statistics: render regression item using QSGNode

Render the confidence area and the regression line into a pixmap
and show that using a QSGNode.

It is unclear whether it is preferred to do it this way or to
triangulate the confidence area into triangles to be drawn by
the shader.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

6 files changed, 124 insertions, 75 deletions

diff --git a/Subsurface-mobile.pro b/Subsurface-mobile.pro
index 588e431d4..8e35c835d 100644
--- a/Subsurface-mobile.pro
+++ b/Subsurface-mobile.pro
@@ -138,6 +138,7 @@ SOURCES += subsurface-mobile-main.cpp \
 	stats/legend.cpp \
 	stats/pieseries.cpp \
 	stats/quartilemarker.cpp \
+	stats/regressionitem.cpp \
 	stats/scatterseries.cpp \
 	stats/statsaxis.cpp \
 	stats/statscolors.cpp \
@@ -289,6 +290,7 @@ HEADERS += \
 	stats/legend.h \
 	stats/pieseries.h \
 	stats/quartilemarker.h \
+	stats/regressionitem.h \
 	stats/scatterseries.h \
 	stats/statsaxis.h \
 	stats/statscolors.h \
diff --git a/stats/CMakeLists.txt b/stats/CMakeLists.txt
index 9048acf97..daeb22146 100644
--- a/stats/CMakeLists.txt
+++ b/stats/CMakeLists.txt
@@ -24,6 +24,8 @@ set(SUBSURFACE_STATS_SRCS
 	pieseries.cpp
 	quartilemarker.h
 	quartilemarker.cpp
+	regressionitem.h
+	regressionitem.cpp
 	scatterseries.h
 	scatterseries.cpp
 	statsaxis.h
diff --git a/stats/regressionitem.cpp b/stats/regressionitem.cpp
new file mode 100644
index 000000000..db02cf688
--- /dev/null
+++ b/stats/regressionitem.cpp
@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "regressionitem.h"
+#include "statsaxis.h"
+#include "zvalues.h"
+
+#include <cmath>
+
+static const QColor regressionItemColor(Qt::red);
+static const double regressionLineWidth = 2.0;
+
+RegressionItem::RegressionItem(StatsView &view, regression_data reg,
+						   StatsAxis *xAxis, StatsAxis *yAxis) :
+	ChartPixmapItem(view, ChartZValue::ChartFeatures),
+	xAxis(xAxis), yAxis(yAxis), reg(reg)
+{
+}
+
+RegressionItem::~RegressionItem()
+{
+}
+
+void RegressionItem::updatePosition()
+{
+	if (!xAxis || !yAxis)
+		return;
+	auto [minX, maxX] = xAxis->minMax();
+	auto [minY, maxY] = yAxis->minMax();
+	auto [screenMinX, screenMaxX] = xAxis->minMaxScreen();
+
+	// Draw the confidence interval according to http://www2.stat.duke.edu/~tjl13/s101/slides/unit6lec3H.pdf p.5 with t*=2 for 95% confidence
+	QPolygonF poly;
+	const int num_samples = 101;
+	poly.reserve(num_samples * 2);
+	for (int i = 0; i < num_samples; ++i) {
+		double x = (maxX - minX) / (num_samples - 1) * static_cast<double>(i) + minX;
+		poly << QPointF(xAxis->toScreen(x),
+			yAxis->toScreen(reg.a * x + reg.b + 2.0 * sqrt(reg.res2 / (reg.n - 2)  * (1.0 / reg.n + (x - reg.xavg) * (x - reg.xavg) / (reg.n - 1) * (reg.n -2) / reg.sx2))));
+	}
+	for (int i = num_samples - 1; i >= 0; --i) {
+		double x = (maxX - minX) / (num_samples - 1) * static_cast<double>(i) + minX;
+		poly << QPointF(xAxis->toScreen(x),
+			yAxis->toScreen(reg.a * x + reg.b - 2.0 * sqrt(reg.res2 / (reg.n - 2)  * (1.0 / reg.n + (x - reg.xavg) * (x - reg.xavg) / (reg.n - 1) * (reg.n -2) / reg.sx2))));
+	}
+	QPolygonF linePolygon;
+	linePolygon.reserve(2);
+	linePolygon << QPointF(screenMinX, yAxis->toScreen(reg.a * minX + reg.b));
+	linePolygon << QPointF(screenMaxX, yAxis->toScreen(reg.a * maxX + reg.b));
+
+	QRectF box(QPointF(screenMinX, yAxis->toScreen(minY)), QPointF(screenMaxX, yAxis->toScreen(maxY)));
+
+	poly = poly.intersected(box);
+	linePolygon = linePolygon.intersected(box);
+	if (poly.size() < 2 || linePolygon.size() < 2)
+		return;
+
+	// Find lowest and highest point on screen. In principle, we need
+	// only check half of the polygon, but let's not optimize without reason.
+	double screenMinY = std::numeric_limits<double>::max();
+	double screenMaxY = std::numeric_limits<double>::lowest();
+	for (const QPointF &point: poly) {
+		double y = point.y();
+		if (y < screenMinY)
+			screenMinY = y;
+		if (y > screenMaxY)
+			screenMaxY = y;
+	}
+	screenMinY = floor(screenMinY - 1.0);
+	screenMaxY = ceil(screenMaxY + 1.0);
+	QPointF offset(screenMinX, screenMinY);
+	for (QPointF &point: poly)
+		point -= offset;
+	for (QPointF &point: linePolygon)
+		point -= offset;
+	ChartPixmapItem::resize(QSizeF(screenMaxX - screenMinX, screenMaxY - screenMinY));
+
+	img->fill(Qt::transparent);
+	QColor col(regressionItemColor);
+	col.setAlphaF(reg.r2);
+	painter->setPen(Qt::NoPen);
+	painter->setBrush(QBrush(col));
+	painter->drawPolygon(poly);
+
+	painter->setPen(QPen(regressionItemColor, regressionLineWidth));
+	painter->drawLine(QPointF(linePolygon[0]), QPointF(linePolygon[1]));
+
+	ChartPixmapItem::setPos(offset);
+}
diff --git a/stats/regressionitem.h b/stats/regressionitem.h
new file mode 100644
index 000000000..607317d08
--- /dev/null
+++ b/stats/regressionitem.h
@@ -0,0 +1,26 @@
+// A regression line and confidence area
+#ifndef REGRESSION_H
+#define REGRESSION_H
+
+#include "chartitem.h"
+
+class StatsAxis;
+class StatsView;
+
+struct regression_data {
+	double a,b;
+	double res2, r2, sx2, xavg;
+	int n;
+};
+
+class RegressionItem : public ChartPixmapItem {
+public:
+	RegressionItem(StatsView &view, regression_data data, StatsAxis *xAxis, StatsAxis *yAxis);
+	~RegressionItem();
+	void updatePosition();
+private:
+	StatsAxis *xAxis, *yAxis;
+	regression_data reg;
+};
+
+#endif
diff --git a/stats/statsview.cpp b/stats/statsview.cpp
index 40d23eed3..455589976 100644
--- a/stats/statsview.cpp
+++ b/stats/statsview.cpp
@@ -6,6 +6,7 @@
 #include "legend.h"
 #include "pieseries.h"
 #include "quartilemarker.h"
+#include "regressionitem.h"
 #include "scatterseries.h"
 #include "statsaxis.h"
 #include "statscolors.h"
@@ -235,8 +236,8 @@ void StatsView::plotAreaChanged(const QSizeF &s)
 		series->updatePositions();
 	for (auto &marker: quartileMarkers)
 		marker->updatePosition();
-	for (RegressionLine &line: regressionLines)
-		line.updatePosition();
+	if (regressionItem)
+		regressionItem->updatePosition();
 	for (auto &marker: histogramMarkers)
 		marker->updatePosition();
 	if (legend)
@@ -347,8 +348,8 @@ void StatsView::reset()
 	legend.reset();
 	series.clear();
 	quartileMarkers.clear();
-	regressionLines.clear();
 	histogramMarkers.clear();
+	regressionItem.reset();
 	grid.reset();
 	axes.clear();
 	title.reset();
@@ -835,61 +836,11 @@ void StatsView::plotDiscreteScatter(const std::vector<dive *> &dives,
 	}
 }
 
