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

[Laurent Pinchart]

Hi David,

Thank you for the patch.

On Mon, Dec 07, 2020 at 06:01:19PM +0000, David Plowman 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>
> ---
>  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) {

Shouldn't this be + eps (and - eps below), based on Pwl::Append() and
Pwm::Prepend() ?

Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>

> +			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.

-- 
Regards,

Laurent Pinchart