opengl: question about glutMainLoop()

问题:

can somebody explain how does glutMainLoop work? and second question, why glClearColor(0.0f, 0.0f, 1.0f, 1.0f); defined after glutDisplayFunc(RenderScene); cause firstly we call glClear(GL_COLOR_BUFFER_BIT); and only then define glClearColor(0.0f, 0.0f, 1.0f, 1.0f);

int main(int argc, char* argv[])
    {
        glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
        glutInitWindowSize(800, 00);
        glutInitWindowPosition(300,50);
    glutCreateWindow("GLRect");
    glutDisplayFunc(RenderScene);
        glutReshapeFunc(ChangeSize);
    glClearColor(0.0f, 0.0f, 1.0f, 1.0f);  <--
    glutMainLoop();

        return 0;
    }

void RenderScene(void)
    {
    // Clear the window with current clearing color
    glClear(GL_COLOR_BUFFER_BIT);

    // Set current drawing color to red
    //         R     G     B
    glColor3f(1.0f, 0.0f, 1.0f);

    // Draw a filled rectangle with current color
    glRectf(0.0f, 0.0f, 50.0f, -50.0f);

    // Flush drawing commands
    glFlush();
    }

回答1:

glutMainLoop() just runs a platform-specific event loop and calls any registered glut*Func() callbacks as needed.

RenderScene() won't be called by GLUT until you call glutMainLoop(). So in reality glClearColor() gets called first, not glClear().

回答2:

glutDisplayFunc(RenderScene);

This only sets the callback function, it doesn't actually call it until it enters the main app loop in the call to glutMainLoop. So glClearColor comes before glClear.