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();
}