圓弧的中點掃描演算法畫圓弧（添加了鍵盤監聽事件）

#include<GL\glut.h>
#include<iostream>
#include<cmath>
#include <string.h>
using namespace std;

int theWindow;
int a, b;
int flag=1;

void init(void)
{
	glClearColor(1.0, 1.0, 1.0, 0.0);//將顯示視窗設定為白色  
	glShadeModel(GL_FLAT);//設定著色模式為恆定著色
}

/*
畫點
*/
void setPixel(int x, int y)
{
	glColor3f(0.0, 0.0, 1.0);       // 藍色
	glPointSize(1.0f);
	glBegin(GL_POINTS);
	glVertex2f(x, y);
	glEnd();
	glFlush();
}

/*
橢圓4路對稱
*/
void Ellipsepot(int x0, int y0, int x, int y)
{
	// 1
	setPixel((x0 + x), (y0 + y));
	// 2
	setPixel((x0 + x), (y0 - y));
	// 3
	setPixel((x0 - x), (y0 - y));
	// 4
	setPixel((x0 - x), (y0 + y));
}

/*
中點畫橢圓演算法
*/
void MidPoint_Ellipse(int x0, int y0, int a, int b)
{
	double sqa = a*a;
	double sqb = b*b;

	double d = sqb + sqa*(0.25 - b);
	int x = 0;
	int y = b;
	Ellipsepot(x0, y0, x, y);
	// 1
	while (sqb*(x + 1) < sqa*(y - 0.5))
	{
		if (d < 0)
		{
			d += sqb*(2 * x + 3);
		}
		else
		{
			d += (sqb*(2 * x + 3) + sqa*((-2)*y + 2));
			--y;
		}
		++x;
		Ellipsepot(x0, y0, x, y);
	}
	d = (b * (x + 0.5)) * 2 + (a * (y - 1)) * 2 - (a * b) * 2;
	// 2
	while (y > 0)
	{
		if (d < 0)
		{
			d += sqb * (2 * x + 2) + sqa * ((-2) * y + 3);
			++x;
		}
		else
		{
			d += sqa * ((-2) * y + 3);
		}
		--y;
		Ellipsepot(x0, y0, x, y);
	}
}

// 視窗大小改變時呼叫的登記函式
void ChangeSize(GLsizei w, GLsizei h)
{

	if (h == 0)     h = 1;

	// 設定視區尺寸
	glViewport(0, 0, w, h);

	// 重置座標系統
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();

	// 建立修剪空間的範圍
	if (w <= h)
		glOrtho(0.0f, 250.0f, 0.0f, 250.0f*h / w, 1.0, -1.0);
	else
		glOrtho(0.0f, 250.0f*w / h, 0.0f, 250.0f, 1.0, -1.0);

}

/*

*/
void display()
{
	// 用當前背景色填充視窗，如果不寫這句會殘留之前的影象
	glClear(GL_COLOR_BUFFER_BIT);
	
	
	int x0 = 120, y0 = 100;
	

	MidPoint_Ellipse(x0, y0, a, b);
}
 

//鍵盤監聽事件
void Specialkeys(int key, int x, int y){
	if (key == GLUT_KEY_UP){
		glutDestroyWindow(theWindow);
		exit(0);
	}
}

void func(){
	glutInitDisplayMode(GLUT_RGB | GLUT_SINGLE);

	glutInitWindowPosition(200, 200);
	glutInitWindowSize(400, 400);
	theWindow = glutCreateWindow("按up鍵退出");
	glutSpecialFunc(Specialkeys);

	glutDisplayFunc(display);
	glutReshapeFunc(ChangeSize);
	glutMainLoop();
}

int main(int argc, char* argv[])
{
	glutInit(&argc, argv);

	init();
	cout << "請輸入a:";
	cin >> a;
	cout << "請輸入b:";
	cin >> b;
	func();
	
	glutMainLoop();
	return 0;
}