绘制路径

路径 QPainterPath,先引用一段 Qt 帮助文档里对路径的描述吧:

A painter path is an object composed of a number of graphical building blocks, such as rectangles, ellipses, lines, and curves. Building blocks can be joined in closed subpaths, for example as a rectangle or an ellipse. A closed path has coinciding start and end points. Or they can exist independently as unclosed subpaths, such as lines and curves.

A QPainterPath object can be used for filling, outlining, and clipping.

也就是说,路径可以由多个图形组成,例如矩形、椭圆、线、曲线、贝塞尔曲线等,一个路径可以和另一个路径合并,也可以从一个路径里扣掉另一个路径,路径可以用来创建复杂的图形,也可以用来限制绘图的区域实现特殊的效果等。

组合路径

下面的程序画了一个很奇怪的图形,用来演示 QPainterPath 添加线、贝塞尔曲线、矩形、扣去其他 QPainterPath 等,只要发挥你的想像,就可以用 QPainterPath 组合出很多复杂有趣的图形:

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void ComplicatedPathWidget::paintEvent(QPaintEvent *) {
QPainter painter(this);
painter.setRenderHint(QPainter::Antialiasing);
QPainterPath path;
path.lineTo(100, 0);
path.cubicTo(200, 0, 100, 50, 200, 100); // 贝塞尔曲线
path.closeSubpath(); // 画一条线到路径的第一个点,闭合路径
path.addRect(50, 50, 50, 50); // 加一个矩形
QPainterPath path2;
path2.addEllipse(80, 30, 50, 50);
path = path.subtracted(path2); // 扣掉圆
painter.translate(20, 20);
painter.setBrush(Qt::lightGray);
painter.drawPath(path);
painter.setBrush(Qt::NoBrush);
painter.drawRect(path.boundingRect());
}

渐变填充文字

QPainterPath 还可以描绘和填充文字,这种效果在开发音乐播放器时,显示歌词常用到:

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void TextPathWidget::paintEvent(QPaintEvent *) {
QPainter painter(this);
painter.setRenderHint(QPainter::Antialiasing);
QString text("jEh 太极");
QFont font("Xingkai SC", 120, QFont::Bold);
QFontMetrics metrics(font);
int x1 = 0;
int y1 = 0;
int x2 = 0;
int y2 = y1 + metrics.height();
// 使用渐变填充
QLinearGradient gradient(x1, y1 + 40, x2, y2);
gradient.setColorAt(0, QColor(255, 255, 255));
gradient.setColorAt(1, QColor(50, 50, 50));
// 用文本创建 QPainterPath,第一、二个参数是 baseline 的坐标
QPainterPath path;
path.addText(x1, y1 + metrics.ascent(), font, text);
painter.translate(20, -20);
painter.setBrush(gradient);
painter.setPen(QPen(QColor(80, 80, 80), 2));
painter.drawPath(path);
}

由于字体的原因,计算出来的结果不够理想,做了一些微调,前面我们也提到过,字体不同时,会有细微的差别,为了实现更好的效果,有时需要手动微调一下,这里就遇到了这种情况。

沿路径运动

QPainterPath::pointAtPercent(qreal t) 是一个很有用的函数,t 的值为 [0, 1.0],可以取得路径上任意一点的坐标,在动画一节里会使用这个函数来实现动画的插值函数,让物体沿着任意的路径运动,这里没有用 Qt 的动画框架实现了让物体沿着任意的路径运动:

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#ifndef ANIMATIONALONGPATHWIDGET_H
#define ANIMATIONALONGPATHWIDGET_H
#include <QWidget>
#include <QPainterPath>
class AnimationAlongPathWidget : public QWidget {
Q_OBJECT
public:
explicit AnimationAlongPathWidget(QWidget *parent = 0);
~AnimationAlongPathWidget();
protected:
void timerEvent(QTimerEvent *event) Q_DECL_OVERRIDE;
void paintEvent(QPaintEvent *event) Q_DECL_OVERRIDE;
private:
QPainterPath path;
float percent;
float step;
int timerId;
};
#endif // ANIMATIONALONGPATHWIDGET_H
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#include "AnimationAlongPathWidget.h"
#include <QPainter>
#include <QtGlobal>
#include <QTimerEvent>
AnimationAlongPathWidget::AnimationAlongPathWidget(QWidget *parent) : QWidget(parent) {
step = 0.02;
percent = 0;
// 构造一个任意的曲线
path.cubicTo(50, 0, 30, 100, 100, 100);
path.cubicTo(150, 100, 250, 0, 200, 100);
path.cubicTo(150, 100, 250, 0, 300, 140);
path.quadTo(150, 310, 150, 100);
// 启动定时器
timerId = startTimer(60);
}
AnimationAlongPathWidget::~AnimationAlongPathWidget() {
}
void AnimationAlongPathWidget::timerEvent(QTimerEvent *event) {
// 不停的更新 percent 的值,刷新界面,实现动画效果
if (event->timerId() == timerId) {
percent += step;
if (percent < 0 || percent > 1) {
step = -step; // 运动方向取反
percent = qMin(percent, 1.0F);
percent = qMax(percent, 0.0F);
}
update();
}
}
void AnimationAlongPathWidget::paintEvent(QPaintEvent *) {
QPainter painter(this);
painter.setRenderHint(QPainter::Antialiasing);
painter.translate(20, 20);
painter.drawPath(path);
painter.setBrush(Qt::red);
painter.drawEllipse(path.pointAtPercent(percent), 4, 4);
}