[libcamera-devel] [PATCH v2 2/6] libcamera: uvcvideo: Update exposure/gain ctrls set with new units

[Naushir Patuck]

Hi Kieran,

On Fri, 20 Mar 2020 at 14:28, Kieran Bingham
<kieran.bingham at ideasonboard.com> wrote:
>
> Hi Naush,
>
> On 09/03/2020 12:33, Naushir Patuck wrote:
> > The ManualExposure control now uses units of 1 micro-second, and UVC
> > uses units of 100 micro-seconds. Correctly map the values before
> > setting V4L2_CID_EXPOSURE_ABSOLUTE on the V4L2 device.
> >
> > The ManualGain control now uses floats to allow fractional gain values.
> > Since UVC has no explicit gain units, map the default gain value to 1.0
> > and linearly map to the requested value.
> >
> > Signed-off-by: Naushir Patuck <naush at raspberrypi.com>
> > ---
> >  src/libcamera/pipeline/uvcvideo.cpp | 17 +++++++++++++++--
> >  1 file changed, 15 insertions(+), 2 deletions(-)
> >
> > diff --git a/src/libcamera/pipeline/uvcvideo.cpp b/src/libcamera/pipeline/uvcvideo.cpp
> > index 29afb121..aff86803 100644
> > --- a/src/libcamera/pipeline/uvcvideo.cpp
> > +++ b/src/libcamera/pipeline/uvcvideo.cpp
> > @@ -245,9 +245,22 @@ int PipelineHandlerUVC::processControls(UVCCameraData *data, Request *request)
> >                       controls.set(V4L2_CID_SATURATION, value);
> >               } else if (id == controls::ManualExposure) {
> >                       controls.set(V4L2_CID_EXPOSURE_AUTO, static_cast<int32_t>(1));> -                       controls.set(V4L2_CID_EXPOSURE_ABSOLUTE, value);
> > +                     /*
> > +                      * controls::ManualExposure is in units of 1 us, and UVC
> > +                      * expects V4L2_CID_EXPOSURE_ABSOLUTE in units of 100 us.
> > +                      * So divide by 100 when setting the control.
>
> Sounds fine, but possibly the last line isn't needed, or I'd at least
> remove the 'So' ... but that's trivial/minor.

Will do.

>
>
> > +                      */
> > +                     controls.set(V4L2_CID_EXPOSURE_ABSOLUTE, value.get<int32_t>() / 100);
> >               } else if (id == controls::ManualGain) {
> > -                     controls.set(V4L2_CID_GAIN, value);
> > +                     /*
> > +                      * controls::ManualGain is specified as an absolute float value.
> > +                      * Map this in a linear way such that 1.0 -> default gain
> > +                      * of the V4L2_CID_GAIN control.
>
> This doesn't seem very clear to me yet. (I get the intent it's just the
> description), and if we're 'mapping' the value like this should it be
> documented at the control level?
>
> Or do you expect this to apply only to UVC? (Presumably if a device
> doesn't specify a default gain, it's just '1.0' ?)

This mapping is explicitly for UVC.

The ManualExposure control is meant to be an explicit exposure time to
use, and likewise, ManualGain is an explicit gain multiplier applied
to all samples.  From Laurent's comment on patch v1, he noted that UVC
doesn't specify units for gain.  I took that to mean that the gain
control is just a range-type control to increase the brightness, and
the mapping in the uvcvideo pipeline is to sanitise the control usage
in this case.

>
> Perhaps going back to patch 1/6 we need to explain more about how
> ManualGain is represented to document it's usage clearly there...
>

I can certainly be more descriptive in the description of the control if needed.

>
> > +                      */
> > +                     ControlRange gainInfo = controls.infoMap()->at(V4L2_CID_GAIN);
> > +                     float requestGain = value.get<float>();
> > +                     int32_t gain = requestGain * gainInfo.def().get<int32_t>();
> > +                     controls.set(V4L2_CID_GAIN, gain);
> >               }
> >       }
> >
> >
>
> --
> Regards
> --
> Kieran

Regards,
Naush