dvigenie strelki

#include <iostream>
#include <list>
#include<cmath>
#include <algorithm>
#include <vector>
#include <GL/glut.h>
#include <GL/gl.h>
#include <GL/glu.h>

using namespace std;

double scale = 1.0;

float update_time = 1000;
float angle_step = M_PI / 30.0 / 1000.0 * update_time;

class vector2d {
public:
    float x ;
    float y;
    vector2d() : x(0) , y(0){}
    vector2d (float a ,float  b ) : x(a) , y(b)  {}
    void print(){
        cout << x <<'\t' << y <<  endl;

    }
    float dot(vector2d other){
        return (x * other.x + y * other.y);
    }
    vector2d operator + (vector2d other){
        return vector2d(x + other.x, y + other.y);
    }
    vector2d operator - (vector2d other){
        return vector2d(x - other.x, y - other.y);
    }
    vector2d operator * (float c){
        return vector2d (x * c , y * c);
    }
    vector2d operator / (float c){
        return vector2d (x / c , y / c);
    }
};

void render_square(double x, double y) {
    glPushMatrix();

    double dx = 0.5;
    double dy = 0.5;
    glBegin(GL_POLYGON);
    glVertex2f(x, y);
    glVertex2f(x + dx, y);
    glVertex2f(x + dx, y - dy);
    glVertex2f(x, y - dy);
    glEnd();
    glPopMatrix();
}

void draw_line(double x, double y, double x1, double y1) {
    glBegin(GL_LINES);
    glVertex2d(x, y);
    glVertex2d(x1, y1);
    glEnd();
}

void draw_arrow(vector2d a){
    draw_line(0, 0, a.x, a.y);
    vector2d n1(-a.y, a.x);
    vector2d n2(a.y, -a.x);
    vector2d ar_1 = (n1 - a * 3) / 10.0;
    vector2d ar_2 = (n2 - a * 3) / 10.0;
    draw_line(a.x, a.y, a.x + ar_1.x, a.y + ar_1.y);
    draw_line(a.x, a.y , a.x + ar_2.x, a.y + ar_2.y);
}





void Render() {
        static float arrow_angle = -0.5 * M_PI;
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
        glLoadIdentity();
        glTranslatef(0, 0, -10);
        glColor3d(1, 1, 1);
        glScalef(scale, scale, scale);
        vector2d a(cos(-arrow_angle),sin(-arrow_angle));
        arrow_angle += angle_step;
        if (arrow_angle > 2 * M_PI) arrow_angle -= 2 * M_PI;
        //draw_line(0,0,1,1);
        draw_arrow(a);


        glFlush();
}

void update(int t) {
    Render();
    glutTimerFunc(update_time, update, 0);
}

void keyb(unsigned char key, int x, int y) {
    if(key == '+') {
        scale *= 1.1;
    }
    if(key == '-') {
        scale *= 0.9;
    }

    Render();
}

void reshape(int w, int h) {

    // предупредим деление на ноль
    // если окно сильно перетянуто будет
    if(h == 0)
        h = 1;
    float ratio = 1.0* w / h;

    // используем матрицу проекции
    glMatrixMode(GL_PROJECTION);

        // Reset матрицы
    glLoadIdentity();

    // определяем окно просмотра
    glViewport(0, 0, w, h);

    // установить корректную перспективу.
    gluPerspective(45,ratio,1,1000);

    // вернуться к модели
    glMatrixMode(GL_MODELVIEW);
}

int main(int argc, char **argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH);
    glutInitWindowSize(800, 600);
    glutCreateWindow("Physics Engine");
    glEnable(GL_DEPTH_TEST);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glClearColor(0., 0.0, 128.0, 0.0);

    glutTimerFunc(50, update, 0);
    glutReshapeFunc(reshape);
    glutDisplayFunc(Render);
    glutKeyboardFunc(keyb);
    glutMainLoop();
}