1 files changed, 37 insertions, 0 deletions
diff --git a/sys/gio/calcomp/rptheta4.x b/sys/gio/calcomp/rptheta4.x
new file mode 100644
index 00000000..b2ee42b7
--- /dev/null
+++ b/sys/gio/calcomp/rptheta4.x
@@ -0,0 +1,37 @@
+# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
+
+include <math.h>
+
+define	PIOVER4		(0.25 * PI)
+define	THREEPIOVER4	(0.75 * TWOPI)
+
+# RPTHETA4 -- Polar angle, Real precision, 4 arguments; from p1(x,y) to p2(x,y):
+# angle between line segment p1-p2 and horizontal +x axis centered on p1; 
+# returned in radians; single precision (see pdtheta4).
+
+real procedure rptheta4 (p1x, p1y, p2x, p2y)
+
+real	p1x,p1y, p2x,p2y	# x,y of each point
+real	dx, dy, ang
+
+begin
+	dx = p2x - p1x
+	dy = p2y - p1y
+
+	if (dx == 0.0) {
+	    if (dy >= 0.0) {
+		ang = HALFPI
+	    } else {
+		ang = THREEPIOVER4
+	    }
+	} else {
+	    ang = atan (dy / dx)
+	    if (dx < 0.0) {		# 2nd or 3rd quadrant
+		ang = ang + PI
+	    } else if (dy < 0.0) {	# 4th quadrant
+		ang = ang + TWOPI
+	    }
+	}
+
+	return (ang)
+end