C ++********************************************************************
C                                                                      *
C                                                                      *
C                                                                      *
C **********************************************************************
C=* FROM: SPIDER - MODULAR IMAGE PROCESSING SYSTEM.   AUTHOR: J.FRANK  *
C=* Copyright (C) 1985-2005  Health Research Inc.                      *
C=*                                                                    *
C=* HEALTH RESEARCH INCORPORATED (HRI),                                *   
C=* ONE UNIVERSITY PLACE, RENSSELAER, NY 12144-3455.                   *
C=*                                                                    *
C=* Email:  spider@wadsworth.org                                       *
C=*                                                                    *
C=* This program is free software; you can redistribute it and/or      *
C=* modify it under the terms of the GNU General Public License as     *
C=* published by the Free Software Foundation; either version 2 of the *
C=* License, or (at your option) any later version.                    *
C=*                                                                    *
C=* This program is distributed in the hope that it will be useful,    *
C=* but WITHOUT ANY WARRANTY; without even the implied warranty of     *
C=* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU  *
C=* General Public License for more details.                           *
C=*                                                                    *
C=* You should have received a copy of the GNU General Public License  *
C=* along with this program; if not, write to the                      *
C=* Free Software Foundation, Inc.,                                    *
C=* 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.      *
C=*                                                                    *
C **********************************************************************
C                                                                      *
C                                                                      *
C                                                                      *
C  PURPOSE:                                                            *
C                                                                      *
C **************************************************************
C CALCULATES, USING LINEAR REGRESSION AND ASSUMING A SINGLE
C TILT AXIS THROUGH THE Y AXIS, THE X, Y, AND Z COORDS OF THE
C MARKERS.
C INPUT:
C     PRJ(2,LS,LV)= COORDS OF MARKER IN PROJECTIONS
C     ANGLE(3,LV)= ANGLES TILTED ABOUT Z,Y,Z
C COMMON INPUT:
C     NUMPT(LV)= MAX INDEX OF POINT USED IN EACH VIEW
C     PTACTIVE(LS,LV)= BOOLEAN ARRAY HOLDING WHETHER A POINT IS IN
C           VIEW OR NOT.
C     NTVW= TOTAL NUMBER OF VIEWS
C     NTPT= TOTAL NUMBER OF MARKERS
C OUTPUT:
C     P3D(3,LS)= COORDS OF POINTS IN 3-D SPACE
C
C  PARAMETERS:                                                         *
C                                                                      *
C23456789012345678901234567890123456789012345678901234567890123456789012
C***********************************************************************

      SUBROUTINE MR2TO3D(P3D, PRJ, ANGLE)

      PARAMETER (LV=300)
      PARAMETER (LS=256)
      LOGICAL*1   PTACTIVE(LS,LV),PARAMQ(4)
      COMMON /GENERAL/PTACTIVE,NUMPTS(LV),NTVW,NTPT,CIR(2),PARAMQ

      DIMENSION   P3D(3,LS)
      DIMENSION   PRJ(2,LS,LV), ANGLE(3,LV), NUMPT(LV)
      DIMENSION   XPR(LV), YPR(LV)

C     PREPARE ARRAYS FOR REGRESSION. 
      DO I=1,NTPT
        YSUM=0.0
	A2=0.0
	B2=0.0
	AB=0.0
	AP=0.0
	BP=0.0
        NYSUM=0
        NDATA=0
        DO  J=1,NTVW
          IF (PTACTIVE(I,J)) THEN
            YSUM=YSUM+PRJ(2,I,J)
            NYSUM=NYSUM+1
		A=COS(ANGLE(2,J))
		B=SIN(ANGLE(2,J))
		P=PRJ(1,I,J)
C Solve system of equations:
C  x*cos(theta(i)) - z*sin(theta(i)) = p(i)
C that is:
C  [ x*cos(theta(i)) - z*sin(theta(i)) - p(i)]^2 ->min
C
		A2=A2+A*A
		B2=B2+B*B
		AB=AB+A*B
		AP=AP+A*P
		BP=BP+B*P
Cc            IF (ABS(SIN(ANGLE(2,J))) .GT. 0.001 ) THEN
              NDATA=NDATA+1
Cc              XPR(NDATA)= 1/TAN(ANGLE(2,J))
Cc              YPR(NDATA)= PRJ(1,I,J)/SIN(ANGLE(2,J))
c the following version worked:
C            IF (ABS(COS(ANGLE(2,J))) .GT. 0.001 ) THEN
C              NDATA=NDATA+1
C              XPR(NDATA)=-TAN(ANGLE(2,J))
C              YPR(NDATA)= PRJ(1,I,J)/COS(ANGLE(2,J))
C            ENDIF
          ENDIF
	ENDDO
	IF(NDATA.NE.0)  THEN
C        X=0
C        ZN=0
C        CALL MRRLINE(XPR,YPR,NDATA,LV,ZN,X,ERZ,ERX,STER)
c        P3D(1,I)=X
c        P3D(3,I)=-ZN
        P3D(1,I)=(AB*BP-B2*AP)/(AB*AB-A2*B2)
        P3D(3,I)=(A2*BP-AB*AP)/(AB*AB-A2*B2)
        P3D(2,I)=YSUM/NYSUM
	ELSE
	  DO    j=1,3
	    P3D(j,i)=0.0
	  ENDDO
	ENDIF
	ENDDO
	END