Rotate OpenGL quad around its center

Question

Sorry, I searched it but couldn't figure how to apply it to my code reading the other answers to similar questions. I what to rotate a quad, my code is the following:

glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, player_texture);
glBegin(GL_QUADS);
    glColor4ub(255, 255, 255, 255);
    glTexCoord2d(0,0); glVertex2f(hero.xValue(), hero.yValue());
    glTexCoord2d(1,0); glVertex2f(hero.xValue()+hero.lValue(), hero.yValue());
    glTexCoord2d(1,1); glVertex2f(hero.xValue()+hero.lValue(), hero.yValue()+hero.hValue());
    glTexCoord2d(0,1); glVertex2f(hero.xValue(), hero.yValue()+hero.hValue());  
glEnd();
glDisable(GL_TEXTURE_2D);

I'm not sure about how to use the glTranslatef function, I tell it what is the center of my object?

In that case it would be: hero.xValue()+hero.lValue()/2 (x) and hero.yValue()+hero.hValue()/2 (y)

I know I have to use both glTranslatef and glRotate, by the way. When I try, setting glTranslatef with the values I mentioned above, I get a texture floating far from where the hero actually is.

Any help on this matter will be appreciated, thank you.

Obs: hValue = height, lValue = lenght

Edit: New information to show the changes I made after @user55564's comment:

I call init() once, on there I set basic openGL and SDL stuff, among them:

glViewport(0, 0, WIDTH, HEIGHT);
glMatrixMode(GL_MODELVIEW);

then I call my draw function, it has in it:

while (running) {
    glLoadIdentity();

.
. stuff non-related to drawing
.

glClear(GL_COLOR_BUFFER_BIT); 
    glClearColor(0, 0, 0, 0); 
    glPushMatrix(); 
    glOrtho(0, WIDTH, HEIGHT, 0, -1, 1);

    // draws the player     
    glEnable(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, player_texture);
    glTranslatef(hero.xValue(), hero.yValue(), 0);
    glRotatef(0, 0, 0, 1);
    glTranslatef(-(hero.xValue()+hero.lValue()/2), -(hero.yValue()+hero.hValue()/2), 0);
    glBegin(GL_QUADS);
        glColor4ub(255, 255, 255, 255);
        glTexCoord2d(0,0); glVertex2f(hero.xValue(), hero.yValue());
        glTexCoord2d(1,0); glVertex2f(hero.xValue()+hero.lValue(), hero.yValue());
        glTexCoord2d(1,1); glVertex2f(hero.xValue()+hero.lValue(), hero.yValue()+hero.hValue());
        glTexCoord2d(0,1); glVertex2f(hero.xValue(), hero.yValue()+hero.hValue());  
    glEnd();
    glDisable(GL_TEXTURE_2D);
    glPopMatrix();

// then I use pushmatrix again to draw the enemies and the health bars of both enemies and player (these don't need rotation);
glPushMatrix();
    glOrtho(0, WIDTH, HEIGHT, 0, -1, 1);
.
.
.

Answer 1

What you need to do to draw a quad with x,y as its top left coords, r as rotation, and cx and cy as the center of rotation (relative to its x,y coord) :

- translate(x,y)
- rotate(r)
- translate(cx, cy);
- draw.
- (reverse/pop the matrix if required. etc)

For your case, try this:

...
glTranslatef(hero.xValue(), hero.yValue(), 0);
glRotatef(0, 0, 0, 1);
glTranslatef(+hero.lValue()/2, +hero.hValue()/2);
glBegin(GL_QUADS);
    glColor4ub(255, 255, 255, 255);
    glTexCoord2d(0,0); glVertex2f(-hero.lValue()/2, -hero.hValue()/2);
    // do the same for other 3 corners.

glEnd();

After the transformations, your (0,0) coord is at where the object's center is. Suppose if object is at (50,50). In your function after the transformation the (0,0) is at (50,50) So if you draw at (50,50) again then in reality you are drawing your object at (100,100).

See http://creativejs.com/2012/01/day-10-drawing-rotated-images-into-canvas/ for more detailed explanation.