# OpenGL 2D rotations

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Hi all. Just registered! I'm starting out with GLUT and OpenGL and have been playing around. Wrote my own little version of Pong and now I'm moving on to 'Asteroids' But I've got a problem. I'm using the 2D view (gluOrtho2D) but don't know how to rotate something (ie. the spaceship) in 2D. I've rotated things in 3D before whilst learning some of the stuff, but glRotate doesn't seem to work in 2D. Is there a specific command to do this? And if not, how can I go about it? Is there some forumla I can use to calculate new x,y co-ords of a shape based on how much I want to rotate it by? Thanks. [Edited by - BigDogUK on May 27, 2005 3:10:44 AM]

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You could certainly do the calculations yourself. However, AFAIK working with 2d graphics in OpenGL ortho mode is basically just like drawing graphics to the xy plane in 3d. All rotations take place about the z axis, so your rotate calls look like this:

glRotate(angle, 0, 0, 1);

I believe there's a glVertex2(), but no glTranslate2(). So your translation calls will just be:

glTranslate(x, y, 0);

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Ahh, ok. My ship rotates now, but I'm not sure how to set the origin of rotation. It seems to be rotating around 0,0 on my 2D view and fiddling with the:

glRotatef(angle, x, y, 1)

x and y doesn't seem to help. One question. How do the float values correspond to places on a say 800x600 2D view. I'm having trouble working out where things are going to appear which is why I switched to 2DOrtho since it seemed to let me use real co-ords within the window. Ie. 400, 300 is the centre.

Here's my draw code, bit of a mess at the moment since I don't fully understand the viewports, matrix modes etc. Any guidance would be welcome :)

void renderGame(void) {

glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glViewport(0, 0, 760, 480);
gluOrtho2D(0, 760, 0, 480);
glPushMatrix();
glColor3f(0.0,0.0,0.0); // set bg colour
glVertex2i(0, 0); // top l
glVertex2i(550, 0); // top r
glVertex2i(550, 400); // bottom r
glVertex2i(0, 400); // bottom l
glEnd();
glColor3f(1.0,1.0,1.0); // set spaceship colour
glRotatef(craftangle, 0, 0, 1);
glBegin(GL_LINE_LOOP); // spacecraft
glVertex2i(260+craftx, 200+crafty); // bottom l
glVertex2i(267+craftx, 224+crafty); // middle
glVertex2i(274+craftx, 200+crafty); // bottom r
glEnd();

glutSwapBuffers();
glPopMatrix();

}

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This:

glRotatef(angle, x, y, 1);

Isn't what you want, but you're doing it correctly in the code below. A couple of other things... You probably need to add glMatrixMode(MODELVIEW) somewhere. And you shouldn't be including your ship's position in the glVertex() calls - use glTranslate() instead. Also, your ship triangle (I assume that's what it is) is currently not centered at the origin. For both those reasons the ship will not behave onscreen as you expect.

Here's your code with a few edits:

glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glViewport(0, 0, 760, 480);
gluOrtho2D(0, 760, 0, 480);
glPushMatrix(); // Is this doing anything?
glColor3f(0.0,0.0,0.0); // set bg colour
glVertex2i(0, 0); // top l
glVertex2i(550, 0); // top r
glVertex2i(550, 400); // bottom r
glVertex2i(0, 400); // bottom l
glEnd();
glColor3f(1.0,1.0,1.0); // set spaceship colour
glRotatef(craftangle, 0, 0, 1);

// You can make this basic shape whatever you want - I
// made it a triangle pointing down the +x axis.
glBegin(GL_LINE_LOOP); // spacecraft
glVertex2i(200, 0); // bottom l
glVertex2i(-200, 200); // middle
glVertex2i(-200, -200); // bottom r
glEnd();

glutSwapBuffers();
glPopMatrix();

Anyway, you might give that a try and see if it behaves any differently.

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Thanks! You've been a great help. I altered my code to your suggestions and it now spins properly on the spot. But I've got one last problem.

At the moment, the spacecraft spins around fine on the spot at the centre where it starts. But when I accelerate it off in a direction and try and rotate it, it still uses the same 0,0 co-ords so the rotation is off again! It's as though its attached to a peice of string connected to 0,0.

Do I need to technically keep it at 0,0 and move view or something? Or is there someway to update the origin it rotates around? Here's my current code:

void renderGame(void) {

glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glViewport(0, 0, 550, 400);
gluOrtho2D(-225, 225, -200, 200);
glColor3f(0.0,0.0,0.0); // set bg colour
glVertex2i(0, 0); // top l
glVertex2i(550, 0); // top r
glVertex2i(550, 400); // bottom r
glVertex2i(0, 400); // bottom l
glEnd();
glColor3f(1.0,1.0,1.0); // set spaceship colour

glRotatef(craftangle, 0, 0, 1);

glBegin(GL_LINE_LOOP); // spacecraft
glVertex2i(7, -12); // bottom l
glVertex2i(0, 12); // middle
glVertex2i(-7, -12); // bottom r
glEnd();

glutSwapBuffers();
glPopMatrix();

}

Thanks again, Oh and just for the record. I'm not actually altering the craft-x. Does this matter? I'm just altering craft-y to accelerate the craft in the direction it is pointing.

[Edited by - BigDogUK on May 26, 2005 3:14:33 AM]

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Have to leave for work in a moment, but very quickly, you need to put the glTranslate() line before glRotate(), not after. Also, you need to adjust both your ship x and y coordinates, i.e. in world space. You probably need something like this:

ship.y += cos(angle) * speed;
ship.x += sin(angle) * speed; // Or maybe -sin(angle)

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Ok. Thanks!

Been playing around with that and can't seem to get it to move in any logical direction!!! I've tried a few things and I've locked it so the angle is always between 0-360 and the angle of the ship does correspond to the correct rotation.

But it's behaving REALLY erratically. For one, it seems to completetly change direction just by adding or taking one degree from the angle. And it sure as hell isn't moving in the right direction :(

Can anyone have a quick look at my source?

http://www.zen77439.zen.co.uk/other/CurrentAsteroids.cpp

There it is! I don't seem to be doing anything strange and I've tried cleaning up the mix of floats and ints but it doesn't seem to help...

Thanks

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Quote:
 But it's behaving REALLY erratically. For one, it seems to completetly change direction just by adding or taking one degree from the angle. And it sure as hell isn't moving in the right direction :(
C++ trig functions take their arguments in radians, not degrees - I suspect that's where your problem lies. You can convert your angles from degrees to radians before submitting them to trig functions by multiplying by pi/180.

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Ahh, I suspected something was off with the angles because I was checking the output of the formula through cout and they were really weird. Sin 90 giving 0.44545 or something. But my knowledge of Maths isn't quite at that level. Getting on to that at A-level hopefully.

Just reworked it and the spaceship moves perfectly! Thanks for all your help :)

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