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IshaqKassam
IshaqKassam

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Drawing a Circle using OpenGL - glfw/glew c++

Hoping that you have already setup glfw & glew, and are ready to learn how to draw 2D shapes, let's dive straight into this. In this article we are going to learn how to draw a circle in openGL using glfw, c++ language.

Here is the code:


void drawCircle(float cx, float cy, float r, int num_segments)
{
    float theta = 3.1415926 * 2 / float(num_segments);
    float tangetial_factor = tanf(theta);//calculate the tangential factor 

    float radial_factor = cosf(theta);//calculate the radial factor 

    float x = r;//we start at angle = 0 

    float y = 0;
    glLineWidth(2);
    glBegin(GL_LINE_LOOP);
    for (int ii = 0; ii < num_segments; ii++)
    {
        glVertex2f(x + cx, y + cy);//output vertex 

        //calculate the tangential vector 
        //remember, the radial vector is (x, y) 
        //to get the tangential vector we flip those coordinates and negate one of them 

        float tx = -y;
        float ty = x;

        //add the tangential vector 

        x += tx * tangetial_factor;
        y += ty * tangetial_factor;

        //correct using the radial factor 

        x *= radial_factor;
        y *= radial_factor;
    }
    glEnd();
}
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The function above can then be called in the main() function as such:

drawCircle(250, 250, 100, 360);

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To change the circle color, you can use the glColor3f() method which takes in 3 parameters, for the RGB values.

Add this line before calling the drawCircle function

glColor3f(0.0, 0.5, 0.5);
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