What's wrong with this OpenGL code?

leadazide · 02-24-2005, 08:19 AM

The following piece of code should draw a sphere in an SDL window, which is opened from a Qt window. The SDL window pops up properly, but the sphere doesn't get displayed. I have little experience with OpenGL, and even less experience with GLU functions. I can't find my error. Can anyone help me?

(The constructor is being called from the main function)

Code:

Simulationwindow::Simulationwindow()
{
  //SDL-Initialisierung
  if(SDL_Init(SDL_INIT_VIDEO) < 0)
  {
    cout << "Could not initialize SDL:" << SDL_GetError() << endl;
    atexit(SDL_Quit);
  }
  //Video-Initialisierung
  SDL_GL_SetAttribute( SDL_GL_RED_SIZE, 5 );
  SDL_GL_SetAttribute( SDL_GL_GREEN_SIZE, 5 );
  SDL_GL_SetAttribute( SDL_GL_BLUE_SIZE, 5 );
  SDL_GL_SetAttribute( SDL_GL_DEPTH_SIZE, 16 );
  SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 );  
  screen = SDL_SetVideoMode(width,height,16, SDL_HWSURFACE | SDL_ANYFORMAT | SDL_OPENGL);
  
  if ( screen == NULL )
  {
     cout << "Couldn't set 640x480x16 video mode: " << SDL_GetError();
     atexit(SDL_Quit);
  }
  
  SDL_WM_SetCaption("Simulation", NULL);
  
  InitGL();
  
  InitDrawScene();
  DrawKugel(300, 200, 10);
  FinalizeDrawGLScene();

}
void Simulationwindow::InitGL()
{

  
  

  glShadeModel(GL_SMOOTH);
   
  glCullFace( GL_BACK );
  glFrontFace( GL_CCW );
  glEnable( GL_CULL_FACE );
  
  
  glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
  
  glViewport(0, 0, width, height);         
  //glEnable(GL_DEPTH_TEST);            
  
  
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  gluPerspective(60.0f, (GLfloat)width/(GLfloat)height, 1.0f, 1024.0f);
  
  
  
  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();
}

void Simulationwindow::DrawKugel(int x, int y, int r)
{
  
  glPushMatrix();

  GLfloat diffuseLight[] = { 0.8f, 0.8f, 0.8f, 1.0f };
  GLfloat specularLight[] = { 1.0f, 1.0f, 1.0f, 1.0f };
  GLfloat LightPosition[] = { 0.4f, 0.0f, 8.0f, 1.0f };
  

  
  glTranslatef(GLfloat(x), GLfloat(y), -5.0f);
  
  glRotatef(0.0f, 0.0f, 0.0f, 0.0f);



  GLUquadric* quad = gluNewQuadric();
  gluQuadricDrawStyle(quad, GLU_FILL);
  
  glEnable(GL_LIGHTING);
               
  glLightfv(GL_LIGHT0, GL_DIFFUSE, diffuseLight);
  glLightfv(GL_LIGHT0, GL_SPECULAR, specularLight);
  glLightfv(GL_LIGHT0, GL_POSITION, LightPosition);
  glLightf(GL_LIGHT0, GL_SPOT_CUTOFF, 10.0f);
  glLightf(GL_LIGHT0, GL_SPOT_EXPONENT, 75.0f);
  
  glEnable(GL_LIGHT0);

  
  glEnable(GL_COLOR_MATERIAL);
  glColorMaterial(GL_FRONT, GL_AMBIENT_AND_DIFFUSE);
  glMaterialfv(GL_FRONT, GL_SPECULAR, specularLight);
  glMateriali(GL_FRONT, GL_SHININESS, 128);
  
  
  gluSphere(quad, r, 20, 20);
  gluDeleteQuadric(quad);

  glPopMatrix();
}

void Simulationwindow::InitDrawScene()
{  
  glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
}

void Simulationwindow::FinalizeDrawGLScene()
{   
  SDL_GL_SwapBuffers();
}

P. S. Thanks for reading the whole piece of code

leadazide · 02-25-2005, 08:06 AM

I took the lighting out and tried to draw some vertices with glBegin(GL_TRIANGLES); glColor3f(), glVertex3f() and glEnd(); and this worked. But why doesn't gluSphere work?

deiussum · 02-25-2005, 08:58 AM

I didn't have time to mess with this much, but some interesting lines that are probably the source of the problem are the following (taken from different functions):

Code:

gluPerspective(60.0f, (GLfloat)width/(GLfloat)height, 1.0f, 1024.0f);

DrawKugel(300, 200, 10);
  
glTranslatef(GLfloat(x), GLfloat(y), -5.0f);

You are using a perspective projection, so your viewing volume is kind of like a pyramid shape with the top cut off. Anything within this volume will be visible, anything not in it will not be visible.

It appears that you are trying to draw your circle at 300, 200, -5. I haven't done the calculations, but I am pretty confident that this location will not fall within your viewing volume. (It is likely clipped by both the top and right clip planes). Try Change the values you pass to DrawKugel to something like 0,0 initially and go from there.

leadazide · 02-25-2005, 10:24 AM

Thank you for your reply, I´m in the library now, will try this when i come home.

leadazide · 02-26-2005, 09:31 AM

Thanks, changing the coordinates in DrawKugel(300, 200, 10); worked and I could see a sphere (althought it looked like a circle) Now I only have to adjust the lights. Although it could be better if the sphere stayed at 300.0f, 200.0f, 10.0f and I could change the gluPerspective to see it (or replace gluPerspective with gluLookAt)

deiussum · 02-27-2005, 09:09 AM

You wouldn't replace gluPerspective with gluLookAt as they do different things. Think of gluPerspective as setting the properties of your camera lens, and gluLookAt as a means pointing your camera in the direction you want. If you want to use gluLookAt, you should use it in the MODELVIEW matrix, and make sure it is the first thing applied after a glLoadIdentity().