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Drawing a Circle in Qt QML three different ways

Published: 05-07-2023 23:59 | Author: Remy van Elst | Text only version of this article


❗ This post is over one years old. It may no longer be up to date. Opinions may have changed.


Qt has no Circle built in to QML as a basic type, as for example the Rectangle or the Button control. This post shows you how to get a Circle in QML, from the most basic method (a Rectangle with a radius of 180) to more advanced methods, using the Canvas JavaScript API (which allows us to draw a partially filled Circle, for a Pie Chart) and a c++ control based on QQuickPaintedItem. I wanted to experiment with the Canvas QML control and the QQuickPaintedItem C++ interface to get a better understanding of Qt and QML drawing interfaces, this post reflects that journey including showing your grouped QML properties exposed from C++.

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With all the controls in it, the program looks like:

qml_circle_all.gif

In this post I'm going to show you QML Profiler screenshots. These are shown as a comparison between the 3 methods on my specific machine. Don't treat them as a benchmark, do that yourself if you experience performance issues.

The QML program has a grid with all the controls in it. Per section I'm commenting out all but one and then taking a screenshot. Without any controls, so just the grid, the QML profiler looks like this:

QML profiler empty

This article also shows how to expose a Grouped QML Property. This allows you to set border.width and border.color in your custom C++ exposed QML control.

For this example I'm using Qt 5.15, the code also works with Qt 6.4.

QML Rectangle Circle

The simplest and most basic method to get a Circle in QML is a Rectangle with a radius property set to width / 2 or 180:

Rectangle {
   width: 150
   height: 150
   color: "dodgerblue"
   radius: 180
}

Which looks like:

rectangle circle

The QML Profiler shows that this is quite fast:

rectangle circle profiler

The rectangle drawing method has a few advantages, namely that you get a border, a fill color and all the other advantages that this QML Control gives you. Later on in the C++ example you'll see that we need to provide that all ourselves.

QML Canvas Circle

Most of the examples you'll find online regarding a Circle in QML refer to the Canvas or Shapes API. The Canvas API is a JavaScript way to draw stuff on screen. The disadvantage is that this requires a lot more memory and resources, but you do get a lot of flexibility.

My QMLCircle.qml file is pasted at the end of this post and supports being used as a Pie chart. One section is filled in with one color and a different section is filled with another color, including an Animation so that it looks nice. Useful for a Chart-like graphics.

The basic circle looks the same as the Rectangle-with-radius circle:

QMLCircle {
  primaryColor: "skyblue"
}

qml circle

The QML profiler shows that a lot more is happening:

qml circle profiler

Is this slower? Probably, and almost certainly on embedded deviced

When using the 'pie-chart' feature it looks like this:

QMLCircle {
    primaryColor: "skyblue"
    secondaryColor: "tomato"
    value: 0.87
}

qml circle filled

Using it combined with the Animation is also quite cool:

qml_circle filled.gif

property int timerDuration: 10
property int timerSecDone: 0  

QMLCircle {
  value: (timerSecDone * 100 / timerDuration) / 100
  primaryColor: "skyblue"
  secondaryColor: "tomato"
}

Timer {
  interval: 1000
  running: true
  triggeredOnStart: true
  repeat: true
  onTriggered: {
    timerSecDone++
    if (timerSecDone > timerDuration+1) {
      timerSecDone = 0;
    }
  }
}

Using Canvas gives you a bunch of flexibility, this Animation was really easy and quick to add. (Especially in comparison to the C++ style in the next section). In one of the Coffee Machines we make at work almost this exact code is used in one of the UI's to show a progress bar of the consumption status. It includes a bit more 'fancyness', styling, animation, but it boils down to the same code. I know because I wrote it.

C++ QML Circle

The last example I want to show is a C++ based Circle. It uses the Qt Drawing API

I've coded up a basic QQuickPaintedItem which draws a Circle. It has two properties, color and antialiassing. The latter is to make it look smooth, the first is for the fill color. It looks like this:

CppCircle {
  width: 150
  height: 150
  color: "greenyellow"
  antialiasing: true
  border.color: "black"
  border.width: 1
}

CppCircle {
  width: 150
  height: 150
  color: "hotpink"
  antialiasing: false
  border.color: "black"
  border.width: 5
}

cpp circle

The second (hotpink) circle is not anti-aliassed. The basic code is a class derived from QQuickPaintedItem. The most important method is the paint method:

void CppCircle::paint(QPainter *painter)
{
    // make it smooth
    if(antialiasing())
        painter->setRenderHint(QPainter::Antialiasing);

