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我需要使用 DDA 绘制一条线，用户输入为 x1、x2、y1、y2，但是当我运行代码并输入 Xs 和 Ys 时，只显示空窗口。出了什么问题，我怎样才能让它画一条线？

这是我第一次使用 Glew 和 Glfw，所以我并不熟悉。如果您向我解释我应该改变什么，将不胜感激。

#include <iostream>
#include <cmath>
#include <GL/glew.h>
#include <GLFW/glfw3.h>

class line{
    int x1, x2, y1, y2;
    void drawline(){
        int dx, dy, step, x, y;
        float xinc, yinc;
        dx = x2 - x1;
        dy = y2 - y1;
        if (abs(dx) > abs(dy))
            step = abs(dx);
        else step = abs(dy);
        xinc = dx/step;
        yinc = dy/step;
        x = x1;
        y = y1;
        glBegin(GL_POINTS);
        glVertex2d(x, y);
        for(int i = 0; i < step; i++){
            x += xinc;
            y += yinc;
            glVertex2d(floor(x), floor(y));
        }
        glEnd();
        glFlush();
    }
public:
    line(int x1, int x2, int y1, int y2){
        this -> x1 = x1;
        this -> x2 = x2;
        this -> y1 = y1;
        this -> y2 = y2;
    }
    line(){
        
    }
    void draw(){
        this -> drawline();
    }
};

void drawtheline(int x1, int x2, int y1, int y2){
    line a;
    a = line(x1, x2, y1, y2);
    a.draw();
}

void init(){
    glClear(GL_COLOR_BUFFER_BIT);
    glClearColor(1.0,1.0,1.0,0.0);
    glColor3f(0.0,0.0,0.0);
    glLoadIdentity();
    gluOrtho2D(0 , 640 , 0 , 480);
}


int main() {
    int x1, x2, y1, y2;
    std::cout << "Enter x1 and y1: " << std::endl;
    std::cin >> x1 >> y1;
    std:: cout << "Enter x2 and y2: " << std::endl;
    std::cin >> x2 >> y2;
    GLFWwindow *window;
    if (!glfwInit()){
        return -1;
    }
    window = glfwCreateWindow(720, 480, "DDA Line Drawing", NULL, NULL);
    if (!window){
        glfwTerminate();
        return -1;
    }
    glfwMakeContextCurrent(window);
    while (!glfwWindowShouldClose(window)){
        glfwPollEvents();
        init();
        drawtheline(x1, x2, y1, y2);
        glfwSwapBuffers(window);
    }
    glfwTerminate();
    return 0;
}
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