# QPolygonF error

• hi, does anyone have a solution for this
I have two polygons (high_polygon, low_polygon), that I want to combine (QPolygonF::united(QPolygonF)), usually it works good.
These are graphs and to combine them, I add -2 flags to the polygons (after combining - remove them)

but in some cases it’s buggs, I don’t have an understanding

I expect this result

but the output is

• @Daniil-S

Maybe it has to do with the negative numbers?!
Just a guess...

Because in the 2nd picture it looks like the polygon leaves the area and is connected to the next point again

• @Pl45m4 i dont think so, it works, but not every time

• @Daniil-S

What means "every time"?
How do you create these polygons? Does anything change from time to time?

• And please provide a minimal, compilable example so we can reproduce the issue.

• @Pl45m4, @Christian-Ehrlicher yes, if i change, for example one point of intersected low

``````QVector<QPointF> Graphic::HtpType(QVector<QPointF> low_graphic, QVector<QPointF> high_graphic)
{
QVector<QPointF> combined, intersected_low, after_high;

QPointF before_enter(-1,-1), after_exit(-1,-1);
auto low_iterator(low_graphic.begin());
while (low_iterator != low_graphic.end()) {

if (low_iterator->x() < high_graphic.first().x()) {

combined.push_back(*low_iterator);
before_enter = *low_iterator;
}
else if (low_iterator->x() >= high_graphic.first().x() && low_iterator->x() <= high_graphic.last().x()) {

intersected_low.push_back(*low_iterator);

} else if (low_iterator->x() > high_graphic.last().x()) {

if (after_exit == QPointF(-1,-1))
after_exit = *low_iterator;

after_high.push_back(*low_iterator);
}
low_iterator++;
}

if (intersected_low.isEmpty()) {

if (before_enter.y() == -1 && after_exit.y() == -1) {

combined.push_back(QPointF(high_graphic.first().x(), -1));
combined.append(high_graphic);
combined.push_back(QPointF(high_graphic.last().x(), -1));
combined.append(after_high);
return combined;
} else {

intersected_low.push_back(QPointF(high_graphic.first().x(), GetTimeProjection(high_graphic.first().x(), before_enter, after_exit)));
intersected_low.push_back(QPointF(high_graphic.last().x(), GetTimeProjection(high_graphic.last().x(), before_enter, after_exit)));
}
}

QPolygonF low_polygon(intersected_low), high_polygon(high_graphic);

if (before_enter.y() != -1) {
low_polygon.prepend(QPointF(high_graphic.first().x(), GetTimeProjection(high_graphic.first().x(), before_enter, intersected_low.first())));
combined.push_back(QPointF(high_graphic.first().x(), GetTimeProjection(high_graphic.first().x(), before_enter, intersected_low.first())));
} else {
low_polygon.prepend(QPointF(high_graphic.first().x(), -1));
combined.push_back(QPointF(high_graphic.first().x(), -1));
}
low_polygon.prepend(QPointF(high_graphic.first().x(), -2));

if (after_exit.y() != -1)
low_polygon.append(QPointF(high_graphic.last().x(), GetTimeProjection(high_graphic.last().x(), intersected_low.last(), after_exit)));
else
low_polygon.append(QPointF(high_graphic.last().x(), -1));
low_polygon.append(QPointF(high_graphic.last().x(), -2));

for(int i(0); i < low_polygon.count(); i++)
if (low_polygon.at(i).y() == -1)
low_polygon[i].ry() = 0;

low_polygon.push_back(low_polygon.first());

high_polygon.prepend(QPointF(high_graphic.first().x(), -2));
high_polygon.append(QPointF(high_graphic.last().x(), -2));

high_polygon.push_back(high_polygon.first());

QPolygonF united_low(low_polygon.united(high_polygon));
united_low.erase(std::remove_if(united_low.begin(), united_low.end(), [](QPointF p) { return p.y() == -2; }), united_low.end());

if (united_low.last() == united_low.first())
united_low.removeLast();

Q_FOREACH(QPointF point, united_low)
if (combined.last() != point)
combined.push_back(point);

QPointF after_high_point(high_graphic.last().x(), -1);
if (after_exit.y() != -1)
after_high_point.ry() = GetTimeProjection(high_graphic.last().x(), intersected_low.last(), after_exit);

if (combined.last() != after_high_point)
combined.push_back(after_high_point);

combined.append(after_high);

return combined;
}
``````

• Again: please provide a minimal, compilable example with the correct points.

• @Christian-Ehrlicher the above is complete code of the combine method,
here are the points of the first call to this method, the result of the combine is passed to the second call, which results in an error, first call works correctly

• I don't see anything but a class function which does some calculation. What input points do you use? Why do you need the function above? You said QPolygonF::united(QPolygonF) triggers this error so write a reproducer which does use exactly only this function and nothing else.

