OpenGL Optimization or...? - c++

Question

I'm making a OpenGL game, and I have problem with optimization. When I start it, it does not respond. If in Update() I just put a for loop and _time += 0.1f, I get a blank screen.
void Update(){
for(; ;){
_time += 0.1f;
Render();
}
}
void Render() {
glClearDepth(1.0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
_colorProgram.use();
GLuint timeLocation = _colorProgram.getUniformLocation("time");
glUniform1f(timeLocation, _time);
_sprite.Render();
_colorProgram.unuse();
glutSwapBuffers();
}
int main(int argc, char** argv) {
std::printf("OpenGL version is %s",glGetString(GL_VERSION));
// Window
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DEPTH | GLUT_DOUBLE | GLUT_RGBA);
glutInitWindowPosition(520, 200);
glutInitWindowSize(800, 600);
glutInitDisplayMode(GLUT_DOUBLE);
glutCreateWindow("OpenGL [ Shader #1 error pt 3 ]");
// Setup GLEW
if( GLEW_OK != glewInit()){
return 1;
} while( GL_NO_ERROR != glGetError() );
// After creating window
Init();
glutDisplayFunc(Render);
Update();
glutMainLoop();
}

Answer 1

The infinite loop in Update() never lets GLUT pump the event queue.
Use glutTimerFunc() or glutIdleFunc() to call Update() instead. That way execution flow periodically returns to GLUT and GLUT can do what it needs to keep the OS happy.

Answer 2

The proper way to run an animation with GLUT is to use a timer function. This way, GLUT can get back to its main loop, and call your display function. You're not supposed to call the display function directly from your own code.
For example, register a timer function during initialization (the first argument is a time in milliseconds):
glutTimerFunc(10, timerFunc, 0);
Then in the timer function:
void timerFunc(int value) {
_time += 0.1f;
glutPostRedisplay();
glutTimerFunc(10, timerFunc, 0);
}
There are two critical pieces in the code fragment above:
You do not call your Render() function directly. Instead, you call glutPostRedisplay() to tell GLUT that a redisplay is needed. It will then call your Render() function because you registered it as the display function with glutDisplayFunc().
You have to register the timer again. glutTimerFunc() fires the timer only once, not periodically. So you have to re-register it every time it fired.
There is one other problem in your code. You have these calls in your main():
glutInitDisplayMode(GLUT_DEPTH | GLUT_DOUBLE | GLUT_RGBA);
...
glutInitDisplayMode(GLUT_DOUBLE);
The flags passed in the second call will override the ones from the first call. While GL_RGBA is the default anyway, you will not get a depth buffer because GL_DEPTH is missing in the second call. You can simply remove the second call, since the first one is most likely what you want.