从菜单中选择时如何只绘制矩形？答案

【问题标题】：How to only draw the rectangle when choosing from the menu?从菜单中选择时如何只绘制矩形？
【发布时间】：2021-07-22 10:57:06
【问题描述】：

我正在编写一个程序，当且仅当用户在菜单中选择它时，才用鼠标绘制一个矩形，如果用户不选择它就不会绘制。到目前为止，我已经成功地用鼠标绘制了矩形，现在如何创建一个菜单让用户可以选择绘制矩形？这是我的程序：

struct Position
{
    Position() : x(0), y(0) {}
    float x, y;
};
Position start, finish;

void mouse(int button, int state, int x, int y)
{
    if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN)
    {
        start.x = finish.x = x;
        start.y = finish.y = y;
    }
    if (button == GLUT_LEFT_BUTTON && state == GLUT_UP)
    {
        finish.x = x;
        finish.y = y;
    }
    glutPostRedisplay();
}

void motion(int x, int y)
{
    finish.x = x;
    finish.y = y;
    glutPostRedisplay();
}

void display()
{
    glClear(GL_COLOR_BUFFER_BIT);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    double w = glutGet(GLUT_WINDOW_WIDTH);
    double h = glutGet(GLUT_WINDOW_HEIGHT);
    glOrtho(0, w, h, 0, -1, 1);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glPushMatrix();
    glBegin(GL_LINE_LOOP);
    glVertex2f(start.x, start.y);
    glVertex2f(finish.x, start.y);
    glVertex2f(finish.x, finish.y);
    glVertex2f(start.x, finish.y);
    glEnd();
    glPopMatrix();

    glutSwapBuffers();
}

void menu(int choice)
{
    switch (choice)
    {
    case 1:
        // What to write in here?
        break;
    }
    glutPostRedisplay();
}

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowSize(640, 480);
    glutInitWindowPosition(100, 100);
    glutCreateWindow("");
    
    glutCreateMenu(menu);
    glutAddMenuEntry("Rectangle", 1);
    glutAttachMenu(GLUT_RIGHT_BUTTON);

    glutMouseFunc(mouse);
    glutMotionFunc(motion);
    glutDisplayFunc(display);

    glutMainLoop();
}

【问题讨论】：

标签： c++ opengl glut

【解决方案1】：

添加一个变量来指示必须绘制的形状：

int shape = 0;

在menu中设置变量：

void menu(int choice)
{
    switch (choice)
    {
    case 1:
        shape = 1
        break;
    }
    glutPostRedisplay();
}

根据shape绘制场景：

void display()
{
    glClear(GL_COLOR_BUFFER_BIT);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    double w = glutGet(GLUT_WINDOW_WIDTH);
    double h = glutGet(GLUT_WINDOW_HEIGHT);
    glOrtho(0, w, h, 0, -1, 1);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    if (shape == 1)
    {
        glPushMatrix();
        glBegin(GL_LINE_LOOP);
        glVertex2f(start.x, start.y);
        glVertex2f(finish.x, start.y);
        glVertex2f(finish.x, finish.y);
        glVertex2f(start.x, finish.y);
        glEnd();
        glPopMatrix();
    }

    glutSwapBuffers();
}

【讨论】：