How do I rotate a polygon around its center on mouse click? - opengl

Question

I want to find out how I would be able to rotate my polygon around its center when I click a mouse button?
glBegin(GL_POLYGON); //drawing a pentagon and coloring it.(using a polygon GL)
glColor3f(0, 0, 1);//Blue
glVertex3f(0.0f, 0.7f, 0.0f);//top point
glColor3f(1, 0.5, 0);//Orange
glVertex3f(-0.5f, 0.3f, 0.0f);//left middle point
glColor3f(0, 1, 0);//Green
glVertex3f(-0.3f, -0.3f, 0.0f);//bottom left point
glColor3f(0.53, 0.12, 0.47);//Purple
glVertex3f(0.3f, -0.3f, 0.0f);//bottom right point
glColor3f(1, 0, 0);//red
glVertex3f(0.5f, 0.3f, 0.0f);//middle right point
glEnd();
glutSwapBuffers();//Sends the Scene to the window
}
void mouse(int button, int state, int x, int y){
if (button == GLUT_RIGHT_BUTTON){
glRotatef(angle, 0, 0, 1);
}
}

Answer 1

The easiest method is to store the rotation angle in a global variable. This would then look like this:
global:
float angle = 0;
draw:
glRotate(angle, 0, 0, 1);
glBegin(GL_POLYGON);
...
void mouse(int button, int state, int x, int y){
if (button == GLUT_RIGHT_BUTTON)
angle += A_VALUE_PER_CLICK;
}
Edit: Note, that that the above rotates whenever you click (as asked). If you want to rotate the object as long as the mouse is pressed you'll have to do it a little bit different:
global:
bool is_mouse_down = false;
float angle = 0;
draw:
if (is_mouse_down)
angle += ANGLE_PER_FRAME;
glRotate(angle, 0, 0, 1);
glBegin(GL_POLYGON);
...
void mouse(int button, int state, int x, int y){
if (button == GLUT_RIGHT_BUTTON) {
if (state == GLUT_DOWN)
is_mouse_down = true;
if (state == GLUT_UP)
is_mouse_down = false;
}
}
Edit 2: To spin about a specific point (in this case the center of the polygon [0, 0.2, 0]), one always has to translate this point to the origin, then rotate and then translate it back. This means one has to use something like this code (the signs might be flipped):
draw:
glTranslate(0, 0.2, 0);
glPushMatrix();
glRotate(angle, 0, 0, 1);
glPushMatrix();
glTranslate(0, -0.2, 0);
//Draw
glPopMatrix();
glPopMatrix();