## DEV Community

IshaqKassam

Posted on • Updated on

# 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 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;

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

}
glEnd();
}
``````

The function above can then be called in the main() function as such:

``````drawCircle(250, 250, 100, 360);

``````

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);
``````  