main/src/georegression/geometry/ConvertRotation3D_F64.java
public static Rodrigues_F64 quaternionToRodrigues( Quaternion_F64 quat,
													   Rodrigues_F64 rodrigues ) {
		if( rodrigues == null )
			rodrigues = new Rodrigues_F64();

		rodrigues.unitAxisRotation.set( quat.x, quat.y, quat.z);
		rodrigues.unitAxisRotation.normalize();

		rodrigues.theta = 2.0 * Math.acos( quat.w);

		return rodrigues;
	}
public static Rodrigues_F64 quaternionToRodrigues( Quaternion_F64 quat,
													   Rodrigues_F64 rodrigues ) {
		if( rodrigues == null )
			rodrigues = new Rodrigues_F64();

		rodrigues.unitAxisRotation.set( quat.x, quat.y, quat.z);
		rodrigues.unitAxisRotation.normalize();

		rodrigues.theta = 2.0 * Math.acos( quat.w);

		return rodrigues;
	}
Converts a unit {@link Quaternion_F64} into {@link Rodrigues_F64}.
@param quat (Input) Unit quaternion
@param rodrigues (Optional) Storage for rodrigues coodinate.  If null a new instance is created.
@return rodrigues