o_mag_m.c
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	#include<stdio.h>
	#include<math.h>
	#include<fonction.h>
	#include<constant.h>
	#include<dimension.h>
	#include<structure.h>

	/****************************************************************/
	/* nom: o_mag_m */
	/* auteur: Jean-Paul Kneib */
	/* date: 10/02/92 */
	/* place: Toulouse */
	/****************************************************************
	* calcul de magnification pour les IMAGES MULTIPLES
	* Compute the deformation parameters thetaPot(thp), distortionPot(dp)
	* and tauPot(tp) for every arclet of the arclet list.
	*
	* arclet thetaPot (thp) = 2Gradxy(Phi)/(Gradyy(Phi) - Gradxx(Phi))
	* arclet distortion (dp) = (K^2 + G^2)/(K^2 - G^2) = trace(a^-2)/2det(a^-1)
	* arclet -tau (tp) = 2KG/(K^2 - G^2)
	*
	* Global variables used :
	* - in e_grad2() : G, lens, lens_table
	*/
	void o_mag_m(int n, struct galaxie *arclet)
	{
	register int i;
	double A, B, C;
	struct matrix MA;


	for (i = 0; i < n; i++)
	{
	MA = e_grad2_gal(&arclet[i], NULL);
	MA.a *= arclet[0].dr;
	MA.b *= arclet[0].dr;
	MA.c *= arclet[0].dr;
	MA.d *= arclet[0].dr;
	A = 1. - MA.a; //Gradxx(Phi)
	B = -MA.b; //Gradxy(Phi)
	C = 1. - MA.c; //Gradyy(Phi)
	arclet[i].A = fabs(A * C - B * B);
	arclet[i].thp = PI / 2. + 0.5 * atan2(2.*B, (A - C));
	arclet[i].dp = (A * A + C * C + 2.B B) / arclet[i].A / 2.;
	arclet[i].tp = sqrt(arclet[i].dp * arclet[i].dp - 1.);
	}

	arclet[n] = arclet[0];
	}