Qt Creator/C++ Resize Drawing

Question

I need a little help in resizing a 2D figure. After 4 hours of searching and trying different methods, I figure I should post a question here.

[Project]: I am trying to draw and resize and rotate a figure using sliders. For some reason it doesn't want to act upon the figure. I want to increase the size by 1 for every time the slider value changes and back.

I don't want to use QPainter::scale or rotate.

[Code]:

#include "dialog.h"
#include "ui_dialog.h"

Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{

    a = 250;
    b = 150;
    c = 200;
    d = 150;


    ui->setupUi(this);

    connect(ui->horizontalSlider,SIGNAL(valueChanged(int)),this,SLOT(change(int)));

}

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

void Dialog::paintEvent(QPaintEvent *e)
{
    QPainter painter(this);

    painter.fillRect(a,b,c,d, Qt::green);

}

void Dialog::change(int value1)
{
    if(value1 > value2)
    {
        next = ++value1;
        a = a;
        b = b;
        c = c + next;
        d = d + next;
    }
}

[Header]:

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>
#include <QtGui>
#include <QtCore>

namespace Ui {
class Dialog;
}

class Dialog : public QDialog
{
    Q_OBJECT

public:
    explicit Dialog(QWidget *parent = 0);
    ~Dialog();

public slots:
    void change(int value1);

private:
    Ui::Dialog *ui;
    int value2, next;

protected:
    void paintEvent(QPaintEvent *e);
    int a,b,c,d;
};

Answer 1

Calling QWidget::update() will update the form and trigger a paint event.

Assuming you have a horizontalSlider working in the range [-100; 100], here is a small example:

dialog.h:

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>

namespace Ui {
class Dialog;
}

class Dialog : public QDialog
{
    Q_OBJECT

public:
    explicit Dialog(QWidget *parent = 0);
    ~Dialog();

protected:
    void paintEvent(QPaintEvent *e);

private:
    Ui::Dialog *ui;
    int a,b,c,d;
};

#endif // DIALOG_H

dialog.cpp:

#include "dialog.h"
#include "ui_dialog.h"

#include <QPainter>

Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    ui->setupUi(this);
    ui->horizontalSlider->setMinimum(-100);
    ui->horizontalSlider->setMaximum(100);

    a = 10;
    b = 10;
    c = 200;
    d = 150;

    connect(ui->horizontalSlider,SIGNAL(valueChanged(int)),this,SLOT(update()));
}

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


void Dialog::paintEvent(QPaintEvent *e)
{
    QPainter painter(this);

    painter.fillRect(a,
                     b,
                     c + ui->horizontalSlider->value(),
                     d + ui->horizontalSlider->value(),
                     Qt::green);

}

If you want to scale the rectangle with its center as pivot point just use:

void Dialog::paintEvent(QPaintEvent *e)
{
    QPainter painter(this);

    painter.fillRect(a - ui->horizontalSlider->value(),
                     b - ui->horizontalSlider->value(),
                     c + ui->horizontalSlider->value() * 2,
                     d + ui->horizontalSlider->value() * 2, Qt::green);

}

In this example the rectangle is resizing around the top-left pivot point. In case this is not what you want let me know and I'll be more explicit in my answer.