Task

Write a function to draw a HSV color wheel completely with code.

This is strictly for learning purposes only. It's highly recommended that you use an image in an actual application to actually draw the color wheel   (as procedurally drawing is super slow). This does help you understand how color wheels work and this can easily be used to determine a color value based on a position within a circle.



// colorwheelwidget.cpp
#include "colorwheelwidget.h"
#include <QPainter>
#include <QPaintEvent>
#include <cmath>
 
namespace {
 
QColor hsvToRgb(int h, double s, double v) {
    double hp = h/60.0;
    double c = s * v;
    double x = c * (1 - std::abs(std::fmod(hp, 2) - 1));
    double m = v - c;
    double r = 0, g = 0, b = 0;
    if (hp <= 1) {
        r = c;
        g = x;
    } else if (hp <= 2) {
        r = x;
        g = c;
    } else if (hp <= 3) {
        g = c;
        b = x;
    } else if (hp <= 4) {
        g = x;
        b = c;
    } else if (hp <= 5) {
        r = x;
        b = c;
    } else {
        r = c;
        b = x;
    }
    r += m;
    g += m;
    b += m;
    return QColor(r * 255, g * 255, b * 255);
}
 
}
 
ColorWheelWidget::ColorWheelWidget(QWidget *parent)
    : QWidget(parent) {
    setWindowTitle(tr("Color Wheel"));
    resize(400, 400);
}
 
void ColorWheelWidget::paintEvent(QPaintEvent *event) {
    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);
    const QColor backgroundColor(0, 0, 0);
    const QColor white(255, 255, 255);
    painter.fillRect(event->rect(), backgroundColor);
    const int margin = 10;
    const double diameter = std::min(width(), height()) - 2*margin;
    QPointF center(width()/2.0, height()/2.0);
    QRectF rect(center.x() - diameter/2.0, center.y() - diameter/2.0,
                diameter, diameter);
    for (int angle = 0; angle < 360; ++angle) {
        QColor color(hsvToRgb(angle, 1.0, 1.0));
        QRadialGradient gradient(center, diameter/2.0);
        gradient.setColorAt(0, white);
        gradient.setColorAt(1, color);
        QBrush brush(gradient);
        QPen pen(brush, 1.0);
        painter.setPen(pen);
        painter.setBrush(brush);
        painter.drawPie(rect, angle * 16, 16);
    }
}