[libcamera-devel] [PATCH v3 1/4] libcamera: utils: Add helper class for std::chrono::duration

[Naushir Patuck]

Hi David,

Thank you for your feedback.

On Fri, 21 May 2021 at 11:20, David Plowman <david.plowman at raspberrypi.com>
wrote:

> Hi Naush
>
> Thanks for the updated patch!
>
> On Fri, 21 May 2021 at 09:05, Naushir Patuck <naush at raspberrypi.com>
> wrote:
> >
> > A new utils::Duration class is defined to represent a
> > std::chrono::duration type with double precision nanosecond timebase.
> > Using a double minimises the loss of precision when converting timebases.
> > This helper class may be used by IPAs to represent variables such as
> frame
> > durations and exposure times.
> >
> > An operator << overload is define to help with displaying
> > utils::Duration value in stream objects. Currently, this will display
> > the duration value in microseconds.
> >
> > Signed-off-by: Naushir Patuck <naush at raspberrypi.com>
> > ---
> >  include/libcamera/internal/utils.h | 42 ++++++++++++++++++++++++++++++
> >  src/libcamera/utils.cpp            | 36 +++++++++++++++++++++++++
> >  2 files changed, 78 insertions(+)
> >
> > diff --git a/include/libcamera/internal/utils.h
> b/include/libcamera/internal/utils.h
> > index 83dada7cc16c..a377d4ea07ab 100644
> > --- a/include/libcamera/internal/utils.h
> > +++ b/include/libcamera/internal/utils.h
> > @@ -316,6 +316,48 @@ auto enumerate(T (&iterable)[N]) ->
> details::enumerate_adapter<T *>
> >  }
> >  #endif
> >
> > +using BaseDuration = std::chrono::duration<double, std::nano>;
> > +
> > +class Duration : public BaseDuration
> > +{
> > +public:
> > +       Duration() = default;
> > +
> > +       template<typename Rep, typename Period>
> > +       constexpr Duration(const std::chrono::duration<Rep, Period> &d)
> > +               : BaseDuration(d)
> > +       {
> > +       }
> > +
> > +       template<typename Period>
> > +       double get() const
> > +       {
> > +               auto const c =
> std::chrono::duration_cast<std::chrono::duration<double, Period>>(*this);
> > +               return c.count();
> > +       }
> > +
> > +       explicit constexpr operator bool() const
> > +       {
> > +               return *this != BaseDuration::zero();
> > +       }
> > +};
> > +
> > +#ifndef __DOXYGEN__
> > +template<class CharT, class Traits>
> > +static inline std::basic_ostream<CharT, Traits>
> &operator<<(std::basic_ostream<CharT, Traits> &os,
> > +                                                           const
> Duration &d)
> > +{
> > +       std::basic_ostringstream<CharT, Traits> s;
> > +
> > +       s.flags(os.flags());
> > +       s.imbue(os.getloc());
> > +       s.setf(std::ios_base::fixed, std::ios_base::floatfield);
> > +       s.precision(2);
> > +       s << d.get<std::micro>() << "us";
> > +       return os << s.str();
> > +}
> > +#endif
>
> One small thing I wonder about is that many classes have a toString
> method. Do we want one here? It could even take an optional argument
> to allow you to format the output differently... but I think that's
> overkill and best left for now!
>

Did you mean that we should do something like:

LOG(IPA, Debug) << "Exposure time is " << exposure.toString();

over

LOG(IPA, Debug) << "Exposure time is " << exposure;

I chose the later way with the overload <<() operator as that is more
similar
to what C++20 will eventually allow with std::chrono::duration formatting.
What do others think?

Regards,
Naush



>
> > +
> >  } /* namespace utils */
> >
> >  } /* namespace libcamera */
> > diff --git a/src/libcamera/utils.cpp b/src/libcamera/utils.cpp
> > index 826144d3c837..3131aa2d6666 100644
> > --- a/src/libcamera/utils.cpp
> > +++ b/src/libcamera/utils.cpp
> > @@ -506,6 +506,42 @@ std::string libcameraSourcePath()
> >   * loop, iterates over an indexed view of the \a iterable
> >   */
> >
> > +/**
> > + * \class Duration
> > + * \brief Helper for std::chrono::duration types. Derived from \a
> BaseDuration
> > + */
> > +
> > +/**
> > + * \fn Duration::Duration(const std::chrono::duration<Rep, Period> &d)
> > + * \brief Copy constructor from a \a std::chrono::duration type.
> > + * \param[in] d The std::chrono::duration object to convert from.
> > + *
> > + * Constructs a \a Duration object from a \a std::chrono::duration
> object,
> > + * converting the representation to \a BaseDuration type.
> > + */
> > +
> > +/**
> > + * \fn Duration::get<Period>()
> > + * \brief Retrieve the tick count, coverted to the timebase provided by
> the
>
> s/coverted/converted/
>
> With this tiny fix:
>
> Reviewed-by: David Plowman <david.plowman at raspberrypi.com>
>
> Thanks!
> David
>
> > + * template argument Period of type \a std::ratio.
> > + *
> > + * A typical usage example is given below:
> > + *
> > + * \code{.cpp}
> > + * utils::Duration d = 5s;
> > + * double d_in_ms = d.get<std::milli>();
> > + * \endcode
> > + *
> > + * \return Tick count
> > + */
> > +
> > +/**
> > + * \fn Duration::operator bool()
> > + * \brief Boolean operator to test if a \a Duration holds a non-zero
> time value.
> > + *
> > + * \return True if \a Duration is a non-zero time value, False
> otherwise.
> > + */
> > +
> >  } /* namespace utils */
> >
> >  } /* namespace libcamera */
> > --
> > 2.25.1
> >
>
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