Solved Draw in QPainter

[ODБOï]

hi,
im trying to periodically generate random lines and append them to each other to create a path. i use QPainter

i'm calling my custom method in the paintEvent

but all i get is

QPainter::setPen: Painter not active
how should i activate it ?

is there a more suited qt class for what i'm doing

thanks

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QPainter>
#include <QTimer>
#include <QDebug>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
protected:
    void paintEvent(QPaintEvent *event)
    {
        Q_UNUSED(event)

        drawLineW(qrand()*100, qrand()*100,(qrand()%10 > 5)?true:false);

    }

    void drawLineW(int x, int y, bool down){
        qDebug()<<down;

        if(down){
            painter->setPen(QPen(Qt::blue, 8, Qt::DashDotLine, Qt::RoundCap));
        }else{
            painter->setPen(QPen(Qt::red, 2, Qt::DashDotLine, Qt::RoundCap));
        }

        painter->drawLine(lastX,lastY,x,y);

        lastX = x;
        lastY = y;
    }

private:
    int lastX=0;
    int lastY=0;
    QPainter* painter;
    QTimer * drawTimer;
    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H

//________________________________________
#include "mainwindow.h"
#include "ui_mainwindow.h"

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

    drawTimer = new QTimer(this);
   
    painter = new QPainter(this);
    
    drawTimer->setInterval(100);

    QObject::connect(drawTimer,&QTimer::timeout,[=](){
        update();
    });

    drawTimer->start();

}

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

[ODБOï]

This was 1000 times easier using Graphics View...

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
private:
    int oldX=0;
    int oldY=0;
    Ui::MainWindow *ui;
    QGraphicsScene *scene;
    QTimer *drawTimer;
    QGraphicsLineItem *vect;

    QVector <QPoint> path;

    int nLine = 0;

    void createPath(){
        for(int i=1;i<20;i++){
              path.append(QPoint(i*10,(i%2)*10));
        }
    }
};
//___ cpp
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    scene = new QGraphicsScene(this);
    ui->graphicsView->setScene(scene);

    drawTimer = new QTimer(this);
    drawTimer->setInterval(50);

    createPath();

    QObject::connect(drawTimer,&QTimer::timeout,[=](){
        if(nLine>=path.length()){

            drawTimer->stop();
            return;
        }

        int newX = path.at(nLine).x();
        int newY = path.at(nLine).y();

        vect = scene->addLine(oldX,oldY,newX,newY);

        oldX= newX;
        oldY= newY;

        nLine++;

    });
    drawTimer->start();
}

[sierdzio]

You need to create the painter in the paint event, I think, just like in the docs: https://doc.qt.io/qt-5/qpainter.html

[ODБOï]

hi @sierdzio
that is the first test thing i did, yes.

but if i create one QPainter on the stack it is deleted every time

i want to keep the old path picture and add new line to it

[J.Hilk]

@LeLev Why don't you then simply append the new lines to a member QVector and simply call drawLines(vector) inside the paint event?

[sierdzio]

@LeLev said in Draw in QPainter:

hi @sierdzio
that is the first test thing i did, yes.

but if i create one QPainter on the stack it is deleted every time

i want to keep the old path picture and add new line to it

Then at the end of paint event, save the painter to a picture (QPixmap). Then in next paint event, first paint the pixmap, then add your new line to it (and save another pixmap).

[sierdzio]

Perhaps you can work around this by using member pointer like you do, but then calling QPainter::begin() in the beginning of your event handler. I'm not sure if it will work, though.

[ODБOï]

@J-Hilk i will try,
but that means every time you add a new line, you re-draw all the lines in the vector (from the beginning) ?

@sierdzio
like this, the first line is drawn and then a crash.

  void paintEvent(QPaintEvent *event)
    {
        Q_UNUSED(event)
        if(!painter->isActive()){
         painter->begin(this);
        }
        drawLineW(qrand()*100, qrand()*100,(qrand()%10 > 5)?true:false);
    }

is there an other type that will not clear the screen, and i can simply add lines ? im not forced to use QPainter

[jsulm]

@LeLev said in Draw in QPainter:

you re-draw all the lines in the vector (from the beginning) ?

You can draw in a QImage when you add a line and draw the image in paintEvent

[ODБOï]

hi
@jsulm said in Draw in QPainter:

You can draw in a QImage

sorry i can't find methods to draw in a QImage nor in QPaintDevice

i have to pass the image to my QPainter so i can draw right ?

