[PATCH v8 3/4] ipa: libipa: pwl: Clean up Pwl class to match libcamera

Paul Elder paul.elder at ideasonboard.com
Tue Jun 11 15:24:29 CEST 2024


Clean up the Pwl class copied from the Raspberry Pi IPA to align it more
with the libcamera style.

Signed-off-by: Paul Elder <paul.elder at ideasonboard.com>
Reviewed-by: Stefan Klug <stefan.klug at ideasonboard.com>
Acked-by: David Plowman <david.plowman at raspberrypi.com>
Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

---
Changes in v8:
- use the updated Vector interface
- remove unused functions (prepend, invert, extendDomain)
- improve class documentation
- checkstyle
- s/PointF/Point/
- make inverse() return pair instead of output parameter
- fix const order
- fix includes

No change in v7

Changes in v6:
- move adding pwl to meson here

Changes in v5:
- fix documentation order
- fix some typos
- add the Vector-based PointF

Changes in v4:
- update to apply to new copy of pwl
- add documentation
- fix doxygen

No change in v3

Changes in v2:
- s/FPoint/PointF/g
- improve documentation
- s/matchDomain/extendDomain/
---
 src/ipa/libipa/meson.build |   2 +
 src/ipa/libipa/pwl.cpp     | 372 ++++++++++++++++++++++++++-----------
 src/ipa/libipa/pwl.h       | 133 +++++--------
 3 files changed, 311 insertions(+), 196 deletions(-)

diff --git a/src/ipa/libipa/meson.build b/src/ipa/libipa/meson.build
index 8b0c8fff901b..3669f8939d3b 100644
--- a/src/ipa/libipa/meson.build
+++ b/src/ipa/libipa/meson.build
@@ -8,6 +8,7 @@ libipa_headers = files([
     'fc_queue.h',
     'histogram.h',
     'module.h',
+    'pwl.h',
     'vector.h',
 ])
 
@@ -19,6 +20,7 @@ libipa_sources = files([
     'fc_queue.cpp',
     'histogram.cpp',
     'module.cpp',
+    'pwl.cpp',
     'vector.cpp',
 ])
 
diff --git a/src/ipa/libipa/pwl.cpp b/src/ipa/libipa/pwl.cpp
index e39123767aa6..4dc59981708d 100644
--- a/src/ipa/libipa/pwl.cpp
+++ b/src/ipa/libipa/pwl.cpp
@@ -1,19 +1,120 @@
 /* SPDX-License-Identifier: BSD-2-Clause */
 /*
  * Copyright (C) 2019, Raspberry Pi Ltd
+ * Copyright (C) 2024, Ideas on Board Oy
  *
- * piecewise linear functions
+ * Piecewise linear functions
  */
 
-#include <cassert>
+#include "pwl.h"
+
+#include <assert.h>
 #include <cmath>
+#include <sstream>
 #include <stdexcept>
 
-#include "pwl.h"
+#include <libcamera/geometry.h>
+
+/**
+ * \file pwl.h
+ * \brief Piecewise linear functions
+ */
+
+namespace libcamera {
+
+namespace ipa {
+
+/**
+ * \class Pwl
+ * \brief Describe a univariate piecewise linear function in two-dimensional
+ * real space
+ *
+ * A piecewise linear function is a univariate function that maps reals to
+ * reals, and it is composed of multiple straight-line segments.
+ *
+ * While a mathematical piecewise linear function would usually be defined by
+ * a list of linear functions and for which values of the domain they apply,
+ * this Pwl class is instead defined by a list of points at which these line
+ * segments intersect. These intersecting points are known as knots.
+ *
+ * https://en.wikipedia.org/wiki/Piecewise_linear_function
+ *
+ * A consequence of the Pwl class being defined by knots instead of linear
+ * functions is that the values of the piecewise linear function past the ends
+ * of the function are constants as opposed to linear functions. In a
+ * mathematical piecewise linear function that is defined by multiple linear
+ * functions, the ends of the function are also linear functions and hence grow
+ * to infinity (or negative infinity). However, since this Pwl class is defined
+ * by knots, the y-value of the leftmost and rightmost knots will hold for all
+ * x values to negative infinity and positive infinity, respectively.
+ */
+
+/**
+ * \typedef Pwl::Point
+ * \brief Describe a point in two-dimensional real space
+ */
+
+/**
+ * \class Pwl::Interval
+ * \brief Describe an interval in one-dimensional real space
+ */
+
+/**
+ * \fn Pwl::Interval::Interval(double _start, double _end)
+ * \brief Construct an interval
+ * \param _start Start of the interval
+ * \param _end End of the interval
+ */
+
+/**
+ * \fn Pwl::Interval::contains
+ * \brief Check if a given value falls within the interval
+ * \param value Value to check
+ */
+
+/**
+ * \fn Pwl::Interval::clamp
+ * \brief Clamp a value such that it is within the interval
+ * \param value Value to clamp
+ */
+
+/**
+ * \fn Pwl::Interval::length
+ * \brief Compute the length of the interval
+ */
+
+/**
+ * \var Pwl::Interval::start
+ * \brief Start of the interval
+ */
 
