C	FUNCTION ARCTAN.FT
C	------------------
C
C
C
	FUNCTION ARCTAN(Y,X)
C
C
C
C	GERSON GROSFELD
C	IMAGE PROCESSING UNIT, DCBD
C	NATIONAL CANCER INSTITUTE
C	NATIONAL INSTITUTES OF HEALTH
C	9000 ROCKVILLE PIKE
C	BETHESDA, MD. 20014
C
C
C
C	WRITTEN 12 DEC 1975
C
C	*** NOTE: THIS PROGRAM REPLACES AN EARLIER PROGRAM WITH
C	THE SAME NAME (LAST DATED 11/21/75) WHICH WAS FOUND TO
C	HAVE BASIC COMPUTATIONAL FLAWS.
C
C
C
C
C	THE FUNCTION RETURNS THE ARCTANGENT, IN RADIANS, BETWEEN
C	POINT (X,Y) AND THE ORIGIN, IN THE RANGE:
C		0=<ARCTAN<=2*PI
C
C	COINCIDENT POINTS [IE (X,Y)=ORIGIN] RETURN THE VALUE ZERO.
C
C
C
C	[ARCTAN USES THE FORTRAN II "ATAN" FUCNCTION WHICH ONLY 
C	RETURNS VALUES IN THE RANGE:  PI/2=<ATAN=<-PI/2]
C
C
C
C
C
C	[0] CONSTANTS
	PI=3.14159265
C
C
C
C	[1.0] COMPUTE ARCTANGENT
C
C	[1.1] TEST X
C	IF X<0, THEN ARCTAN=ATAN+PI [OPPOSITE QUADRANT]
C	IF X=0, THEN ARCTAN=PI/2 OR 3*PI/2: TEST Y [1.2]
C	IF X>0, THEN TEST Y FOR QUADRANT [1.3]
C
	IF(X)180,12,13
C
C
C
C	[1.2] TEST Y
C	IF Y<0, THEN ARCTAN=3*PI/2
C	IF Y=0, THEN POINT (X,Y)=ORIGIN
C	IF Y>0, THEN ARCTAN=PI/2
C
12	IF(Y)270,2001,90
C
C
C	[1.3] TEST Y
C	IF Y<0, THEN ARCTAN=ATAN+2*PI
C	IF Y>=0, THEN ARCTAN=ATAN
C
13	IF(Y)360,1000,1000
C
C
C	[1.4] ARCTAN=PI/2
C
90	ARCTAN=PI/2.
	RETURN
C
C
C	[1.5] ARCTAN=ATAN+PI
C
180	ARCTAN=ATAN(Y/X)+PI
	RETURN
C
C
C	[1.6] ARCTAN=3*PI/2
C
270	ARCTAN=3.*PI/2.
	RETURN
C
C
C	[1.7] ARCTAN=ATAN+2*PI
C
360	ARCTAN=ATAN(Y/X)+2.*PI
	RETURN
C
C
C	[1.8] ARCTAN=ATAN
C
1000	ARCTAN=ATAN(Y/X)
	RETURN
C
C
C
C	[2.0] POINT=ORIGIN
C	SET ARCTAN TO ZERO 
2001	ARCTAN=0
	RETURN
C
C
C
	END
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