• @Christian-Ehrlicher I wrote what points I use, in this method call "united", don't understand what else is needed

``````QPolygonF united_low(low_polygon.united(high_polygon));
united_low.erase(std::remove_if(united_low.begin(), united_low.end(), [](QPointF p) { return p.y() == -2; }), united_low.end());
``````

this function searches for the intersection of the upper (overlayed) graph with the lower, creates two polygons from the intersection and the upper graph and unite them, after which it clears the extra flags and adds the final result of the merge

three steps of the loop, the first does not use the "HtrType" method, the other two use

first step
high QVector(QPointF(79,31), QPointF(182.303,139), QPointF(643,0), QPointF(673,31))
low QVector(QPointF(0,-1), QPointF(1000,-1))

second step
high QVector(QPointF(293,0), QPointF(406.102,225), QPointF(556,0))
low QVector(QPointF(0,-1), QPointF(79,-1), QPointF(79,31), QPointF(182.303,139), QPointF(182.303,139), QPointF(643,0), QPointF(643,0), QPointF(673,31), QPointF(673,-1), QPointF(1000,-1))

third step
high QVector(QPointF(293,0), QPointF(406.102,225), QPointF(556,0))
low QVector(QPointF(0,-1), QPointF(79,-1), QPointF(79,31), QPointF(182.303,139), QPointF(182.303,139), QPointF(293,105), QPointF(338.857,91.2255), QPointF(406.102,225), QPointF(534.345,32.5048), QPointF(556,26), QPointF(643,0), QPointF(643,0), QPointF(673,31), QPointF(673,-1), QPointF(1000,-1))

• @Daniil-S said in QPolygonF error:

don't understand what else is needed

If you want us to take a look on the problem then

• create two QPolygonF with your points
• call unite() on them
• tells us what's wrong

Noone will step through your code above, especially when the input is unknown.

• @Christian-Ehrlicher

move the slider and graph will sometimes bug

.pro file

``````QT  += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += c++11

# You can make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
main.cpp \
mainwindow.cpp

mainwindow.h

FORMS += \
mainwindow.ui

# Default rules for deployment.
qnx: target.path = /tmp/\$\${TARGET}/bin
else: unix:!android: target.path = /opt/\$\${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

``````

main.cpp

``````#include "mainwindow.h"

#include <QApplication>
#include <QWidget>

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
``````

mainwindow.h

``````#pragma once

#include <QMainWindow>
#include <QPainterPath>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
Q_OBJECT

static QVector<QPointF> HtpType(QVector<QPointF> low_graphic, QVector<QPointF> high_graphic);
static int GetTimeProjection(qreal x_coord, QPointF left_point, QPointF right_point);

public:
MainWindow(QWidget *parent = nullptr);
~MainWindow();

public slots:
void recalculate(int value);

private:
Ui::MainWindow *ui;
};
``````

