Help with QPainter arc math

[sammonius]

I'm trying to make a ball-character that shows scrolling arcs when it moves to create the illusion of a 3d ball. The ball is drawn as an 80x80 pixmap with this code, (f is a global variable that refers to the frame count):

QPixmap *Character::getPixmap(){
	QPixmap *pixmap = new QPixmap(80, 80);
	pixmap->fill(Qt::transparent);
	
	QPainter painter(pixmap);
	painter.setRenderHint(QPainter::Antialiasing);
	painter.setRenderHint(QPainter::SmoothPixmapTransform);
	
	painter.setBrush(QBrush(QColor(255, 255, 255));
	painter.drawEllipse(QRectF(0, 0, 79, 79));
	
	for(int i = (f % 10); i <= 80; i += 10){
		if(i<50) painter.drawArc(0, i, 79, 80-i*1.5, 0, 180*16);
		else painter.drawArc(0, 50-(i-50), 79, (i-55)*2, 180*16, 180*16);
	}
	
	painter.end();
	return pixmap;
}

The main loop draws a green background and then the character at a rate of about 30fps, and the result looks like this: <YouTube link>

Can anyone help me adjust the arc animation? (It doesn't really matter how many arcs there are, although a version that uses a macro to determine the arc number would be very appreciated!)

[sammonius]

Sorry, I should have provided some code to test it.

#include <QtWidgets>

unsigned long f = 0;

class Character : public QWidget {
public:
	Character();
	~Character();
	
	QPixmap *getPixmap();
protected:
	void paintEvent(QPaintEvent *e);
};

Character::Character() : QWidget(){}
Character::~Character(){}

void Character::paintEvent(QPaintEvent *event){
	Q_UNUSED(event);

	QPainter painter(this);
	painter.setRenderHint(QPainter::Antialiasing);
	painter.setRenderHint(QPainter::SmoothPixmapTransform);
	
	painter.setBrush(QBrush(QColor(255, 255, 255)));
	painter.drawEllipse(QRectF(0, 0, 79, 79));
	
	for(int i = (f % 10); i <= 80; i += 10){
		if(i<50) painter.drawArc(0, i, 79, 80-i*1.5, 0, 180*16);
		else painter.drawArc(0, 50-(i-50), 79, (i-55)*2, 180*16, 180*16);
	}
	
	painter.end();
	f++;
}

int main(int argc, char *argv[]){
	QApplication app(argc, argv);
	Character *c = new Character();
	
	QTimer t;
	QObject::connect(&t, &QTimer::timeout, c, static_cast<void(QWidget::*)()>(&QWidget::update));
	t.start(30);
	
	c->show();
	return app.exec();
}