-using namespace RPiController;
+/**
+ * \var Pwl::Interval::end
+ * \brief End of the interval
+ */
 
-int Pwl::read(const libcamera::YamlObject &params)
+/**
+ * \fn Pwl::Pwl(std::vector<Point> const &points)
+ * \brief Construct a piecewise linear function from a list of 2D points
+ * \param points Vector of points from which to construct the piecewise linear function
+ *
+ * \a points must be in ascending order of x-value.
+ */
+
+/**
+ * \brief Populate the piecewise linear function from yaml data
+ * \param params Yaml data to populate the piecewise linear function with
+ *
+ * Any existing points in the piecewise linear function will *not* be
+ * overwritten.
+ *
+ * The yaml data is expected to be a list with an even number of numerical
+ * elements. These will be parsed in pairs into x and y points in the piecewise
+ * linear function, and added in order. x must be monotonically increasing.
+ *
+ * \return 0 on success, negative error code otherwise
+ */
+int Pwl::readYaml(const libcamera::YamlObject &params)
 {
 	if (!params.size() || params.size() % 2)
 		return -EINVAL;
@@ -24,64 +125,109 @@ int Pwl::read(const libcamera::YamlObject &params)
 		auto x = it->get<double>();
 		if (!x)
 			return -EINVAL;
-		if (it != list.begin() && *x <= points_.back().x)
+		if (it != list.begin() && *x <= points_.back().x())
 			return -EINVAL;
 
 		auto y = (++it)->get<double>();
 		if (!y)
 			return -EINVAL;
 
-		points_.push_back(Point(*x, *y));
+		points_.push_back(Point({ *x, *y }));
 	}
 
 	return 0;
 }
 
+/**
+ * \brief Append a point to the end of the piecewise linear function
+ * \param x x-coordinate of the point to add to the piecewise linear function
+ * \param y y-coordinate of the point to add to the piecewise linear function
+ * \param eps Epsilon for the minimum x distance between points (optional)
+ *
+ * The point's x-coordinate must be greater than the x-coordinate of the last
+ * (= greatest) point already in the piecewise linear function.
+ */
 void Pwl::append(double x, double y, const double eps)
 {
-	if (points_.empty() || points_.back().x + eps < x)
-		points_.push_back(Point(x, y));
+	if (points_.empty() || points_.back().x() + eps < x)
+		points_.push_back(Point({ x, y }));
 }
 
+/**
+ * \brief Prepend a point to the beginning of the piecewise linear function
+ * \param x x-coordinate of the point to add to the piecewise linear function
+ * \param y y-coordinate of the point to add to the piecewise linear function
+ * \param eps Epsilon for the minimum x distance between points (optional)
+ *
+ * The point's x-coordinate must be less than the x-coordinate of the first
+ * (= smallest) point already in the piecewise linear function.
+ */
 void Pwl::prepend(double x, double y, const double eps)
 {
-	if (points_.empty() || points_.front().x - eps > x)
-		points_.insert(points_.begin(), Point(x, y));
+	if (points_.empty() || points_.front().x() - eps > x)
+		points_.insert(points_.begin(), Point({ x, y }));
 }
 
+/**
+ * \brief Get the domain of the piecewise linear function
+ * \return An interval representing the domain
+ */
 Pwl::Interval Pwl::domain() const
 {
-	return Interval(points_[0].x, points_[points_.size() - 1].x);
+	return Interval(points_[0].x(), points_[points_.size() - 1].x());
 }
 
+/**
+ * \brief Get the range of the piecewise linear function
+ * \return An interval representing the range
+ */
 Pwl::Interval Pwl::range() const
 {
-	double lo = points_[0].y, hi = lo;
+	double lo = points_[0].y(), hi = lo;
 	for (auto &p : points_)
-		lo = std::min(lo, p.y), hi = std::max(hi, p.y);
+		lo = std::min(lo, p.y()), hi = std::max(hi, p.y());
 	return Interval(lo, hi);
 }
 
