glTranslatef messing up previously called glRotatef

Newbie & Debugging Questions
#1

im trying to rotate a simple cube round on origin to get a bit of experience in animation with openGL, but it aint working ???
it seems ti still rotate around hte point 0,0
heeres my code


glPushMatrix();
    glTranslatef(-px,-py,0);
    glRotatef(angle,0.0,0.0,1.0);
    glTranslatef(px,py,0);
    glBegin(GL_QUADS);
        glVertex2f(px,py);
        glVertex2f(px+50,py);
        glVertex2f(px+50,py+50);
        glVertex2f(px,py+50);
    glEnd();
    angle+=1;
glPopMatrix();

thanks for any help in advance, P.S im actually using C++ on this but i believe the openGl is just the same as in Java

#2

But is it really rotating, or it does nothing?

#3

Think we need a bit more info mate, px and py, then perhaps how you set things up.

#4

well currently px and py are set to 90, but it should work whatever number you make them, the issue is that the cube is at x=90 and y=90 but is not rotating around the origin of the cube like i wanted it to

#5

[s]yep.

To rotate around the origin you have to translate to the middle of the cube! :slight_smile:

(Such a COMMON mistake. The web helps you a LOT: Stackoverflow)…

But just handing out links isn’t okey too… so:[/s]

Just noticed you don’t draw it really like you should… You should draw after translating and without calculating the x and y position for the vertices in the calls to glVertex2f…

soooo:


int w = 50; // The width of the cube
int h = 50; // The height of the cube
glPushMatrix();
    // Translate:
    glTranslatef(-px, -py, 0); // Translate to the position of the cube
    // Rotate:
    glTranslatef(-w/2, -h/2, 0); // Translate even further to the middle of the cube
    glRotatef(angle, 0, 0, 1); // Rotate around z-axis by angle
    glTranslatef(w/2, h/2, 0); // Translate to the corner of the cube
    // Draw:
    glBegin(GL_QUADS);
        glVertex2f(0, 0);
        glVertex2f(w, 0);
        glVertex2f(w, h);
        glVertex2f(0, h);
    glEnd();
glPopMatrix();
#6

Yes but isnt that going to draw the cube at -90x -90y, ive played about with the code you sent me and it just wont work

#7

Sorry. this should be glTranslatef(px, py, 0);, right? Did you try that out?

#8

Thanks, i dont know why i didnt try that, but you still made a mistake, it should translate like this :


        glTranslatef(w/2, h/2, 0);
        glRotatef(angle, 0, 0, 1);
        glTranslatef(-w/2, -h/2, 0);

not


        glTranslatef(-w/2, -h/2, 0);
        glRotatef(angle, 0, 0, 1);
        glTranslatef(w/2, h/2, 0);

Anyway thanks so much for your help, everythings working now and i greatly appreciate all responses :slight_smile:

#9

Yup… just messed up with negating…

It was the translating for the viewport where you needed negation… messed that up :slight_smile: