有方向：

E:\graduate_two\zy_evaluation_8gray\01Algorithm\cornerity

function [ output_args ] = theta_direction( input_args )
calculate_theta(0,0,1,-1,-1,-1)

function [theta] = calculate_theta(x1,y1,x2,y2,x3,y3)
xf=x2-x1;
yf=y2-y1;

xb=x3-x1;
yb=y3-y1;

A=[xf,yf];
B=[xb,yb];

D=dot(A,B);
N=norm(A)norm(B);
rho=xfyb-xbyf; %%%%%根据正负判断方向 12 —>13 逆时针
ptheta=acos(D/N)/pi180;

if rho<0
theta= 180+ptheta;
else
theta=ptheta;
end

无方向：

https://www.cnblogs.com/lovebay/p/11411512.html
%计算夹角：计算以x1为顶点的两个向量的夹角
function [theta] = calculate_theta(x1,y1,x2,y2,x3,y3)
vector1 = [x2,y2,0] - [x1,y1,0];
vector2 = [x3,y3,0] - [x1,y1,0];
theta = atan2d(norm(cross(vector1, vector2)), dot(vector1, vector2));