-StatsView::RegressionLine::RegressionLine(const struct regression_data reg, QBrush brush, QGraphicsScene *scene, StatsAxis *xAxis, StatsAxis *yAxis) :
-	item(createItemPtr<QGraphicsPolygonItem>(scene)),
-	central(createItemPtr<QGraphicsPolygonItem>(scene)),
-	xAxis(xAxis), yAxis(yAxis),
-	reg(reg)
-{
-	item->setZValue(ZValues::chartFeatures);
-	item->setPen(Qt::NoPen);
-	item->setBrush(brush);
-
-	central->setZValue(ZValues::chartFeatures+1);
-	central->setPen(QPen(Qt::red));
-}
-
-void StatsView::RegressionLine::updatePosition()
-{
-	if (!xAxis || !yAxis)
-		return;
-	auto [minX, maxX] = xAxis->minMax();
-	auto [minY, maxY] = yAxis->minMax();
-
-	QPolygonF line;
-	line << QPoint(xAxis->toScreen(minX), yAxis->toScreen(reg.a * minX + reg.b))
-		<< QPoint(xAxis->toScreen(maxX), yAxis->toScreen(reg.a * maxX + reg.b));
-
-	// Draw the confidence interval according to http://www2.stat.duke.edu/~tjl13/s101/slides/unit6lec3H.pdf p.5 with t*=2 for 95% confidence
-	QPolygonF poly;
-	for (double x = minX; x <= maxX + 1; x += (maxX - minX) / 100)
-		poly << QPointF(xAxis->toScreen(x),
-			yAxis->toScreen(reg.a * x + reg.b + 2.0 * sqrt(reg.res2 / (reg.n - 2)  * (1.0 / reg.n + (x - reg.xavg) * (x - reg.xavg) / (reg.n - 1) * (reg.n -2) / reg.sx2))));
-	for (double x = maxX; x >= minX - 1; x -= (maxX - minX) / 100)
-		poly << QPointF(xAxis->toScreen(x),
-			yAxis->toScreen(reg.a * x + reg.b - 2.0 * sqrt(reg.res2 / (reg.n - 2)  * (1.0 / reg.n + (x - reg.xavg) * (x - reg.xavg) / (reg.n - 1) * (reg.n -2) / reg.sx2))));
-	QRectF box(QPoint(xAxis->toScreen(minX), yAxis->toScreen(minY)), QPoint(xAxis->toScreen(maxX), yAxis->toScreen(maxY)));
-
-	item->setPolygon(poly.intersected(box));
-	central->setPolygon(line.intersected(box));
-}
-
 void StatsView::addHistogramMarker(double pos, QColor color, bool isHorizontal, StatsAxis *xAxis, StatsAxis *yAxis)
 {
 	histogramMarkers.push_back(createChartItem<HistogramMarker>(pos, isHorizontal, color, xAxis, yAxis));
 }
 