    // create rect which will be used to draw circle in
    QRectF rect(0 + border()->width(), 0 + border()->width(), width() - 1 - (border()->width()*2), height() - 1 - (border()->width()*2));

    // create brush based on QML color property
    QBrush brush(m_color);
    // use brush to fill figures
    painter->setBrush(brush);


    // create pen
    QPen pen;
    if(border()->width() > 0) {
        pen.setBrush(border()->color());
        pen.setWidth(border()->width());
        pen.setStyle(Qt::SolidLine);
    }
    else {
        pen.setStyle(Qt::NoPen);
    }
    painter->setPen(pen);


    // Draw the circle
    painter->drawEllipse(rect);
}

The full code is at the end of the article but the gist should be clear. A Pen is used to draw lines and outlines, a Brush is used to fill figures. The full code at the end of this page shows border as a QML Property Group so that I can set border.width and border.color.

The QML profiler seems to show that this custom control is even faster than the Rectangle (which was 28 microseconds compared to 21):

cpp circle profiler

The C++ method is the fastest of the bunch, but it is also the most limited. Adding the border property took way more time than the Animation in the Javascript QML Control, so this is a tradeoff you need to make for yourself.

Full source code

Here below you'll find the full source code for the program. First an animated gif of the full program. Any artifacts or stuttering are caused by the recording software. It's super smooth on my local machine.

qml_circle_all.gif

main.qml

/*
 * Copyright (c) 2023 Remy van Elst
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, version 3.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 *
 */


import QtQuick 2.15
import QtQuick.Controls 2.15
import QtQuick.Window 2.15
import org.raymii.shapes 1.0

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("QML Circles demo by Raymii.org")

    property int timerDuration: 10
    property int timerSecDone: 0


    Grid {
        anchors.fill: parent
        anchors.margins: 10
        spacing: 10
        columns: 3
        rows: 3

        Rectangle {
            id: rectCircle
            width: 150
            height: 150
            color: "dodgerblue"
            radius: 180
        }

        QMLCircle {
            primaryColor: "skyblue"
            secondaryColor: "tomato"
            value: 0.87
        }

        CppCircle {
            width: 150
            height: 150
            color: "greenyellow"
            antialiasing: true
            border.color: "black"
            border.width: 1
        }

        CppCircle {
            width: 150
            height: 150
            color: "hotpink"
            antialiasing: false
            border.color: "black"
            border.width: 5
        }



        QMLCircle {
            value: (timerSecDone * 100 / timerDuration) / 100
            primaryColor: "skyblue"
            secondaryColor: "tomato"
        }

        QMLCircle {
            value: 0.15
        }
    }


    Timer {
        interval: 1000
        running: true
        triggeredOnStart: true
        repeat: true
        onTriggered: {
            timerSecDone++
            if (timerSecDone > timerDuration+1) {
                timerSecDone = 0;
            }
        }
    }   
}

main.cpp

/*
 * Copyright (c) 2023 Remy van Elst
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, version 3.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 *
 */
#include "cppcircle.h"

#include <QGuiApplication>
#include <QQmlApplicationEngine>


int main(int argc, char *argv[])
{
#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;

    qmlRegisterType<CppCircle>("org.raymii.shapes", 1, 0, "CppCircle");
    // You MUST make this type know to QML otherwise you'll receive an error:
    // Invalid grouped property access: Property "border" with type "BorderGroupedProperty*", which is not a value type
    qmlRegisterType<BorderGroupedProperty>("org.raymii.shapes", 1, 0, "BorderGroupedProperty");

    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
        &app, [url](QObject *obj, const QUrl &objUrl) {
            if (!obj && url == objUrl)
                QCoreApplication::exit(-1);
        }, Qt::QueuedConnection);
    engine.load(url);
    return app.exec();
}

cppCircle.h

/*
 * Copyright (c) 2023 Remy van Elst
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, version 3.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 *
 */

#pragma once

#include <QObject>
#include <QQuickPaintedItem>
#include <QPainter>


class BorderGroupedProperty : public QObject
{
    Q_OBJECT
    Q_PROPERTY(int width READ width WRITE setWidth NOTIFY widthChanged)
    Q_PROPERTY(QColor color READ color WRITE setColor NOTIFY colorChanged)
public:
    BorderGroupedProperty(QObject* parent = nullptr);
    int width() const;
    void setWidth(int newWidth);
    QColor color() const;
    void setColor(const QColor &newColor);

signals:
    void widthChanged();
    void colorChanged();

private:
    int m_width = 0;
    QColor m_color = QColor(0,0,0);
};



