Qt/C++ - Lesson 019. How to paint triangle in Qt5. Positioning shapes in QGraphicsScene

QGraphicsItem, QPainter, QPolygon, Qt, Qt5, Рисование Qt, Рисование Qt5, треугольник

Drawing interfaces, the formation of the database tables, work with the network - it's all good, but sometimes you want to just draw something, such as a triangle. And then of course, revive this place, so that they can be controlled, and subsequently to turn this project into a little game. Well, who does not want to write your own game, even the most simple?

Let us then take the first step toward the unpretentious games, namely deal with drawing objects to Qt, trying to draw a triangle.

Project structure

  • Triangle.pro - Project profile, created by default, and in this project does not require the correction;
  • main.cpp - file, which starts with the application, the file is called a widget, which will be located in the graphic scene with a triangle;
  • widget.h - header file of a widget with a graphic scene;
  • widget.cpp - source file of widget;
  • triangle.h - Triangle header file class that inherits from QGraphicsItem;
  • triangle.cpp - source file of Triangle class;

mainwindow.ui

The interface design simply throws QGraphicsView in the widget. Nothing more is required.

widget.h

This file is only for defining a graphical scene and the triangle object with which we work.

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QGraphicsScene>

#include <triangle.h>

namespace Ui {
class Widget;
}

class Widget : public QWidget
{
    Q_OBJECT

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

private:
    Ui::Widget      *ui;
    QGraphicsScene  *scene;    
    Triangle        *triangle;  
};

#endif // WIDGET_H

widget.cpp

In this file are configured objects QGraphicsView, QGraphicsScene , and the triangle object is created and installed on the graphic scene.

#include "widget.h"
#include "ui_widget.h"

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    ui->setupUi(this);
    this->resize(600,600);          // Set sizes of widget
    this->setFixedSize(600,600);    // Fix sizes of widget
    scene = new QGraphicsScene();   // Init graphic scene
    triangle = new Triangle();      // Init Triangle

    ui->graphicsView->setScene(scene);  // Set graphics scene into graphicsView
    ui->graphicsView->setRenderHint(QPainter::Antialiasing);    
    ui->graphicsView->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff); 
    ui->graphicsView->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff); 

    scene->setSceneRect(-250,-250,500,500); 

    scene->addLine(-250,0,250,0,QPen(Qt::black));   // Add horizontal line via center
    scene->addLine(0,-250,0,250,QPen(Qt::black));   // Add vertical line via center

    scene->addItem(triangle);   
    triangle->setPos(0,0);      


}

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

triangle.h

But now is the time to work out the class itself, which creates a triangle. In this case, we are inheriting from QGraphicsItem .

#ifndef TRIANGLE_H
#define TRIANGLE_H

#include <QGraphicsItem>
#include <QPainter>

// Наследуемся от QGraphicsItem
class Triangle : public QGraphicsItem
{
public:
    Triangle();
    ~Triangle();

protected:
    QRectF boundingRect() const;    /* We define a virtual method that returns the area in which the triangle is
                                     * */
    /* Define a method for rendering a triangle
     * */
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);
};

#endif // TRIANGLE_H

triangle.cpp

Now draw a triangle in our class. Here, there is one important moment. Coordinate System QGraphicsItem object - a concept different from the coordinate system of the graphic scenes. That is, each QGraphicsItem object or inherited from this class has its own coordinate system, which is translated in QGraphicsScene coordinate system. When we assign a position where there will be an object in the graphic scene, we indicate where it will be located point of the graphic object that has the coordinates 0 on the X-axis and 0 on the Y-axis in the coordinate system of the object, so it is important that this point was the center of the graphic. This will facilitate further work, unless of course you are not consciously intend to other options.

#include "triangle.h"

Triangle::Triangle() :
    QGraphicsItem()
{

}

Triangle::~Triangle()
{

}

QRectF Triangle::boundingRect() const
{
    return QRectF(-25,-40,50,80);   // We are limiting the area of triangle
}

void Triangle::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
        QPolygon polygon;   // Using Polygon class, to draw the triangle
        // We put the coordinates of points in the polygonal model
        polygon << QPoint(0,-40) << QPoint(25,40) << QPoint(-25,40);
        painter->setBrush(Qt::red);     // We set the brush, which will render the object
        painter->drawPolygon(polygon);  // Draw a triangle on a polygonal model
        Q_UNUSED(option);
        Q_UNUSED(widget);
}

Result

As a result, you should get an application that displays a red triangle in the center of the graphic scenes at the intersection of the two lines, as shown in the figure.

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Last comments
April 8, 2020, 9:12 a.m.
ogustbiller

Круто! Немного начинает проясняться что к чему. Спасибо.
K
April 7, 2020, 8:55 a.m.
KULINAR847

А вот уже и на python... #!/usr/bin/env python# -'''- coding: utf-8 -'''-import sysfrom PySide2.QtWidgets import *from PySide2.QtQuick import *from PySide2.QtCore import *from PySide2.…
April 3, 2020, 8:06 a.m.
Konstantin Grudnitskiy

Я надеюсь вы уже разобрались в чем дело, но если вдруг нет, то проблема состоит в том, что вы пытаетесь запустить программу из интерпретатора питона. Файл main.py это уже готова…
April 3, 2020, 6:18 a.m.
Konstantin Grudnitskiy

>>> text = 'hello world'>>> ' '.join(word for word in text.split()[:-1])'hello'>>> def remove_last_word(text):... return text and ' '.join(word for word in text.s…
March 27, 2020, 2:40 p.m.
Evgenij Legotskoj

Добрый день. В конце пятой статьи скачать можете.
Now discuss on the forum
DK
April 9, 2020, 3:31 a.m.
Dzhon Kofi

Привет. Делаю реализацию перемещения строк на QTableView с моделью QSqlTableModel. Буду в этой теме спрашивать нужное. Пока такой вопрос при создании модели: как мне узнать rowCount? int…
April 8, 2020, 5:42 p.m.
elyana

Подскажите как правильно реализовать... Нужно создать массив в C++ с переменными типа сhar или же int. В Qml будет переменная, например text, которая будет изменять своё значение в зависим…
DK
April 8, 2020, 6:29 a.m.
Dzhon Kofi

:-D блин, без слов, пойду пройдусь))
s
April 8, 2020, 4:59 a.m.
slava_d2000

Всем привет. Прошу помощи.есть статическая сборка 5.14.2 (без SSL) компилируется чистый шаблон на QML и Widgets если войти на компьютер в терминальной сессии и запустить программ…
s
April 6, 2020, 8:06 a.m.
shuric

Добрый день. Объясните пожалуйста ... ... допиливать стилевое оформление в прокси классе ... где именно копать ? В каком виртуальном методе лучше допиливать (если можно н…
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