-void StatsView::addLinearRegression(const struct regression_data reg, StatsAxis *xAxis, StatsAxis *yAxis)
-{
-	QColor red = QColor(Qt::red);
-	red.setAlphaF(reg.r2);
-	QPen pen(red);
-	QBrush brush(red);
-	brush.setStyle(Qt::SolidPattern);
-
-	regressionLines.emplace_back(reg, brush, &scene, xAxis, yAxis);
-}
-
 // Yikes, we get our data in different kinds of (bin, value) pairs.
 // To create a category axis from this, we have to templatify the function.
 template<typename T>
@@ -1194,5 +1145,5 @@ void StatsView::plotScatter(const std::vector<dive *> &dives, const StatsVariabl
 	// y = ax + b
 	struct regression_data reg = linear_regression(points);
 	if (!std::isnan(reg.a))
-		addLinearRegression(reg, xAxis, yAxis);
+		regressionItem = createChartItem<RegressionItem>(reg, xAxis, yAxis);
 }
diff --git a/stats/statsview.h b/stats/statsview.h
index f472e1e16..b0a199200 100644
--- a/stats/statsview.h
+++ b/stats/statsview.h
@@ -9,7 +9,6 @@
 #include <QImage>
 #include <QPainter>
 #include <QQuickItem>
-#include <QGraphicsPolygonItem>
 
 struct dive;
 struct StatsBinner;
@@ -17,7 +16,6 @@ struct StatsBin;
 struct StatsState;
 struct StatsVariable;
 
-class QGraphicsLineItem;
 class QGraphicsSimpleTextItem;
 class StatsSeries;
 class CategoryAxis;
@@ -26,6 +24,7 @@ class CountAxis;
 class HistogramAxis;
 class HistogramMarker;
 class QuartileMarker;
+class RegressionItem;
 class StatsAxis;
 class StatsGrid;
 class Legend;
@@ -36,13 +35,6 @@ enum class ChartSubType : int;
 enum class ChartZValue : int;
 enum class StatsOperation : int;
 
-struct regression_data {
-	double a,b;
-	double res2, r2, sx2, xavg;
-	int n;
-};
-
-
 class StatsView : public QQuickItem {
 	Q_OBJECT
 public:
@@ -127,17 +119,6 @@ private:
 	// Helper functions to add feature to the chart
 	void addLineMarker(double pos, double low, double high, const QPen &pen, bool isHorizontal);
 
-	// A regression line
-	struct RegressionLine {
-		std::unique_ptr<QGraphicsPolygonItem> item;
-		std::unique_ptr<QGraphicsPolygonItem> central;
-		StatsAxis *xAxis, *yAxis;
-		const struct regression_data reg;
-		void updatePosition();
-		RegressionLine(const struct regression_data reg, QBrush brush, QGraphicsScene *scene, StatsAxis *xAxis, StatsAxis *yAxis);
-	};
-
-	void addLinearRegression(const struct regression_data reg, StatsAxis *xAxis, StatsAxis *yAxis);
 	void addHistogramMarker(double pos, QColor color, bool isHorizontal, StatsAxis *xAxis, StatsAxis *yAxis);
 
 	StatsState state;
@@ -147,9 +128,9 @@ private:
 	std::vector<std::unique_ptr<StatsSeries>> series;
 	std::unique_ptr<Legend> legend;
 	std::vector<std::unique_ptr<QuartileMarker>> quartileMarkers;
-	std::vector<RegressionLine> regressionLines;
 	std::vector<std::unique_ptr<HistogramMarker>> histogramMarkers;
 	std::unique_ptr<QGraphicsSimpleTextItem> title;
+	std::unique_ptr<RegressionItem> regressionItem;
 	StatsSeries *highlightedSeries;
 	StatsAxis *xAxis, *yAxis;
 	Legend *draggedItem;