class CppCircle : public QQuickPaintedItem
{
    Q_OBJECT
    Q_PROPERTY(QColor color READ color WRITE setColor NOTIFY colorChanged)
    Q_PROPERTY(bool antialiasing READ antialiasing WRITE setAntialiasing NOTIFY antialiasingChanged)
    Q_PROPERTY(BorderGroupedProperty* border READ border)

public:
    explicit CppCircle(QQuickItem *parent = nullptr);
    virtual void paint(QPainter *painter);
    QColor color() const;
    void setColor(const QColor &newColor);
    bool antialiasing() const;
    void setAntialiasing(bool newAntialiasing);
    BorderGroupedProperty *border() const;

signals:
    void colorChanged();
    void antialiasingChanged();

private:
    QColor m_color;
    bool m_antialiasing;
    BorderGroupedProperty *m_border = nullptr;
};

cppCircle.cpp

/*
 * Copyright (c) 2023 Remy van Elst
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, version 3.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 *
 */

#include "cppcircle.h"
#include <QPen>

CppCircle::CppCircle(QQuickItem *parent)
    : QQuickPaintedItem{parent}, m_border(new BorderGroupedProperty(this))
{

}


QColor CppCircle::color() const
{
    return m_color;
}

void CppCircle::setColor(const QColor &newColor)
{
    if (m_color == newColor)
        return;
    m_color = newColor;
    emit colorChanged();
}


bool CppCircle::antialiasing() const
{
    return m_antialiasing;
}

void CppCircle::setAntialiasing(bool newAntialiasing)
{
    if (m_antialiasing == newAntialiasing)
        return;
    m_antialiasing = newAntialiasing;
    emit antialiasingChanged();
}


void CppCircle::paint(QPainter *painter)
{
    // make it smooth
    if(antialiasing())
        painter->setRenderHint(QPainter::Antialiasing);

    // create rect which will be used to draw circle in
    QRectF rect(0 + border()->width(), 0 + border()->width(), width() - 1 - (border()->width()*2), height() - 1 - (border()->width()*2));

    // create brush based on QML color property
    QBrush brush(m_color);
    // use brush to fill figures
    painter->setBrush(brush);


    // create pen
    QPen pen;
    if(border()->width() > 0) {
        pen.setBrush(border()->color());
        pen.setWidth(border()->width());
        pen.setStyle(Qt::SolidLine);
    }
    else {
        pen.setStyle(Qt::NoPen);
    }
    painter->setPen(pen);


    // Draw the circle
    painter->drawEllipse(rect);
}



BorderGroupedProperty::BorderGroupedProperty(QObject *parent) : QObject(parent)
{

}

int BorderGroupedProperty::width() const
{
    return m_width;
}

void BorderGroupedProperty::setWidth(int newWidth)
{
    if (m_width == newWidth)
        return;
    m_width = newWidth;
    emit widthChanged();
}

QColor BorderGroupedProperty::color() const
{
    return m_color;
}

void BorderGroupedProperty::setColor(const QColor &newColor)
{
    if (m_color == newColor)
        return;
    m_color = newColor;
    emit colorChanged();
}

BorderGroupedProperty *CppCircle::border() const
{
    return m_border;
}

QMLCircle.qml

/*
 * Copyright (c) 2023 Remy van Elst
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, version 3.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 *
 */

import QtQuick 2.15

Item {
    id: root
    property int size: 150
    property real value: 0
    property color primaryColor: "#ff6725"
    property color secondaryColor: "#52adff"
    property int animationTime: 1000
    width: size
    height: size

    onValueChanged: c.degree = value * 360

    Canvas {
        id: c
        property real degree: 0

        anchors.fill: parent
        antialiasing: true
        onDegreeChanged: requestPaint()

        onPaint: {
            var ctx = getContext("2d");

            var x = root.width / 2;
            var y = root.height / 2;

            var radius = root.size / 2
            var startAngle = (Math.PI / 180) * 270;
            var fullAngle = (Math.PI / 180) * (270 + 360);
            var progressAngle = (Math.PI / 180) * (270 + degree);

            ctx.reset()

            ctx.fillStyle = root.secondaryColor;
            ctx.beginPath();
            ctx.moveTo(x,y);
            ctx.arc(x, y, radius-1, startAngle, fullAngle);
            ctx.lineTo(x, y)
            ctx.fill();

            ctx.fillStyle = root.primaryColor;
            ctx.beginPath();
            ctx.moveTo(x,y);
            ctx.arc(x, y, radius, startAngle, progressAngle);
            ctx.lineTo(x, y)
            ctx.fill();
        }

        Behavior on degree {
            NumberAnimation {
                duration: root.animationTime
            }
        }
    }
}
Tags: articles , c++ , cpp , debugging , development , javascript , performance , qml , qt , qt5 , qt6