How to draw grid line only in a circle using QGraphicsItem

Question

Please forgive the poor English in advance.

hello! I am currently implementing view widget using QGraphicsView & QGraphicsItem.

Is there any way to draw gridlines only inside a circle?

Rectangles are fine, but trying to draw them inside a circle is a pain.

Below is sample code.

    for(int x = rect.left(); x <= rect.right(); ++x) {
        painter->drawLine(QPointF(x, rect.top()), QPointF(x, rect.bottom()));
    }

    for(int y = rect.top(); y <= rect.bottom(); ++y) {
        painter->drawLine(QPointF(rect.left(), y), QPointF(rect.right(), y));
    }

here is my current state --> Current grid line result

result what i want is looks like --> Reuslt example what I want

I don't want to show grid line out of circle bound.

If you have any good ideas, please reply.

thank you!

Answer 1

It's just a matter of tigronometry calculation. From the position on the X or Y axis you can calculate the angle to go to the point on the circle using arc sin and arc cosine. The code below should work

#include <math.h>

class Point
{
public:
    double X = 0.0;
    double Y = 0.0;
};

int main() 
{
    double radius = 10.0;
    double stepx = 2.0;
    double stepy = 2.0;
    
    int stepsInX = radius / stepx;
    int stepsInY = radius / stepy;

    // this is just for positive X (need another loop for negative X)
    for (int i = 0; i <= stepsInX; i++)
    {
        double angle = acos(i*stepx / radius);
        double y = radius * sin(angle);
        Point p1;
        p1.X = i * stepx;
        p1.Y = y;
        Point p2;
        p2.X = i * stepx;
        p2.Y = -y;
        drawLine(p1, p2);
    }
    // this is just for positive Y (need another loop for negative Y)
    for (int i = 0; i <= stepsInY; i++)
    {
        double angle = asin(i * stepy / radius);
        double x = radius * cos(angle);
        Point p1;
        p1.X = x;
        p1.Y = i * stepy;
        Point p2;
        p2.X = -x;
        p2.Y = i * stepy;
        drawLine(p1, p2);
    }
}

void drawLine(Point const& p1, Point const& p2)
{
    // your code to draw line here
}