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.