Draw and Rotate an Arrow

Question

I am trying to draw an arrow using the style below but this arrow should be also rotated according to a condition (need to pass the degree for each condition). I can draw the rectangle and a triangle but I cannot draw the triangle as an arrowhead. Also, how can I include the rotation degree into the code? Is there any easier way to draw an arrow and rotate it?

int triangleRect=4, triangleTri=3, lineWidth=3;
double twicePi = 2.0f * M_PI, angle_offsetR =1.5* M_PI/2, radius = 0.05,
       xR=m_start.x(), y=m_start.y(), xT=m_start.x()+ m_rect_width;


glColor3f(0,1,0);
glLineWidth(lineWidth);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_MULTISAMPLE);
glEnable(GL_POINT_SMOOTH);
glEnable(GL_LINE_SMOOTH);
glBegin(GL_TRIANGLE_FAN);

//RECTANGLE
for(int i = 0; i <= triangleRect; i++) {
    glVertex2f((xR + (radius * cos(i *  twicePi / triangleRect + angle_offsetR)))* m_parent_width_function(),
               (y + (radius * sin(i * twicePi / triangleRect + angle_offsetR)))* m_parent_height_function());
}
// TRIANGLE
for(int i = 0; i <= triangleTri; i++) {
    glVertex2f((xT + (radius * cos(i * twicePi / triangleTri + angle_offsetR)))* m_parent_width_function(),
               (y + (radius * sin(i * twicePi / triangleTri + angle_offsetR)))* m_parent_height_function());
}

glEnd();

Answer 1

You accidentally add angle_offsetR to the angle for the triangle vectors. Furthermore you've to restart a GL_TRIANGLE_FAN primitiv when you draw an new shape (see Triangle primitives).
If you want to rotate the model, then set the add a rotation around the z axis to the model view matrix by glRotatef.
Do not translate and scale the vertex coordinates. Use glScale and glTranslate. The matrix transformations are not commutative, the order matters:

float angle_of_roation = 30.0; // 30°

glPushMatrix();
// scale
glScalef( m_parent_width_function(), m_parent_height_function(), 1.0f);
// move triangle and rectangle to the position in the world
glTranslatef(xR, y, 0.0f);
// roatate triangle and rectangle
glRotatef(angle_of_roation, 0, 0, 1);
              
//RECTANGLE
glBegin(GL_TRIANGLE_FAN);
for(int i = 0; i <= triangleRect; i++) {
    float angle = i * twicePi / triangleRect + angle_offsetR;
    glVertex2f(radius * cos(angle), radius * sin(angle));
}
glEnd();

glPushMatrix();
// translate triangle relative to rectangle
glTranslatef(xT-xR, 0.0f, 0.0f); 
    
// TRIANGLE
glBegin(GL_TRIANGLE_FAN);
for(int i = 0; i <= triangleTri; i++) {
    float angle = i * twicePi / triangleTri;
    glVertex2f(radius * cos(angle), radius * sin(angle));
}
glEnd();

glPopMatrix();
glPopMatrix();