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statistics: render regression item using QSGNode

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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>
This commit is contained in:
Berthold Stoeger 2021-01-15 18:39:14 +01:00 • committed by bstoeger
parent faf3e7079d
commit 2008857660
6 changed files with 124 additions and 75 deletions
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87
stats/regressionitem.cpp Normal file
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// 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);
}