+/**
+ * \brief Check if the piecewise linear function is empty
+ * \return True if there are no points in the function, false otherwise
+ */
 bool Pwl::empty() const
 {
 	return points_.empty();
 }
 
-double Pwl::eval(double x, int *spanPtr, bool updateSpan) const
+/**
+ * \brief Evaluate the piecewise linear function
+ * \param[in] x The x value to input into the function
+ * \param[inout] span Initial guess for span
+ * \param[in] updateSpan Set to true to update span
+ *
+ * Evaluate Pwl, optionally supplying an initial guess for the
+ * "span". The "span" may be optionally be updated. If you want to know
+ * the "span" value but don't have an initial guess you can set it to
+ * -1.
+ *
+ *  \return The result of evaluating the piecewise linear function at position \a x
+ */
+double Pwl::eval(double x, int *span, bool updateSpan) const
 {
-	int span = findSpan(x, spanPtr && *spanPtr != -1 ? *spanPtr : points_.size() / 2 - 1);
-	if (spanPtr && updateSpan)
-		*spanPtr = span;
-	return points_[span].y +
-	       (x - points_[span].x) * (points_[span + 1].y - points_[span].y) /
-		       (points_[span + 1].x - points_[span].x);
+	int index = findSpan(x, span && *span != -1
+					? *span
+					: points_.size() / 2 - 1);
+	if (span && updateSpan)
+		*span = index;
+	return points_[index].y() +
+	       (x - points_[index].x()) * (points_[index + 1].y() - points_[index].y()) /
+		       (points_[index + 1].x() - points_[index].x());
 }
 
 int Pwl::findSpan(double x, int span) const
 {
 	/*
 	 * Pwls are generally small, so linear search may well be faster than
-	 * binary, though could review this if large PWls start turning up.
+	 * binary, though could review this if large Pwls start turning up.
 	 */
 	int lastSpan = points_.size() - 2;
 	/*
@@ -89,65 +235,43 @@ int Pwl::findSpan(double x, int span) const
 	 * control point
 	 */
 	span = std::max(0, std::min(lastSpan, span));
-	while (span < lastSpan && x >= points_[span + 1].x)
+	while (span < lastSpan && x >= points_[span + 1].x())
 		span++;
-	while (span && x < points_[span].x)
+	while (span && x < points_[span].x())
 		span--;
 	return span;
 }
 
-Pwl::PerpType Pwl::invert(Point const &xy, Point &perp, int &span,
-			  const double eps) const
-{
-	assert(span >= -1);
-	bool prevOffEnd = false;
-	for (span = span + 1; span < (int)points_.size() - 1; span++) {
-		Point spanVec = points_[span + 1] - points_[span];
-		double t = ((xy - points_[span]) % spanVec) / spanVec.len2();
-		if (t < -eps) /* off the start of this span */
-		{
-			if (span == 0) {
-				perp = points_[span];
-				return PerpType::Start;
-			} else if (prevOffEnd) {
-				perp = points_[span];
-				return PerpType::Vertex;
-			}
-		} else if (t > 1 + eps) /* off the end of this span */
-		{
-			if (span == (int)points_.size() - 2) {
-				perp = points_[span + 1];
-				return PerpType::End;
-			}
-			prevOffEnd = true;
-		} else /* a true perpendicular */
-		{
-			perp = points_[span] + spanVec * t;
-			return PerpType::Perpendicular;
-		}
-	}
-	return PerpType::None;
-}
-
-Pwl Pwl::inverse(bool *trueInverse, const double eps) const
+/**
+ * \brief Compute the inverse function
+ * \param[in] eps Epsilon for the minimum x distance between points (optional)
+ *
+ * The output includes whether the resulting inverse function is a proper
+ * (true) inverse, or only a best effort (e.g. input was non-monotonic).
+ *
+ * \return A pair of the inverse piecewise linear function, and whether or not
+ * the result is a proper/true inverse
+ */
+std::pair<Pwl, bool> Pwl::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 (std::abs(inverse.points_.back().x - p.y) <= eps ||
-			 std::abs(inverse.points_.front().x - p.y) <= eps)
+		if (inverse.empty()) {
+			inverse.append(p.y(), p.x(), eps);
+		} else if (std::abs(inverse.points_.back().x() - p.y()) <= eps ||
+			   std::abs(inverse.points_.front().x() - p.y()) <= eps) {
 			/* do nothing */;
-		else if (p.y > inverse.points_.back().x) {
-			inverse.append(p.y, p.x, eps);
+		} else if (p.y() > inverse.points_.back().x()) {
+			inverse.append(p.y(), p.x(), eps);
 			appended = true;
-		} else if (p.y < inverse.points_.front().x) {
-			inverse.prepend(p.y, p.x, eps);
+		} else if (p.y() < inverse.points_.front().x()) {
+			inverse.prepend(p.y(), p.x(), eps);
 			prepended = true;
-		} else
+		} else {
 			neither = true;
+		}
 	}
 
