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>

1 files changed, 87 insertions, 0 deletions

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);
+}