How are menus and submenus created in OpenGL? Illustrate with an example.

6.B] How are menus and submenus created in OpenGL? Illustrate with an example.

Answer:

Menus are an important feature of any application program. OpenGL provides a feature called “Pop-up-menus” using which sophisticated interactive applications can be created.

Creating Menus and Submenus in OpenGL

1. Define the Menu Callback Function
   • Create a callback function that will handle menu item selections. This function will be called whenever a menu item is selected by the user.
2. Create Menu Items
   • Use the glutCreateMenu function to create a menu and specify the callback function. Then, use glutAddMenuEntry to add individual menu items.
3. Create Submenus
   • Create a submenu using glutCreateMenu and add items to it using glutAddMenuEntry. Attach this submenu to a parent menu using glutAddSubMenu.
4. Attach the Menu to a Mouse Button
   • Use glutAttachMenu to attach the menu to a mouse button (usually the right mouse button).

// Function to create the menus
void createMenu() {
    int submenu;

    // Create a submenu
    submenu = glutCreateMenu(menu);
    glutAddMenuEntry("Submenu Item 1", 3);
    glutAddMenuEntry("Submenu Item 2", 4);

    // Create the main menu
    glutCreateMenu(menu);
    glutAddMenuEntry("Menu Item 1", 1);
    glutAddMenuEntry("Menu Item 2", 2);
    glutAddSubMenu("Submenu", submenu);
    glutAddMenuEntry("Exit", 5);

    // Attach the menu to the right mouse button
    glutAttachMenu(GLUT_RIGHT_BUTTON);
}

// Function to handle menu item selections
void menu(int item) {
    switch (item) {
        case 1:
            printf("Menu Item 1 selected\n");
            break;
        case 2:
            printf("Menu Item 2 selected\n");
            break;
        case 3:
            printf("Submenu Item 1 selected\n");
            break;
        case 4:
            printf("Submenu Item 2 selected\n");
            break;
        case 5:
            printf("Exit selected\n");
            exit(0);
            break;
    }
}

Full Code:-

#include <GL/glut.h>
#include <stdio.h>

// Function to handle menu item selections
void menu(int item) {
    switch (item) {
        case 1:
            printf("Menu Item 1 selected\n");
            break;
        case 2:
            printf("Menu Item 2 selected\n");
            break;
        case 3:
            printf("Submenu Item 1 selected\n");
            break;
        case 4:
            printf("Submenu Item 2 selected\n");
            break;
        case 5:
            printf("Exit selected\n");
            exit(0);
            break;
    }
}

// Function to create the menus
void createMenu() {
    int submenu;

    // Create a submenu
    submenu = glutCreateMenu(menu);
    glutAddMenuEntry("Submenu Item 1", 3);
    glutAddMenuEntry("Submenu Item 2", 4);

    // Create the main menu
    glutCreateMenu(menu);
    glutAddMenuEntry("Menu Item 1", 1);
    glutAddMenuEntry("Menu Item 2", 2);
    glutAddSubMenu("Submenu", submenu);
    glutAddMenuEntry("Exit", 5);

    // Attach the menu to the right mouse button
    glutAttachMenu(GLUT_RIGHT_BUTTON);
}

// Display function (for rendering)
void display() {
    glClear(GL_COLOR_BUFFER_BIT);
    glFlush();
}

// Main function
int main(int argc, char** argv) {
    // Initialize GLUT
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowSize(500, 500);
    glutCreateWindow("OpenGL Menu Example");

    // Set up the display function
    glutDisplayFunc(display);

    // Create the menu
    createMenu();

    // Start the GLUT event loop
    glutMainLoop();
    return 0;
}

Explanation

1. Menu Callback Function (menu)
   • This function is called whenever a menu item is selected. It handles different cases based on the item selected.
2. Creating Menus (createMenu)
   • A submenu is created first using glutCreateMenu and populated with items using glutAddMenuEntry.
   • The main menu is created and populated with items, including the submenu using glutAddSubMenu.
   • Finally, the menu is attached to the right mouse button using glutAttachMenu.
3. Display Function (display)
   • This function clears the screen and is required by GLUT, though it doesn’t do much in this example.
4. Main Function (main)
   • Initializes GLUT, sets up the window, and starts the event loop.