Question

I made a fancy shape with OpenGL and I draw that shape this function:

drawShape(const Point & center, char radius, int points, int rotation)

Inside the function I have code that tells OpenGL where the vertexes are:

glBegin(GL_LINE_LOOP);
  glColor3f(1.0, 1.0, 1.0);
  glVertex2f(center.getX() + 0.0, center.getY() + 1.0);
  // more vertices
glEnd();

Now when I add glRotatef(rotation, 0.0, 0.0, 1.0), I would like just this shape that I have drawn to rotate on the screen. However, if I add it above glBegin() it rotates everything in the window. If I include all the code between glPushMatrix() and glPopMatrix() rotates the object, but around the center of the window. How can I rotate just the object that I have drawn?

Answer 1

You’re doing OpenGL’s job by having center.getX and such added to the values.

What you want is this:

glPushMatrix();
glTranslatef(center.getX(), center.getY(), 0.0f);
glRotatef(rotation, 0.0, 0.0, 1.0);

glBegin(GL_LINE_LOOP);
  glColor3f(1.0, 1.0, 1.0);
  glVertex2f(0.0, 1.0);
  // more vertices
glEnd();

glPopMatrix();

You can probably apply the radius by using a glScale matrix and assuming a radius of 1.0 in your glVertex calls.