二维图元生成：五角星绘制实例

OpenGL程序设计结构

以老版的opengl为例

// OpenGLOld.cpp : 定义控制台应用程序的入口点。
//
#include <GL/glew.h>
#include <GL/glut.h>
void init(void){
	glClearColor(1.0, 1.0, 1.0, 1.0);//设置背景色白色
}

void display(void){
	glClear(GL_COLOR_BUFFER_BIT);

	glBegin(GL_TRIANGLES);//图形类型，GL_LINES为线段
		glColor3f(1.0f,0.0f,0.0f);//指定顶点颜色
		glVertex2f(100.0f,100.0f);//指定顶点坐标
		glColor3f(0.0f,1.0f,0.0f);
		glVertex2f(500.0f,100.0f);
		glColor3f(0.0f,0.0f,1.0f);
		glVertex2f(300.0f,400.0f);
	glEnd();

	//glFlush();//单缓冲时必须要，说明绘图命令（函数）结束
	glutSwapBuffers();//交换缓冲（双缓冲时使用）
}

void reshape(int width, int height){
	glViewport(0, 0, width, height);

	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	gluOrtho2D(0, width, 0, height);
	glMatrixMode(GL_MODELVIEW);
}

void keyboard(unsigned char key, int x, int y){
}

int main(int argc, char* argv[]){
	glutInitWindowPosition(200, 200);
	glutInitWindowSize(800, 600);
	
	glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE);//双缓冲，单缓冲为GLUT_SINGLE
	glutCreateWindow("OpenGL Old");
	init();

	glutDisplayFunc(display);//图形绘制
	glutReshapeFunc(reshape);//窗口大小变化
	glutKeyboardFunc(keyboard);//键盘交互

	glutMainLoop();

	return 0;
}

遇到的问题：gluOrtho2D(0, width, 0, height);有问题，查了很多资料也解决不了

所以就改成glOrtho(-width,width,-height,height, -1,1); 在原理上等于glOrtho(-width,width,-height,height)

这个函数的功能：待更

绘图实例

线段

• glBegin(GL_LINES)//画线
• glBegin(GL_LINE_STRIP)
• glBegin(GL_LINE_LOOP)

//一条线
glColor3f(0,0,1);
glBegin(GL_LINES);
glVertex2f(-110,0);
glVertex2f(-200,30);		
glEnd();
//图形线围成图形
glBegin(GL_LINE_LOOP); 
glVertex2f(30,30); 
glVertex2f(100,100); 
glVertex2f(100,200); 
glVertex2f(50,100); 
glEnd();

填充图形

• glBegin(GL_POLYGON)

glColor3f(0,1,0);
glBegin(GL_POLYGON);
glVertex2f(110,0);
glVertex2f(200,0);
glVertex2f(200,30);
glVertex2f(110,30);
glEnd();

五角星原理

其实是一大一小的圆，然后按照五等分得到的点

我们需要知道大小圆的半径\(R\)和\(r\)，圆心坐标\((x_c,y_c)\),和夹角\(\theta\)

先确定第一个小三角形的两个坐标，然后旋转

旋转是通过弧度制，弧度加上五等分的角\(\alpha\) 就行

实现代码

Π可以直接通过cos(-1.0)得到

外圆点的角度初值为p=Π/2，内圆的初始值为p=Π/2+theta

为了两种颜色的质感，奇偶不同颜色

#include <iostream>
#include <cmath>
// GLEW
#define GLEW_STATIC
#include <GL/glew.h>

// GLFW
#include <GLFW/glfw3.h>

#include <GL/glut.h>

using namespace std;
const float pai = acos(-1.0);
const float alpha = 0.4*pai;
//初始化背景
void init() {
    glClearColor(0, 0, 0, 0);
    glMatrixMode(GL_PROJECTION);
}


void draw(int cx,int cy, float R, float r, float theta){
    float *x,*y;
    x = new float[10],y=new float[10];
    float p = pai/2;
    for(int i = 0;i < 10;i += 2){
        x[i] = R*cos(p);
        y[i] = R*sin(p);
        p += alpha;
    }
    p = pai/2+theta;
    for(int i = 1;i < 10;i += 2){
        x[i] = r*cos(p);
        y[i] = r*sin(p);
        p += alpha;
    }
    for(int i = 0;i < 10;++i){
        glColor3f(1 ,0 ,0);
        glBegin(GL_LINES);
        glVertex2f(x[i],y[i]);
        glVertex2f(x[(i + 1) % 10],y[(i + 1) % 10]);
        glEnd();
    }
    for(int i = 0;i < 10;++i){
       if(i & 1) glColor3f(1 ,0 ,0);
       else glColor3f(1 ,1 ,0);
        glBegin(GL_POLYGON);
        glVertex2f(x[i],y[i]);
        glVertex2f(x[(i + 1) % 10],y[(i + 1) % 10]);
        glVertex2f(cx,cy);
        glEnd();
    }
    glutSwapBuffers();
    delete []x;
    delete []y;

}
//生成五角星
void display() {
    draw(0,0,200,100,0.2*pai);
}
void reshape(int width, int height) {
    glViewport(0, 0, width, height);
    glOrtho(-width,width,-height,height, -1,1);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glMatrixMode(GL_MODELVIEW);
}

int main(int argc, char *argv[]) {
   glutInit(&argc, argv);
    glutInitWindowPosition(200,200);
    glutInitWindowSize(800, 600);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutCreateWindow("DEMO PHY");

    init();

    glutDisplayFunc(display);
    glutReshapeFunc(reshape);

    glutMainLoop();
    return 0;
}