 	/*
@@ -155,50 +279,58 @@ Pwl Pwl::inverse(bool *trueInverse, const double eps) const
 	 * onto both ends of the inverse, or if there were points that couldn't
 	 * go on either.
 	 */
-	if (trueInverse)
-		*trueInverse = !(neither || (appended && prepended));
+	bool trueInverse = !(neither || (appended && prepended));
 
-	return inverse;
+	return { inverse, trueInverse };
 }
 
+/**
+ * \brief Compose two piecewise linear functions together
+ * \param[in] other The "other" piecewise linear function
+ * \param[in] eps Epsilon for the minimum x distance between points (optional)
+ *
+ * The "this" function is done first, and "other" after.
+ *
+ * \return The composed piecewise linear function
+ */
 Pwl Pwl::compose(Pwl const &other, const double eps) const
 {
-	double thisX = points_[0].x, thisY = points_[0].y;
+	double thisX = points_[0].x(), thisY = points_[0].y();
 	int thisSpan = 0, otherSpan = other.findSpan(thisY, 0);
-	Pwl result({ { thisX, other.eval(thisY, &otherSpan, false) } });
+	Pwl result({ Point({ thisX, other.eval(thisY, &otherSpan, false) }) });
+
 	while (thisSpan != (int)points_.size() - 1) {
-		double dx = points_[thisSpan + 1].x - points_[thisSpan].x,
-		       dy = points_[thisSpan + 1].y - points_[thisSpan].y;
+		double dx = points_[thisSpan + 1].x() - points_[thisSpan].x(),
+		       dy = points_[thisSpan + 1].y() - points_[thisSpan].y();
 		if (std::abs(dy) > eps &&
 		    otherSpan + 1 < (int)other.points_.size() &&
-		    points_[thisSpan + 1].y >=
-			    other.points_[otherSpan + 1].x + eps) {
+		    points_[thisSpan + 1].y() >= other.points_[otherSpan + 1].x() + eps) {
 			/*
 			 * next control point in result will be where this
 			 * function's y reaches the next span in other
 			 */
-			thisX = points_[thisSpan].x +
-				(other.points_[otherSpan + 1].x -
-				 points_[thisSpan].y) *
+			thisX = points_[thisSpan].x() +
+				(other.points_[otherSpan + 1].x() -
+				 points_[thisSpan].y()) *
 					dx / dy;
-			thisY = other.points_[++otherSpan].x;
+			thisY = other.points_[++otherSpan].x();
 		} else if (std::abs(dy) > eps && otherSpan > 0 &&
-			   points_[thisSpan + 1].y <=
-				   other.points_[otherSpan - 1].x - eps) {
+			   points_[thisSpan + 1].y() <=
+				   other.points_[otherSpan - 1].x() - eps) {
 			/*
 			 * next control point in result will be where this
 			 * function's y reaches the previous span in other
 			 */
-			thisX = points_[thisSpan].x +
-				(other.points_[otherSpan + 1].x -
-				 points_[thisSpan].y) *
+			thisX = points_[thisSpan].x() +
+				(other.points_[otherSpan + 1].x() -
+				 points_[thisSpan].y()) *
 					dx / dy;
-			thisY = other.points_[--otherSpan].x;
+			thisY = other.points_[--otherSpan].x();
 		} else {
 			/* we stay in the same span in other */
 			thisSpan++;
-			thisX = points_[thisSpan].x,
-			thisY = points_[thisSpan].y;
+			thisX = points_[thisSpan].x(),
+			thisY = points_[thisSpan].y();
 		}
 		result.append(thisX, other.eval(thisY, &otherSpan, false),
 			      eps);
@@ -206,32 +338,47 @@ Pwl Pwl::compose(Pwl const &other, const double eps) const
 	return result;
 }
 
