c++ - C++ Opengl 圆函数是如何工作的

Question

我正在阅读一本关于 opengl 的书，其中有一个画圆的函数，但我不知道如何将这个函数放入我的代码中并运行它，我也不知道我在其中输入了什么参数。

我是 opengl 的新手，我正在努力弄明白。

代码

#include <stdlib.h>
#include <GL/glut.h>
#include <cmath>

void keyboard(unsigned char key, int x, int y);
void display(void);
void drawCircle(float cx, float cy, float r, int num_segments);

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutKeyboardFunc(&keyboard);
    glutDisplayFunc(&display);
    glutMainLoop();

    return EXIT_SUCCESS;
}

void keyboard(unsigned char key, int x, int y)
{
    switch (key)
    {
    case '\x1B':
        exit(EXIT_SUCCESS);
        break;
    }
}

void display()
{
    glClear(GL_COLOR_BUFFER_BIT);
    //drawCircle(, , , );
    glFlush();
}

void drawCircle(float cx, float cy, float r, int num_segments)
{
    glBegin(GL_LINE_LOOP);
    for (int i = 0; i < num_segments; i++)
    {
        float theta = i * (2.0f * PI / num_segments); // get the current angle
        float x = r * cos(theta); // calculate the x component
        float y = r * sin(theta); // calculate the y component
        glVertex2f(x + cx, y + cy); // output vertex
    }
    glEnd();
}

Answer 1

您缺少窗口创建和设置您所在的圆圈的颜色绘图:

#include <stdlib.h>
#include <gl/glut.h>
#include <math.h>
#define M_PI 3.14159265359


void keyboard(unsigned char key, int x, int y);
void display(void);
void drawCircle(float cx, float cy, float r, int num_segments);

int main(int argc, char** argv)
{
  int width = 1280;
  int height = 720;
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_RGB | GLUT_SINGLE);
  glutInitWindowSize(width, height);
  glutCreateWindow("circle");
  glutKeyboardFunc(&keyboard);
  glutDisplayFunc(&display);
  glutMainLoop();
  return EXIT_SUCCESS;
}

void keyboard(unsigned char key, int x, int y)
{
  switch (key)
  {
  case '\x1B':
    exit(EXIT_SUCCESS);
    break;
  }
}

void display()
{
  glColor3f(1.0, 0, 0);
  glClear(GL_COLOR_BUFFER_BIT);
  gluOrtho2D(0.0, 1280, 0.0, 720);
  glColor3f(1.0, 1.0, 1.0);
  drawCircle(640, 360, 100, 200);
  glFlush();
}

void drawCircle(float cx, float cy, float r, int num_segments)
{
  glBegin(GL_LINE_LOOP);
  for (int i = 0; i < num_segments; i++)
  {
    float theta = i * (2.0f * M_PI / num_segments); // get the current angle
    float x = r * cos(theta); // calculate the x component
    float y = r * sin(theta); // calculate the y component
    glVertex2f(x + cx, y + cy); // output vertex
  }
  glEnd();
}