OpenGL从1.0开始--GLUT菜单

我们现在来学习如何利用GLUT添加弹出式菜单，直接看示例：

#include <Gl/glut.h>
#include <stdio.h>
const GLint pickBuffsize = 32;
GLsizei winWidth = 400, winHeight = 400;
GLfloat red = 1.0, green = 1.0, blue = 1.0;
GLenum fillMode = GL_SMOOTH;
void init()
{
    glClearColor(0.6, 0.,0.6, 1.0);
    glMatrixMode(GL_PROJECTION);
    gluOrtho2D(0.0, 300.0, 0.0, 300.0);
}
void fillOption(GLint selectedOption)//菜单消息响应函数
{
    switch (selectedOption)
    {
    case 1:
        fillMode = GL_FLAT;//同色填充
        break;
    case 2:
        fillMode = GL_SMOOTH;//顶点颜色插值
        break;
    default:
        break;
    }
    glutPostRedisplay();//消息响应后必须被重绘
}
void displayTriangle()
{
    glClear(GL_COLOR_BUFFER_BIT);
    glShadeModel(fillMode);//多边形填充方式
    glColor3f(red, green, blue);
    glBegin(GL_TRIANGLES);//两个白点
    glVertex2i(280, 20);
    glVertex2i(160, 280);
    glColor3f(red, 0.0, 0.0);//一个红点
    glVertex2i(20, 100);
    glEnd();
    glFlush();
}

void winReshapeFcn(GLint newWidth, GLint newHeight)
{
    glViewport(0, 0, newWidth, newHeight);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0.0, GLdouble(newWidth), 0.0, GLdouble(newHeight));
    winWidth = newWidth;
    winHeight = newHeight;
}

void main(int argc, char**argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowPosition(100, 100);
    glutInitWindowSize(winWidth, winHeight);
    glutCreateWindow("An Example OpenGL Program");
    init();
    glutDisplayFunc(displayTriangle);
    glutCreateMenu(fillOption);//创建菜单并绑定回调函数
    glutAddMenuEntry("Solid-Color Fill", 1);//第一个菜单项
    glutAddMenuEntry("Color-Interpolation Fill", 2);//第二个菜单
    glutAttachMenu(GLUT_RIGHT_BUTTON);//指定鼠标右键来弹出菜单项
    glutReshapeFunc(winReshapeFcn);

    glutMainLoop();//启动主循环，等待消息
}

程序默认显示的是一个插值变化的三角形，当我们点击鼠标右键时，在光标位置出现弹出式菜单可以改变插值方式，当我们选择第一个选项时，三角形变为红色。下面我们来讲解下相应菜单函数。
以下语句创建一个弹出式菜单;

glutCreateMenu(menuFcn);

其参数是一个回调函数：

void menuFcn(GLint menuItemNumber)

函数的整型参数对应于选中项的位置。我们用下面的函数来设定每个菜单项的名称和位置：

glutAddMenuEntry("First Menu Item",1);
glutAddMenuEntry("Second Menu Item",2);

然后，我们必须通过以下函数指定来选择菜单项的鼠标键

glutAttachMenu(button);

button即鼠标键的符号常量。
非常简单地我们也可以在菜单项中添加子菜单。

#include <Gl/glut.h>
#include <stdio.h>
const GLint pickBuffsize = 32;
GLsizei winWidth = 400, winHeight = 400;
GLfloat red = 1.0, green = 1.0, blue = 1.0;
GLenum fillMode = GL_SMOOTH;
void init()
{
    glClearColor(0.6, 0.,0.6, 1.0);
    glMatrixMode(GL_PROJECTION);
    gluOrtho2D(0.0, 300.0, 0.0, 300.0);
}
void mainMenu(GLint selectedOption)
{
    switch (selectedOption)
    {
    case 1:
        fillMode = GL_FLAT;
        break;
    case 2:
        fillMode = GL_SMOOTH;
        break;
    default:
        break;
    }
    glutPostRedisplay();
}
void colorSubMenu(GLint colorOption)
{
    switch (colorOption)
    {
    case  1:
        red = 0.0, green = 0.0, blue = 1.0;
        break;
    case  2:
        red = 0.0, green = 1.0, blue =0.0;
        break;
    case  3:
        red = 1.0, green = 0.0, blue = 0.0;
        break;
    default:
        break;
    }
    glutPostRedisplay();
}
void displayTriangle()
{
    glClear(GL_COLOR_BUFFER_BIT);
    glShadeModel(fillMode);
    glColor3f(red, green, blue);
    glBegin(GL_TRIANGLES);
    glVertex2i(280, 20);
    glVertex2i(160, 280);
    glColor3f(red, 0.0, 0.0);
    glVertex2i(20, 100);
    glEnd();
    glFlush();
}
void winReshapeFcn(GLint newWidth, GLint newHeight)
{
    glViewport(0, 0, newWidth, newHeight);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0.0, GLdouble(newWidth), 0.0, GLdouble(newHeight));
    winWidth = newWidth;
    winHeight = newHeight;
}
void main(int argc, char**argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowPosition(100, 100);
    glutInitWindowSize(winWidth, winHeight);
    glutCreateWindow("An Example OpenGL Program");
    init();
    glutDisplayFunc(displayTriangle);
    GLint subMenu = glutCreateMenu(colorSubMenu);//子菜单项和普通菜单项创建一样
    glutAddMenuEntry("Blue", 1);
    glutAddMenuEntry("Green", 2);
    glutAddMenuEntry("Red", 3);
    glutCreateMenu(mainMenu);//主菜单项
    glutAddMenuEntry("Solid-Color Fill", 1);
    glutAddMenuEntry("Color-Interpolation Fill", 2);
    glutAddSubMenu("color", subMenu);//给主菜单添加子菜单项
    glutAttachMenu(GLUT_RIGHT_BUTTON);
    glutReshapeFunc(winReshapeFcn);
    glutMainLoop();//启动主循环，等待消息
}

可以看到整个过程是非常简单的，只是利用glutAddSubMenu将一菜单项添加到了另一菜单项下面作为子菜单项。