Question

Here I created one simple OpenGLES iPad sample.

//----------------------------------------
glMatrixMode(GL_PROJECTION);
glLoadIdentity();

glFrustumf(-1, 1, -1, 1, 0, 20);
glMatrixMode(GL_MODELVIEW);

 static const GLfloat squareVertices[] = {
    -0.5f,  -0.33f,1.6,
     0.5f,  -0.33f,1.6,
    -0.5f,   0.33f,1.6,
     0.5f,   0.33f,1.6
 };

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
. . .  . . . .
. . .  . . . .
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

//----------------------------------------

I used gluLookAt in mac sample, but it is not found in iPad. What is the basic mistake in above code? why square is not visible?

if I change z value in vertex array then works fine.

  static const GLfloat squareVertices[] = {
    -0.5f,  -0.33f,0.5,
     0.5f,  -0.33f,0.5,
    -0.5f,   0.33f,0.5,
     0.5f,   0.33f,0.5
  };

I thought z value range should be 0-20, here 1.6, its in range. I don’t like to change vertex value now. Help me to set glFrustumf.

Answer 1

The near and far values passed to glFrustum are not absolute coordinates in object space. They are the distances from the camera to the near and far planes. That means that you can’t say that a z coordinate of 1.6 is necessarily in your frustum unless you know where the camera position is and what direction it is pointing in.

For example if your camera is at (0,0,-19) pointed at the origin and you pass 0 and 20 to glFrustum for near and far then in object space the near plane is at -19 (-19+0) and the far plane is at 1 (-19+20). In such a case the square with z=1.6 would be past the far plane but the one with z=0.5 would be inside the frustum. Similarly if your camera was at (0,0,1) with the same values for glFrustum then in object space the near plane would be at 1 and the far would be at -19 so 0.5 would be in and 1.6 would be out.