如何用Qt QPainter绘制线性渐变弧?

时间:2014-04-30 18:03:44

标签: qt linear-gradients qpainter

我正在尝试开发一个类似下图的自定义QProgressBar

enter image description here

我创建了一个扩展QProgressBar的类并实现了paintEvent():

void CircularProgressBar::paintEvent(QPaintEvent*) {

int progress = this->value();
int progressInDegrees = (double)(progress*360)/100;

int barWidth = 20;

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

painter.setPen(QPen(Qt::black, barWidth, Qt::SolidLine,Qt::RoundCap));

painter.drawArc(barWidth/2, barWidth/2, this->width() - barWidth, this->height() - barWidth,
                90*16, progressInDegrees*-16);}

这很适合绘制圆形进度条,但是我遇到了条形线性渐变颜色的问题。我尝试使用QPen对象创建QLinearGradient,并尝试将QPainter画笔设置为QLinearGradient对象,但两种策略均无效。是否可以绘制具有线性渐变颜色的QPainter的圆弧?

2 个答案:

答案 0 :(得分:9)

我知道这是一个老问题,但我几天前遇到过它,我想我有一个解决方案。您想要的是创建一个圆锥形渐变并剪切您想要用作圆形加载栏的磁盘。这是一个例子:

widget.h:

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

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 <QPaintEvent>
#include <QPainter>
#include <QConicalGradient>
#include <QPen>

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中:

#include "widget.h"
#include <QApplication>

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

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

    return a.exec();
}

结果是:

enter image description here

当然,这不是完整而准确的解决方案,但我认为这是您需要知道的一切,以实现您想要的。其余的细节并不难实现。

答案 1 :(得分:4)

这个解决方案并不完全是你所追求的;渐变从上到下,而不是围绕圆圈:

#include <QtWidgets>

class Widget : public QWidget
{
public:
    Widget() {
        resize(200, 200);
    }

    void paintEvent(QPaintEvent *) {
        QPainter painter(this);
        painter.setRenderHint(QPainter::Antialiasing);

        const QRectF bounds(0, 0, width(), height());
        painter.fillRect(bounds, "#1c1c1c");

        QPen pen;
        pen.setCapStyle(Qt::RoundCap);
        pen.setWidth(20);

        QLinearGradient gradient;
        gradient.setStart(bounds.width() / 2, 0);
        gradient.setFinalStop(bounds.width() / 2, bounds.height());
        gradient.setColorAt(0, "#1c1c1c");
        gradient.setColorAt(1, "#28ecd6");

        QBrush brush(gradient);
        pen.setBrush(brush);
        painter.setPen(pen);

        QRectF rect = QRectF(pen.widthF() / 2.0, pen.widthF() / 2.0, width() - pen.widthF(), height() - pen.widthF());
        painter.drawArc(rect, 90 * 16, 0.65 * -360 * 16);
    }
};

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

    Widget w;
    w.show();

    return app.exec();
}

然而,一个带有线性渐变的弧! :P