How to draw a linear gradient arc with Qt QPainter?

Question

I\'m trying to develop a custom QProgressBar that will look like the following image :

Answer 1

I know this is an old question but I came across it some days ago and I think I have a solution. What you want is to create a conical gradient and clip the disk you want to use as circular loading bar. Here is an example:

widget.h:

#ifndef WIDGET_H
#define WIDGET_H

#include 

class QPaintEvent;

class Widget : public QWidget
{
    Q_OBJECT

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

    void setLoadingAngle(int loadingAngle);
    int loadingAngle() const;

    void setDiscWidth(int width);
    int discWidth() const;

protected:
    void paintEvent(QPaintEvent *);

private:
    int m_loadingAngle;
    int m_width;
};

#endif // WIDGET_H

widget.cpp:

#include "widget.h"

#include 
#include 
#include 
#include 

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    m_loadingAngle(0),
    m_width(0)
{
}

Widget::~Widget()
{
}

void Widget::setLoadingAngle(int loadingAngle)
{
    m_loadingAngle = loadingAngle;
}

int Widget::loadingAngle() const
{
    return m_loadingAngle;
}

void Widget::setDiscWidth(int width)
{
    m_width = width;
}

int Widget::discWidth() const
{
    return m_width;
}

void Widget::paintEvent(QPaintEvent *)
{
    QRect drawingRect;
    drawingRect.setX(rect().x() + m_width);
    drawingRect.setY(rect().y() + m_width);
    drawingRect.setWidth(rect().width() - m_width * 2);
    drawingRect.setHeight(rect().height() - m_width * 2);

    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);

    QConicalGradient gradient;
    gradient.setCenter(drawingRect.center());
    gradient.setAngle(90);
    gradient.setColorAt(0, QColor(178, 255, 246));
    gradient.setColorAt(1, QColor(5, 44, 50));

    int arcLengthApproximation = m_width + m_width / 3;
    QPen pen(QBrush(gradient), m_width);
    pen.setCapStyle(Qt::RoundCap);
    painter.setPen(pen);
    painter.drawArc(drawingRect, 90 * 16 - arcLengthApproximation, -m_loadingAngle * 16);
}

main.cpp:

#include "widget.h"
#include 

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    Widget w;
    w.setDiscWidth(20);
    w.setLoadingAngle(270);
    w.show();

    return a.exec();
}

And the result is:

Of course, it is not the complete and exact solution but I think it is everything you need to know in order to achieve what you want. The rest are details not hard to implement.