[libcamera-devel] [PATCH v2 4/6] src: ipa: raspberrypi: Compute inverse of piecewise linear function

[Naushir Patuck]

Hi David,

Thank you for your patch.


On Mon, 7 Dec 2020 at 18:02, David Plowman <david.plowman at raspberrypi.com>
wrote:

> Add a method to the piecewise linear function (Pwl) class to compute
> the inverse of a given Pwl. If the input function is non-monotonic we
> can only produce a best effort "pseudo" inverse, and we signal this to
> the caller.
>
> Signed-off-by: David Plowman <david.plowman at raspberrypi.com>
>

Reviewed-by: Naushir Patuck <naush at raspberrypi.com>


> ---
>  src/ipa/raspberrypi/controller/pwl.cpp | 27 ++++++++++++++++++++++++++
>  src/ipa/raspberrypi/controller/pwl.hpp |  3 +++
>  2 files changed, 30 insertions(+)
>
> diff --git a/src/ipa/raspberrypi/controller/pwl.cpp
> b/src/ipa/raspberrypi/controller/pwl.cpp
> index aa134a1f..70206418 100644
> --- a/src/ipa/raspberrypi/controller/pwl.cpp
> +++ b/src/ipa/raspberrypi/controller/pwl.cpp
> @@ -114,6 +114,33 @@ Pwl::PerpType Pwl::Invert(Point const &xy, Point
> &perp, int &span,
>         return PerpType::None;
>  }
>
> +Pwl Pwl::Inverse(bool *true_inverse, const double eps) const
> +{
> +       bool appended = false, prepended = false, neither = false;
> +       Pwl inverse;
> +
> +       for (Point const &p : points_) {
> +               if (inverse.Empty())
> +                       inverse.Append(p.y, p.x, eps);
> +               else if (p.y > inverse.points_.back().x - eps) {
> +                       inverse.Append(p.y, p.x, eps);
> +                       appended = true;
> +               } else if (p.y < inverse.points_.front().x + eps) {
> +                       inverse.Prepend(p.y, p.x, eps);
> +                       prepended = true;
> +               } else
> +                       neither = true;
> +       }
> +
> +       // This is not a proper inverse if we found ourselves putting
> points
> +       // onto both ends of the inverse, or if there were points that
> couldn't
> +       // go on either.
> +       if (true_inverse)
> +               *true_inverse = !(neither || (appended && prepended));
> +
> +       return inverse;
> +}
> +
>  Pwl Pwl::Compose(Pwl const &other, const double eps) const
>  {
>         double this_x = points_[0].x, this_y = points_[0].y;
> diff --git a/src/ipa/raspberrypi/controller/pwl.hpp
> b/src/ipa/raspberrypi/controller/pwl.hpp
> index 4f168551..484672f6 100644
> --- a/src/ipa/raspberrypi/controller/pwl.hpp
> +++ b/src/ipa/raspberrypi/controller/pwl.hpp
> @@ -80,6 +80,9 @@ public:
>         };
>         PerpType Invert(Point const &xy, Point &perp, int &span,
>                         const double eps = 1e-6) const;
> +       // Compute the inverse function. Indicate if it is a proper (true)
> +       // inverse, or only a best effort (e.g. input was non-monotonic).
> +       Pwl Inverse(bool *true_inverse = nullptr, const double eps = 1e-6)
> const;
>         // Compose two Pwls together, doing "this" first and "other" after.
>         Pwl Compose(Pwl const &other, const double eps = 1e-6) const;
>         // Apply function to (x,y) values at every control point.
> --
> 2.20.1
>
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