[VEX] rotate

e.g.1)

float angle = @Time;
vector axis = rand(@ptnum);
matrix3 m = ident();
rotate(m, angle, axis);
@orient = quaternion(m);

e.g.2)

float angle = @Time;
vector axis = rand(@ptnum);
@orient = quaternion(angle, axis);

e.g.3)

float angle = chf("angle");
vector4 rot = quaternion(radian(angle), {0, 1, 0});
@N = qrotate(rot, @N);

http://www.sidefx.com/docs/houdini/vex/snippets.html
https://www.sidefx.com/docs/houdini/dopparticles/attributes.html

vector4 quaternion(matrix3 rotations)
Creates a vector4 representing a quaternion from a 3×3 rotational matrix.

vector4 quaternion(float angle, vector axis)
Creates a vector4 representing a quaternion from an angle and axis. The angle is specified in radians.

vector4 quaternion(vector angleaxis)

Thanks for sharing knowledge! 
http://www.tokeru.com/cgwiki/index.php?title=HoudiniVex
https://www.sidefx.com/forum/topic/34836/