Hi,

I'm struggling with something that must be really simple to do.

I'm 'making' a sphere from particles [a texture on a quad]. And I don't seem to be able to force those particles to always face the exterior of the sphere.

Each particles is a 3d vector. So I'm using Math.atan2 [via the methods headingXY(), headingXZ() and headingYZ() from toxi's geom lib] to get the rotation angles [for each planes] that a particle needs depending of its coord.

Here is what I do:

Code:

gl.glTranslatef(pPos.x, pPos.y, pPos.z);

gl.glRotatef(p.degrees(pPos.headingXY()), 0, 0, 1);
gl.glRotatef(p.degrees(pPos.headingXZ()), 0, 1, 0);
gl.glRotatef(p.degrees(pPos.headingYZ()), 1, 0, 0);

 

The following, on its own, works:

Code:

gl.glTranslatef(pPos.x, pPos.y, pPos.z);

gl.glRotatef(p.degrees(pPos.headingXY()), 0, 0, 1);
 

but the the rotations on the y and x axes gives strange results.

Thanks,

here's what i do to rotate a plane so its normal faces another vector:

your particle would by in the XY plane correct so ill use its normal (0,0,1)

U vector is (0,1,0) (normalized)

V is your sphere normal at a certain point (normalized)


N = U.cross( V ) // axis of rotation
N.normalize()
float dot = U.dot( V );// cos angle
float angle = acos(dot); // angle of rotation

// quat from an angle and a rotation axis
Quaternion quat = new Quaternion();
quat.rotateAxis( N, angle);
// transform vertex
Vector3 dv = quat.mul( P );

_position.set( dv );
// translate to right position
_position.add( offset );


i used quaternions to rotate the vector, but you could create a matrix using the UVN vectors.
offset is the translation vector. which means, the position where u want the particle to be

so then transform your plane's vertices and u should be ok

Anything here that might be helpful?
http://seltar.org/projects/MarchingTriangleWar2/