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Calculating

Started by Node Dec 21, 2006 at 10:15 AM 2 replies 1k views

Original Post

Dec 21, 2006 at 10:15 AM

I have to spheres in my opengl window (S and T), and I want to draw a cylinder (C) between them. The sphere is connected to S via a translation, however I am not sure how to to calculate the required rotations to make the other end of C connect to T. My code currently looks like this: dx = Tx - Sx; dy = Ty - Sy; dz = Tz - Sz; len = sqrt((dx * dx) + (dy * dy) + (dz * dz)); angle = atan2(dz, dx); angle = (180 * angle) / PI; glRotatef(angle, 0.0f, 1.0f, 0.0f); angle = atan2(dy, dz); angle = (180 * angle) / PI; glRotatef(angle, 1.0f, 0.0f, 0.0f); angle = atan2(dy, dx); angle = (180 * angle) / PI; glRotatef(angle, 0.0f, 0.0f, 1.0f);

SuperNerd

Dec 25, 2006 at 9:39 PM

glRotatef(0.0f, dx / len, dy / len, dz / len);

I havn't used opengl but from what I just looked up the documetation that should work. Depending on the orientation of your cylinder you may have to switch some of the x, y, and z parameters in the function to get it to work right.

Vorpy

Dec 25, 2006 at 10:14 PM

This sounds similiar to this recent thread except that you're using opengl, while that thread was asking for a solution for quaternions.

I'm guessing right now the cylinder is aligned to one of the axes, and you want a rotation to align it with the direction from one sphere to the other.

Let v1 be a unit vector in the direction of the axis the cylinder is aligned with, and let v2 be (dx,dy,dz) in your code, the vector from the center of one sphere to the center of the other.

The axis of rotation is normalize(cross(v1,v2)).

The angle of rotation is acos(dot(v1,normalize(v2))).

I'm sure that like in the other thread there's some clever and more direct way of calculating the rotation you want, but I don't feel like trying to come up with it right now and this method is probably good enough.

Zakwayda

1,120

Zakwayda

Dec 26, 2006 at 2:13 AM

I think this thread was an accidental double post; the original thread is here.

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