How to delete points from QPainterPath?

[Christina123]

Hi, I am wondering is it possible to delete points from QPainterPath. I declare a QPainterPath variable and use it to draw lines by using moveTo and lineTo when the mouse is pressed or released. I wish to create a button that when it is pressed, it will delete the last line in the variable.

[SGaist]

There's no need to allocate QVector on the heap. You are only making it more complicated than necessary.

[SGaist]

Hi,

From the looks of it you can't.

If you only want one level of undo, then you could keep a copy of the path up to the last operation so you can copy it back to the "work path" if undone.

[Christina123]

I've decided to use QVector<QPainterPath> to store my lines. But when I try to pop the last item out, it is not working. Also, when I use size() to print the size of my vector, it seems to increase despite nothing is drawn on the window.

/*header file*/
#ifndef WINDOW_H
#define WINDOW_H

#include <QWidget>
#include <QFrame>
#include <QPainterPath>
#include <QMouseEvent>


class window : public QWidget
{
    Q_OBJECT

public:
    window(QWidget *parent = nullptr);
    void mousePressEvent(QMouseEvent *event);
    void mouseReleaseEvent(QMouseEvent *event);

public slots:

private slots:
    void reversePath();

private:
    QVector<QPainterPath> *path;
    QPainterPath *temp;
    void paintEvent(QPaintEvent *event);
};
#endif // WINDOW_H

/*cpp file*/
#include "window.h"
#include <QDebug>
#include <QtCore>
#include <QBoxLayout>
#include <QDebug>
#include <QVectorIterator>
#include <QVector>
#include <QPainter>
#include <QPushButton>


window::window(QWidget *parent)
    : QWidget(parent)
{
    setMinimumSize(QSize(500,500));
    path = new QVector<QPainterPath>;
    temp = new QPainterPath;

    QPushButton *reverse = new QPushButton("Reverse");
    connect(reverse, &QPushButton::clicked, this, &window::reversePath);

    QVBoxLayout *vbox = new QVBoxLayout(this);
    QHBoxLayout *hbox = new QHBoxLayout;

    hbox -> addWidget(reverse);
    hbox -> addStretch();

    vbox -> addLayout(hbox);
    vbox -> addStretch();
    setLayout(vbox);
}


void window::mousePressEvent(QMouseEvent *e)
{
    temp -> moveTo(QPoint(e->x(),e->y()));
}

void window::mouseReleaseEvent(QMouseEvent *e)
{
    temp -> lineTo(QPoint(e->x(),e->y()));
    update();
}

void window::reversePath(void)
{
    path -> pop_back();
    qDebug()<<path -> size();
}

void window::paintEvent(QPaintEvent *event)
{
    QPainter painter(this);
    path->append(*temp);
    QVectorIterator<QPainterPath> iter(*path);
    while(iter.hasNext()){
        painter.drawPath(iter.next());
    }
}

Printed result when I press the reverse button
28 41 53 65 95

[SGaist]

There's no need to allocate QVector on the heap. You are only making it more complicated than necessary.

[Christina123]

@SGaist I assumes that what you mean is for me to delete the line path = new QVector<QPainterPath>, but when I remove this line and run the program. It shows "The program has unexpectedly finished." What might be wrong?

[SGaist]

Because you are now using an invalid pointer.

Use the stack for QVector.

[Christina123]

Even though I've changed the vector declaration from QVector<QPainterPath> *path to QVector<QPainterPath> path, but the output when I press the button is still increasing each time.
16 28 35...

[SGaist]

Well, you are adding new points to temp on every click and only adding its content to your vector on paintEvent which is a bad idea because paintEvent might get called a lot of time. Therefore temp acts just the same as before you added your vector of QPainterPath.

[Christina123]

Thank you! I've solved my problem now. It is just my interest to know why paintEvent will be called a lot of time, as from what I learn, it will only be called when the signal update() is detected.

[SGaist]

There are a lot of reasons for paintings to happen. Moving the window, having something else coming through said window, etc.