CODEWITHSUNDEEP
openglgeometry
jomo
jomo

Reputation: 11

Drawing in OpenGL

How can I draw a half circle in OpenGl? I have tried this :

float step=5.0;
glBegin(GL_LINE_STRIP);
 for(float angle=0.0f; angle <= 360; angle+=step)
 {
   float rad  = 2*angle/180;
   x  = radius*sin(rad);
   y  = radius*cos(rad);
   glVertex3f(x,y,0.0f);

}

glEnd();

but I obtained a half circle that is not on a straight line..it is inclined.How can i resolve this?

Upvotes: 1

Views: 5226

Answers (3)

Andon M. Coleman
Andon M. Coleman

Reputation: 43319

Your question is hard to understand, particularly the last part.

If I understand correctly, what you are trying to say is that you are drawing something like this:


(source: mathopenref.com)

But what you want is this:


(source: js at abyss.uoregon.edu)

The reason for this is that you are using a line strip, which connects each line segment but does not create a segment that loops back to the first point. Use GL_LINE_LOOP instead, and you will have a half-circle that is "on a straight line."

Upvotes: 3

Sefo Noaman
Sefo Noaman

Reputation: 36

those 2 functions draw 2 half circles, 1 is full and another is hollow.(in c++)

void drawLeftCircle()   // the filled one
{
    float radius = 70;
    float twoPI = 2 * PI;

    glBegin(GL_TRIANGLE_FAN);

    for (float i = PI; i <= twoPI; i += 0.001)
        glVertex2f((sin(i)*radius), (cos(i)*radius));

    glEnd();
    glFlush();
}


void drawRightHalfCircle()  // the empty one
{
    float radius = 70;
    float twoPI = 2 * PI;

    glBegin(GL_POINTS);

    for (float i = 0.0; i <= twoPI / 2; i += 0.001)
        glVertex2f((sin(i)*radius), (cos(i)*radius));

    glEnd();
    glFlush();
}

Upvotes: 0

rems4e
rems4e

Reputation: 3172

Your conversion fro degrees to radians is wrong, you have to multiply your degree value by PI/180 to get the correct value. And to obtain a "up facing" circle, swap your usage of sin and cos functions.

Upvotes: 1

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