OpenGL rotation

[ay_okay]

I'm trying to make a little 2D rectangle rotate. But, it can also move up. When it does move up, it rotates around the center of the window, like its in orbit. I want it to rotate on its own center point help! I've been using

glRotate2f(x, 0.0f, 0.0f 1.0f);

[-=SoKrA=-]

Is that the actual call you are usingd? It shouldn't compile.

Something like

glRotate3f(x, 0.0, 0.0)

should work for your case. The number does not correspond to how many things you're passing, and there's a comma missing.
The last param, you can omit, because it defaults to 1 if not used.

[Shakti]

It is

glRotatef(angle, x, y, z);

[ay_okay]

i think I read something that glRotate() rotates the whole plane, not just the object. how do i make only the object rotate. If you give me any math equations, could you explain them please, im only in grade 9.

[linuxdude]

/*draw stuff*/
glTranslatef(0.0,x,0.0);/*move up like you said*/
glPushMatrix();
     glRotatef(x,0.0,1.0,0.0);
     /*draw stuff to rotate*/
glPopMatrix();

you have to get used to using push pop matrix like that. Or else you are rotating everything. Does this help if not show me the code.

[Darkness]

OpenGL rotates the coordinate system. If you rotate first, then translate, the direciton it translates is now with respect to the *rotated* coordinate system.

Say, for example, you want to rotate 45 degrees about the z axis, then you wan to translate down the x axis 5 units. Well, if you *just* translated down the x axis 5 units, then your position would be:

(5,0,0)

However, if you rotate 45 degrees about the z axis (glRotatef(45,0,0,1)) and THEN translate down the x axis 5 units (glTranslatef(5,0,0)) your position will *actually* be about:

(3.5, 3.5, 0)

So, in general, if you want an object to *just* rotate, you need to translate it *first*, and then rotate the object. Linuxdude's code works, but you don't need to push and then pop the matrix, just as long as you always translate before you rotate.

Note that there are some times when it is convenient to do it the other way around. For example, if you want a planet to orbit another one, you can just rotate out through the current angle and then always translate down a constant axis (because that axis will actually be rotated if you call glRotate first).

If something I or someone else has said (we're all saying basically the same thing) doesn't make sense, just say what doesn't make sense and we'll reiterate or draw some pics. Good luck.

[Magos]

Unless you already knew, matrix transformations take effect in a reverse order. If you call Translate then Rotate the rotation will be made first, then the translation. Quite mysterious at first, but once you use push/pop-matrix to its full power it'll seem natural.

[Darkness]

To be honest I'm kind of confused right now and something I said earlier might not be correct.

EDIT:
I didn't really say anything that was incorrect on the last page, but I might've been a bit ambiguous. I am sorry to say that this is probably all very confusing. Just in general, if you want an object to look like it is rotating around itself, always call glTranslate first, and then call glRotate. However, technically speaking, what it does is it rotates the object first, and then translates it out to the world position, but dont' worry about those details until you get used to doing basic first.

glTranslate(pos.x,pos.y,pos.z);

glRotate(angle,axis.x,axis.y,axis.z);

that will always have the effect of an object rotating around itself

[ay_okay]

Thanks darkness, I figured it out

float temp;
temp = theta * (3.14 / 180);
side = side + (inc * cos(temp));
up = up + (inc * sin(temp));
glTranslate(side, up, 0.0f);

You've been a lot of help

[Darkness]

Very nice work. If you can continue to work problems out on your own you *will* be successful with this stuff!

I was honestly afriad I had confused you (because admittidely some of the stuff I was saying was ambiguous and confusing).

Keep up the good work.