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QPainter>
#include <QTimer>
#include <QVector>
#include <QDebug>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
protected:
    void paintEvent(QPaintEvent *event)
    {
        Q_UNUSED(event)
        if(!painter->isActive()){
         painter->begin(this);
        }

        drawLineW(qrand()*100, qrand()*100,(qrand()%10 > 5)?true:false);
    }

    void drawLineW(int x, int y, bool down){
       
        painter->drawLine(lastX,lastY,x,y);
        lastX = x;
        lastY = y;
    }

private:
    int lastX=0;
    int lastY=0;
    //QVector <QPoint> lines; 
    QImage pathImg;

    QPainter* painter;
    QTimer * drawTimer;
    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H

//__________________________________
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    drawTimer = new QTimer(this);

    painter = new QPainter(&pathImg);

    drawTimer->setInterval(100);

    QObject::connect(drawTimer,&QTimer::timeout,[=](){

        update();
    });

    drawTimer->start();

}

sorry im struggling so hard now, i can't get this done fo 3 houres.. I m not used to QWidgets at all.
i did the same thing in qml/js in about 10minutes

[sierdzio]

@LeLev said in Draw in QPainter:

i have to pass the image to my QPainter so i can draw right ?

You can use setBackground() or drawPixmap() first, then proceed with your drawLine() call.

[jsulm]

@LeLev said in Draw in QPainter:

sorry i can't find methods to draw in a QImage

Sorry, it's drawPixmap.

"i have to pass the image to my QPainter so i can draw right ?" - no, use one of the drawPixmap() methods..

[ODБOï]

@sierdzio

i call begin() as you suggested
and i also have to call end() at the end of my method.

now i can see my randomly generated lines, but every time i add a line the rest of lines are deleted/cleaned.

@sierdzio said in Draw in QPainter:

Then at the end of paint event, save the painter to a picture (QPixmap). Then in next paint event, first paint the pixmap

now i'm trying to do this.
There is a QPainter::drawPixmap to draw the saved image, but i can't find how to save the QPainter content to a QPixmap first, can you please give me a hint ?

[ODБOï]

almost working, just need to fix the save and reload feature

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
protected:
    void paintEvent(QPaintEvent *event)
    {

        Q_UNUSED(event)
        if(!painter->isActive()){
            painter->begin(this);
        }

        drawLineW(qrand()%250, qrand()%250,(qrand()%10 > 5)?true:false);
    }

    void drawLineW(int x, int y, bool down){

        // load last path/picture
        painter->drawPixmap(0,0,this->height(),this->width(),oldPath);

        if(down){
            painter->setPen(QPen(Qt::blue, 3, Qt::DashDotLine, Qt::RoundCap));
        }else{
            painter->setPen(QPen(Qt::red, 3, Qt::DashDotLine, Qt::RoundCap));
        }

        painter->drawLine(lastX,lastY,x,y);
        painter->end();

        lastX = x;
        lastY = y;

        // save current path/picture
        oldPath = QPixmap::fromImage(oldPathImg);

    }

private:
    int lastX=0;
    int lastY=0;

    QImage oldPathImg;

    QPixmap oldPath;

    QPainter* painter;
    QTimer * drawTimer;
    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H


// ___

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

    drawTimer = new QTimer(this);

    painter = new QPainter(&oldPathImg);

    drawTimer->setInterval(1000);

    QObject::connect(drawTimer,&QTimer::timeout,[=](){
        update();
    });

    drawTimer->start();
}

[ODБOï]

This was 1000 times easier using Graphics View...

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
private:
    int oldX=0;
    int oldY=0;
    Ui::MainWindow *ui;
    QGraphicsScene *scene;
    QTimer *drawTimer;
    QGraphicsLineItem *vect;

    QVector <QPoint> path;

    int nLine = 0;

    void createPath(){
        for(int i=1;i<20;i++){
              path.append(QPoint(i*10,(i%2)*10));
        }
    }
};
//___ cpp
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    scene = new QGraphicsScene(this);
    ui->graphicsView->setScene(scene);

    drawTimer = new QTimer(this);
    drawTimer->setInterval(50);

    createPath();

    QObject::connect(drawTimer,&QTimer::timeout,[=](){
        if(nLine>=path.length()){

            drawTimer->stop();
            return;
        }

        int newX = path.at(nLine).x();
        int newY = path.at(nLine).y();

        vect = scene->addLine(oldX,oldY,newX,newY);

        oldX= newX;
        oldY= newY;

        nLine++;

    });
    drawTimer->start();
}