized is quite sensible...

Thanks
  j