+/**
+ * \brief Apply function to (x,y) values at every control point
+ * \param f Function to be applied
+ */
 void Pwl::map(std::function<void(double x, double y)> f) const
 {
 	for (auto &pt : points_)
-		f(pt.x, pt.y);
+		f(pt.x(), pt.y());
 }
 
+/**
+ * \brief Apply function to (x, y0, y1) values wherever either Pwl has a
+ * control point.
+ */
 void Pwl::map2(Pwl const &pwl0, Pwl const &pwl1,
 	       std::function<void(double x, double y0, double y1)> f)
 {
 	int span0 = 0, span1 = 0;
-	double x = std::min(pwl0.points_[0].x, pwl1.points_[0].x);
+	double x = std::min(pwl0.points_[0].x(), pwl1.points_[0].x());
 	f(x, pwl0.eval(x, &span0, false), pwl1.eval(x, &span1, false));
+
 	while (span0 < (int)pwl0.points_.size() - 1 ||
 	       span1 < (int)pwl1.points_.size() - 1) {
 		if (span0 == (int)pwl0.points_.size() - 1)
-			x = pwl1.points_[++span1].x;
+			x = pwl1.points_[++span1].x();
 		else if (span1 == (int)pwl1.points_.size() - 1)
-			x = pwl0.points_[++span0].x;
-		else if (pwl0.points_[span0 + 1].x > pwl1.points_[span1 + 1].x)
-			x = pwl1.points_[++span1].x;
+			x = pwl0.points_[++span0].x();
+		else if (pwl0.points_[span0 + 1].x() > pwl1.points_[span1 + 1].x())
+			x = pwl1.points_[++span1].x();
 		else
-			x = pwl0.points_[++span0].x;
+			x = pwl0.points_[++span0].x();
 		f(x, pwl0.eval(x, &span0, false), pwl1.eval(x, &span1, false));
 	}
 }
 
+/**
+ * \brief Combine two Pwls
+ *
+ * Create a new Pwl where the y values are given by running f wherever either
+ * has a knot.
+ */
 Pwl Pwl::combine(Pwl const &pwl0, Pwl const &pwl1,
 		 std::function<double(double x, double y0, double y1)> f,
 		 const double eps)
@@ -243,27 +390,32 @@ Pwl Pwl::combine(Pwl const &pwl0, Pwl const &pwl1,
 	return result;
 }
 
-void Pwl::matchDomain(Interval const &domain, bool clip, const double eps)
-{
-	int span = 0;
-	prepend(domain.start, eval(clip ? points_[0].x : domain.start, &span),
-		eps);
-	span = points_.size() - 2;
-	append(domain.end, eval(clip ? points_.back().x : domain.end, &span),
-	       eps);
-}
-
+/**
+ * \brief Multiply the piecewise linear function
+ * \param d Scalar multiplier to multiply the function by
+ * \return This function, after it has been multiplied by \a d
+ */
 Pwl &Pwl::operator*=(double d)
 {
 	for (auto &pt : points_)
-		pt.y *= d;
+		pt[1] *= d;
 	return *this;
 }
 
-void Pwl::debug(FILE *fp) const
+/**
+ * \brief Assemble and return a string describing the piecewise linear function
+ * \return A string describing the piecewise linear function
+ */
+std::string Pwl::toString() const
 {
-	fprintf(fp, "Pwl {\n");
+	std::stringstream ss;
+	ss << "Pwl { ";
 	for (auto &p : points_)
-		fprintf(fp, "\t(%g, %g)\n", p.x, p.y);
-	fprintf(fp, "}\n");
+		ss << "(" << p.x() << ", " << p.y() << ") ";
+	ss << "}";
+	return ss.str();
 }
+
+} /* namespace ipa */
+
+} /* namespace libcamera */
diff --git a/src/ipa/libipa/pwl.h b/src/ipa/libipa/pwl.h
index 7d5e7e4d3fda..a2cbad6c1597 100644
--- a/src/ipa/libipa/pwl.h
+++ b/src/ipa/libipa/pwl.h
@@ -2,126 +2,87 @@
 /*
  * Copyright (C) 2019, Raspberry Pi Ltd
  *
- * piecewise linear functions interface
+ * Piecewise linear functions interface
  */
 #pragma once
 
+#include <algorithm>
+#include <cmath>
 #include <functional>
-#include <math.h>
+#include <string>
+#include <utility>
 #include <vector>
 
+#include <libcamera/geometry.h>
+
 #include "libcamera/internal/yaml_parser.h"
 