mainwindow.cpp

``````#include "mainwindow.h"
#include "ui_mainwindow.h"

QVector<QPointF> MainWindow::HtpType(QVector<QPointF> low_graphic, QVector<QPointF> high_graphic)
{
QVector<QPointF> combined, intersected_low, after_high;

QPointF before_enter(-1,-1), after_exit(-1,-1);
auto low_iterator(low_graphic.begin());
while (low_iterator != low_graphic.end()) {

if (low_iterator->x() < high_graphic.first().x()) {

combined.push_back(*low_iterator);
before_enter = *low_iterator;
}
else if (low_iterator->x() >= high_graphic.first().x() && low_iterator->x() <= high_graphic.last().x()) {

intersected_low.push_back(*low_iterator);

} else if (low_iterator->x() > high_graphic.last().x()) {

if (after_exit == QPointF(-1,-1))
after_exit = *low_iterator;

after_high.push_back(*low_iterator);
}
low_iterator++;
}

if (intersected_low.isEmpty()) {

if (before_enter.y() == -1 && after_exit.y() == -1) {
high_graphic.push_front(QPointF(high_graphic.first().x(), -1));
high_graphic.push_back(QPointF(high_graphic.last().x(), -1));

Q_FOREACH(QPointF point, high_graphic)
if (combined.last() != point)
combined.push_back(point);

Q_FOREACH(QPointF point, after_high)
if (combined.last() != point)
combined.push_back(point);

return combined;
} else {

intersected_low.push_back(QPointF(high_graphic.first().x(), GetTimeProjection(high_graphic.first().x(), before_enter, after_exit)));
intersected_low.push_back(QPointF(high_graphic.last().x(), GetTimeProjection(high_graphic.last().x(), before_enter, after_exit)));
}
}

QPolygonF low_polygon(intersected_low), high_polygon(high_graphic);

QPointF prepend_point(before_enter.y() != -1 ? QPointF(high_graphic.first().x(), GetTimeProjection(high_graphic.first().x(), before_enter, intersected_low.first()))
: QPointF(high_graphic.first().x(), -1));

low_polygon.prepend(prepend_point);
if (combined.last() != prepend_point)
combined.push_back(prepend_point);

low_polygon.prepend(QPointF(high_graphic.first().x(), -2));
low_polygon.append(after_exit.y() != -1 ? QPointF(high_graphic.last().x(), GetTimeProjection(high_graphic.last().x(), intersected_low.last(), after_exit))
: QPointF(high_graphic.last().x(), -1));
low_polygon.append(QPointF(high_graphic.last().x(), -2));

for(int i(0); i < low_polygon.count(); i++)
if (low_polygon.at(i).y() == -1)
low_polygon[i].ry() = 0;

high_polygon.prepend(QPointF(high_graphic.first().x(), -2));
high_polygon.append(QPointF(high_graphic.last().x(), -2));

QPolygonF united_low(low_polygon.united(high_polygon));
united_low.erase(std::remove_if(united_low.begin(), united_low.end(), [](QPointF p) { return p.y() == -2; }), united_low.end());

if (united_low.last() == united_low.first())
united_low.removeLast();

QPointF after_high_point(high_graphic.last().x(), -1);
if (after_exit.y() != -1)
after_high_point.ry() = GetTimeProjection(high_graphic.last().x(), intersected_low.last(), after_exit);
united_low.push_back(after_high_point);

Q_FOREACH(QPointF point, united_low)
if (combined.last() != point)
combined.push_back(point);

Q_FOREACH(QPointF point, after_high)
if (combined.last() != point)
combined.push_back(point);

return combined;
}

int MainWindow::GetTimeProjection(qreal x_coord, QPointF left_point, QPointF right_point)
{
if (left_point.y() == -1)
left_point.ry() = 0;
if (right_point.y() == -1)
right_point.ry() = 0;

qreal x_min = (( right_point.x() <= left_point.x() ) ? right_point.x() : left_point.x());
qreal x_max = (( right_point.x() >= left_point.x() ) ? right_point.x() : left_point.x());
qreal y_min = (( right_point.y() <= left_point.y() ) ? right_point.y() : left_point.y());
qreal y_max = (( right_point.y() >= left_point.y() ) ? right_point.y() : left_point.y());

int direction = 0;

if (left_point.y() != right_point.y())
direction = ((left_point.y() < right_point.y()) ? 1 : -1);

qreal delta_x = x_max - x_min;
qreal delta_y = y_max - y_min;

int value = 0;

direction == -1 ? value = (delta_x ? y_min + delta_y - (delta_y / delta_x) * (x_coord - x_min) : y_min + delta_y)
: value = (delta_x ? y_min + (delta_y / delta_x) * (x_coord - x_min) : y_min);

return value;
}

MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::MainWindow)
{
ui->setupUi(this);
ui->graphicsView->setScene(new QGraphicsScene(ui->graphicsView));

ui->horizontalSlider->setRange(0, 255);
connect(ui->horizontalSlider, &QSlider::valueChanged, this, &MainWindow::recalculate);
}

MainWindow::~MainWindow()
{
delete ui;
}

void MainWindow::recalculate(int value)
{
QVector<QPointF> high, low1;
high << QPointF(293,0) << QPointF(406.102, 225) << QPointF(556,0);
low1 << QPointF(0,-1) << QPointF(79,-1) << QPointF(79,31) << QPointF(182.303,value) << QPointF(643,0) << QPointF(673,31) << QPointF(673,-1) << QPointF(1000,- 1);

QVector<QPointF> result(HtpType(HtpType(low1, high), high));

QPointF first(result.first());
first.ry() = std::abs(255 - first.y());
QPainterPath path(first);
for(int i(1); i < result.count(); i++) {

QPointF next(result.at(i));
next.ry() = std::abs(255 - next.y());
path.lineTo(next);
}

ui->graphicsView->scene()->clear();
}
``````

mainwindow.ui

``````<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
<class>MainWindow</class>
<widget class="QMainWindow" name="MainWindow">
<property name="geometry">
<rect>
<x>0</x>
<y>0</y>
<width>800</width>
<height>600</height>
</rect>
</property>
<property name="windowTitle">
<string>MainWindow</string>
</property>
<widget class="QWidget" name="centralwidget">
<widget class="QGraphicsView" name="graphicsView">
<property name="geometry">
<rect>
<x>50</x>
<y>50</y>
<width>681</width>
<height>461</height>
</rect>
</property>
</widget>
<widget class="QSlider" name="horizontalSlider">
<property name="geometry">
<rect>
<x>290</x>
<y>520</y>
<width>160</width>
<height>16</height>
</rect>
</property>
<property name="orientation">
<enum>Qt::Horizontal</enum>
</property>
</widget>
</widget>
<property name="geometry">
<rect>
<x>0</x>
<y>0</y>
<width>800</width>
<height>20</height>
</rect>
</property>
</widget>
<widget class="QStatusBar" name="statusbar"/>
</widget>
<resources/>
<connections/>
</ui>

``````