Rotation

OpenGL Development JOGL Development
#1

Being stupid here, but I can’t figure this out.

I am working in JOGL, and the universe I am modelling is a unit cube, centered on the origin. I want to use the mouse to spin the cube about its centre, to view it from an angle. This is exactly he same as if the view is orbiting the cube at a constant radius.

My mouse handler accumulates x and y movements to build up an XRot and YRot value. Then in my display method I do:

    gl.glRotated(gs.getXRotation(), 1.0f, 0.0f, 0.0f);
    gl.glRotated(gs.getYRotation(), 0.0f, 1.0f, 0.0f);
    gl.glRotated(gs.getZRotation(), 0.0f, 0.0f, 1.0f);
    gl.glTranslated(axes.getXStart() - axes.getRange()/2, axes.getYStart() - axes.getRange()/2, axes.getZStart() - axes.getRange()/2);
    gl.glScaled(axes.getRange(), axes.getRange(), axes.getRange());

(Z rotation is always 0, x/y/z start is 0, range is 1). Now the bottom corner of the cube rotates about the origin, but unfortunately the cube itself also rotates about the corner. So, I can see the cube from any angle, but it isn’t always in the centre of the screen.

Anyone know of a simple tutorial to get a cube to rotate about its own centre?

#2

Rotation is always about the origin, but you can move the origin with glTranslate(). If don’t want the view of the object to be translated after you’ve done the rotation just translate back.


glTranslate(center.x,center.y,center.z);
glRotate(angle,axis.x,axis.y,axis.z);
glTranslate(-center.x,-center.y,-center.z);
#3

Thanks, but that’s not quite what I want to do. I want to draw a 1 unit cube which is centred on the origin (ie the x/y/z faces are all at +/- 0.5).

I want that cube to rotate around the origin - the cube should never leave its original enclosing sphere.

But, I also want the cube to have its own coordinate system, so that I can draw a point inside the cube using coordinate values between 0 and 1. I hope that is clear, its a bit confusing to describe.

My thinking was, first rotate (about the origin) then translate to move the origin to (-.5, -.5, -.5), but that doesn’t work at all. Possibly I am misunderstanding how translate works?

#4

I think you have the right idea, can you define what exactly you mean by doesn’t work at all? If it’s a black screen it could be problems with the way the frustum is set up. One easy way to do it is forget about translate/rotate and just use gluLookAt().

#5

Fixed it now - the coordinates were actually working properly, the problem was with the way I was drawing the cube (using further rotates which were not set up properly).

I guess I should have used a better 3D model to test with. Thanks for your help.