给出一个输入为两个三维点和半径的画一个圆柱体的m函数，并给出使用例子

以下是一个MATLAB的函数，可以通过两个三维点和半径来画出一个圆柱体：

function [X,Y,Z] = draw_cylinder(p1,p2,r)
% p1: 3-element vector representing the first point
% p2: 3-element vector representing the second point
% r: radius of the cylinder
% X,Y,Z: output matrices for plotting

% calculate the axis and height of the cylinder
axis = p2 - p1;
h = norm(axis);

% calculate the angle between the axis and the z-axis
theta = acos(axis(3) / h);

% calculate the rotation matrix
R = vrrotvec2mat(vrrotvec([0 0 1], axis / h));

% generate the coordinates of a circle
theta_circ = linspace(0, 2*pi, 100);
x_circ = r * cos(theta_circ);
y_circ = r * sin(theta_circ);

% create the cylinder
[X_cyl,Y_cyl,Z_cyl] = cylinder(r, 100);
Z_cyl = Z_cyl * h;

% transform the cylinder to the correct orientation
for i = 1:numel(X_cyl)
    p = [X_cyl(i); Y_cyl(i); Z_cyl(i)];
    p = R * p;
    X_cyl(i) = p(1) + p1(1);
    Y_cyl(i) = p(2) + p1(2);
    Z_cyl(i) = p(3) + p1(3);
end

% generate the end caps
[X_cap1,Y_cap1] = meshgrid(x_circ, y_circ);
Z_cap1 = zeros(size(X_cap1)) + p1(3);
[X_cap2,Y_cap2] = meshgrid(x_circ, y_circ);
Z_cap2 = zeros(size(X_cap2)) + p2(3);

% transform the end caps to the correct orientation
for i = 1:numel(X_cap1)
    p1_cap = [X_cap1(i); Y_cap1(i); Z_cap1(i)];
    p2_cap = [X_cap2(i); Y_cap2(i); Z_cap2(i)];
    p1_cap = R * p1_cap;
    p2_cap = R * p2_cap;
    X_cap1(i) = p1_cap(1) + p1(1);
    Y_cap1(i) = p1_cap(2) + p1(2);
    Z_cap1(i) = p1_cap(3) + p1(3);
    X_cap2(i) = p2_cap(1) + p2(1);
    Y_cap2(i) = p2_cap(2) + p2(2);
    Z_cap2(i) = p2_cap(3) + p2(3);
end

% combine the cylinder and end caps
X = [X_cyl, X_cap1, X_cap2];
Y = [Y_cyl, Y_cap1, Y_cap2];
Z = [Z_cyl, Z_cap1, Z_cap2];

% plot the cylinder
surf(X,Y,Z);
end

使用例子：

p1 = [0, 0, 0];
p2 = [1, 1, 1];
r = 0.5;
draw_cylinder(p1, p2, r);

这将会画出一个以(0,0,0)和(1,1,1)为中心的半径为0.5的圆柱体。