function m = rv2m(rv) 
%  等效旋转矢量转换为旋转矩阵
%
% Input: rv - 旋转矢量
% Output: m -旋转矩阵 Cb2n 严龚敏 2.2.23
%     m = I + sin(|rv|)/|rv|*(rvx) + [1-cos(|rv|)]/|rv|^2*(rvx)^2
%     where rvx is the askew matrix or rv.

	nm2 = rv'*rv;  % 旋转矢量的模方
    if nm2<1.e-8   % 如果模方很小，则可用泰勒展开前几项求三角函数
        a = 1-nm2*(1/6-nm2/120); b = 0.5-nm2*(1/24-nm2/720);  % a->1, b->0.5
    else
        nm = sqrt(nm2);
        a = sin(nm)/nm;  b = (1-cos(nm))/nm2;
    end
    VX = askew(rv);
    m = eye(3) + a*VX + b*VX^2;