-namespace RPiController {
+#include "vector.h"
+
+namespace libcamera {
+
+namespace ipa {
 
 class Pwl
 {
 public:
+	using Point = Vector<double, 2>;
+
 	struct Interval {
 		Interval(double _start, double _end)
-			: start(_start), end(_end)
-		{
-		}
-		double start, end;
+			: start(_start), end(_end) {}
+
 		bool contains(double value)
 		{
 			return value >= start && value <= end;
 		}
-		double clip(double value)
-		{
-			return value < start ? start
-					     : (value > end ? end : value);
-		}
-		double len() const { return end - start; }
-	};
-	struct Point {
-		Point() : x(0), y(0) {}
-		Point(double _x, double _y)
-			: x(_x), y(_y) {}
-		double x, y;
-		Point operator-(Point const &p) const
-		{
-			return Point(x - p.x, y - p.y);
-		}
-		Point operator+(Point const &p) const
-		{
-			return Point(x + p.x, y + p.y);
-		}
-		double operator%(Point const &p) const
+
+		double clamp(double value)
 		{
-			return x * p.x + y * p.y;
+			return std::clamp(value, start, end);
 		}
-		Point operator*(double f) const { return Point(x * f, y * f); }
-		Point operator/(double f) const { return Point(x / f, y / f); }
-		double len2() const { return x * x + y * y; }
-		double len() const { return sqrt(len2()); }
+
+		double length() const { return end - start; }
+
+		double start, end;
 	};
+
 	Pwl() {}
-	Pwl(std::vector<Point> const &points) : points_(points) {}
-	int read(const libcamera::YamlObject &params);
+	Pwl(const std::vector<Point> &points)
+		: points_(points) {}
+	int readYaml(const libcamera::YamlObject &params);
+
 	void append(double x, double y, const double eps = 1e-6);
-	void prepend(double x, double y, const double eps = 1e-6);
+
+	bool empty() const;
 	Interval domain() const;
 	Interval range() const;
-	bool empty() const;
-	/*
-	 * Evaluate Pwl, optionally supplying an initial guess for the
-	 * "span". The "span" may be optionally be updated.  If you want to know
-	 * the "span" value but don't have an initial guess you can set it to
-	 * -1.
-	 */
-	double eval(double x, int *spanPtr = nullptr,
+
+	double eval(double x, int *span = nullptr,
 		    bool updateSpan = true) const;
-	/*
-	 * Find perpendicular closest to xy, starting from span+1 so you can
-	 * call it repeatedly to check for multiple closest points (set span to
-	 * -1 on the first call). Also returns "pseudo" perpendiculars; see
-	 * PerpType enum.
-	 */
-	enum class PerpType {
-		None, /* no perpendicular found */
-		Start, /* start of Pwl is closest point */
-		End, /* end of Pwl is closest point */
-		Vertex, /* vertex of Pwl is closest point */
-		Perpendicular /* true perpendicular found */
-	};
-	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 *trueInverse = 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. */
+
+	std::pair<Pwl, bool> inverse(const double eps = 1e-6) const;
+	Pwl compose(const Pwl &other, const double eps = 1e-6) const;
+
 	void map(std::function<void(double x, double y)> f) const;
-	/*
-	 * Apply function to (x, y0, y1) values wherever either Pwl has a
-	 * control point.
-	 */
-	static void map2(Pwl const &pwl0, Pwl const &pwl1,
-			 std::function<void(double x, double y0, double y1)> f);
-	/*
-	 * Combine two Pwls, meaning we create a new Pwl where the y values are
-	 * given by running f wherever either has a knot.
-	 */
+
 	static Pwl
-	combine(Pwl const &pwl0, Pwl const &pwl1,
+	combine(const Pwl &pwl0, const Pwl &pwl1,
 		std::function<double(double x, double y0, double y1)> f,
 		const double eps = 1e-6);
-	/*
-	 * Make "this" match (at least) the given domain. Any extension my be
-	 * clipped or linear.
-	 */
-	void matchDomain(Interval const &domain, bool clip = true,
-			 const double eps = 1e-6);
+
 	Pwl &operator*=(double d);
-	void debug(FILE *fp = stdout) const;
+
+	std::string toString() const;
 
 private:
+	void prepend(double x, double y, const double eps = 1e-6);
+	static void map2(const Pwl &pwl0, const Pwl &pwl1,
+			 std::function<void(double x, double y0, double y1)> f);
 	int findSpan(double x, int span) const;
 	std::vector<Point> points_;
 };
 
-} /* namespace RPiController */
+} /* namespace ipa */
+
+} /* namespace libcamera */
-- 
2.39.2



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