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// 08.08.12
// 10.01.02

function X3=Zperspt(P)

  global FocusPoint EyePoint
  
  Tmp=P-FocusPoint;
  X1=Tmp(1); Y1=Tmp(2); Z1=Tmp(3);
  Tmp=EyePoint-FocusPoint;
  E1=Tmp(1); F1=Tmp(2); G1=Tmp(3);
  Ca=E1/sqrt(E1^2+F1^2);
  Sa=F1/sqrt(E1^2+F1^2);
  X2=X1*Ca+Y1*Sa; Y2=-X1*Sa+Y1*Ca; Z2=Z1;
  E2=E1*Ca+F1*Sa; F2=-E1*Sa+F1*Ca; G2=G1;
  Cb=E2/sqrt(E2^2+G2^2);
  Sb=G2/sqrt(E2^2+G2^2);
  X3=X2*Cb+Z2*Sb;
endfunction