Calculate angle between vector and cam
#include <s3d.h>
float s3d_angle_to_cam(const float obj_pos[], const float cam_pos[], float *angle_rad);
Given obj_pos and cam_pos in the format float[3], angle_rad about which angle the object should be rotated around the y-axis so that it faces the camera. This might become handy if you have some text floating in space and want it to face the camera. http://en.wikipedia.org/wiki/Vector_(spatial)
Simon Wunderlich
Author of s3d