Last change
on this file since 9472 was 731, checked in by tbretz, 24 years ago |
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1 | #include "slalib.h"
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2 | #include "slamac.h"
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3 | void slaInvf ( double fwds[6], double bkwds[6], int *j )
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4 | /*
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5 | ** - - - - - - - -
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6 | ** s l a I n v f
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7 | ** - - - - - - - -
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8 | **
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9 | ** Invert a linear model of the type produced by the slaFitxy routine.
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10 | **
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11 | ** Given:
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12 | ** fwds double[6] model coefficients
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13 | **
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14 | ** Returned:
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15 | ** bkwds double[6] inverse model
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16 | ** *j int status: 0 = OK, -1 = no inverse
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17 | **
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18 | ** The models relate two sets of [x,y] coordinates as follows.
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19 | ** Naming the elements of fwds:
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20 | **
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21 | ** fwds[0] = a
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22 | ** fwds[1] = b
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23 | ** fwds[2] = c
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24 | ** fwds[3] = d
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25 | ** fwds[4] = e
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26 | ** fwds[5] = f
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27 | **
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28 | ** Where two sets of coordinates [x1,y1] and [x2,y1] are related
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29 | ** thus:
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30 | **
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31 | ** x2 = a + b*x1 + c*y1
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32 | ** y2 = d + e*x1 + f*y1
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33 | **
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34 | ** The present routine generates a new set of coefficients:
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35 | **
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36 | ** bkwds[0] = p
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37 | ** bkwds[1] = q
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38 | ** bkwds[2] = r
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39 | ** bkwds[3] = s
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40 | ** bkwds[4] = t
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41 | ** bkwds[5] = u
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42 | **
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43 | ** Such that:
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44 | **
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45 | ** x1 = p + q*x2 + r*y2
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46 | ** y1 = s + t*x2 + u*y2
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47 | **
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48 | ** Two successive calls to slaInvf will thus deliver a set
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49 | ** of coefficients equal to the starting values.
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50 | **
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51 | ** See also slaFitxy, slaPxy, slaXy2xy, slaDcmpf
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52 | **
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53 | ** Last revision: 30 October 1993
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54 | **
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55 | ** Copyright P.T.Wallace. All rights reserved.
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56 | */
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57 | {
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58 | double a, b, c, d, e, f, det;
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59 |
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60 | a = fwds[0];
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61 | b = fwds[1];
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62 | c = fwds[2];
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63 | d = fwds[3];
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64 | e = fwds[4];
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65 | f = fwds[5];
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66 | det = b * f - c * e;
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67 |
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68 | if ( det != 0.0 ) {
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69 | bkwds[0] = ( c * d - a * f ) / det;
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70 | bkwds[1] = f / det;
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71 | bkwds[2] = - c / det;
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72 | bkwds[3] = ( a * e - b * d ) / det;
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73 | bkwds[4] = - e / det;
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74 | bkwds[5] = b / det;
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75 | *j = 0;
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76 | } else {
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77 | *j = -1;
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78 | }
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79 | }
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