function [A] = kostrafo(achse,winkel)

a = winkel;

if achse == 'x'
    v = [1 0 0]';
elseif achse == 'y'
    v = [0 1 0]';
elseif achse == 'z'
    v = [0 0 1]';
elseif length(achse) == 3
    v = achse;
end

X = v(1);
Y = v(2);
Z = v(3);

A = [X^2*(1-cos(a))+cos(a)     X*Y*(1-cos(a))+Z*sin(a)     X*Z*(1-cos(a))-Y*sin(a);
     X*Y*(1-cos(a))-Z*sin(a)   Y^2*(1-cos(a))+cos(a)       Y*Z*(1-cos(a))+X*sin(a);
     X*Z*(1-cos(a))+Y*sin(a)   Y*Z*(1-cos(a))-X*sin(a)     Z^2*(1-cos(a))+cos(a)];

end