Pseudo-angle method

pseudoangle.h

// angle relative to the x axis. 
inline float pseudoangle(float2 p1, float2 p2) {
	float dx, dy, t;
	dx = p2.x - p1.x;
	dy = p2.y - p1.y;
	if ((dx == 0.0) && (dy == 0.0)) {
		return -1; // indicating error
	}
	else {
		t = dy / (fabs(dx) + fabs(dy));
		/* Now correct for quadrant -- first quadrant: [0,1] */
		if (dx < 0.0)
			return 2.0 - t;
		/* Inside second or third quadrant (1,3)*/
		else if (dy < 0.0)
			return 4.0 + t;
		return t;
	}
}