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I'm doing this OpenGL project for my Computer Graphics class, where I display an object and I rotate it and stuff, the thing is that at the beginning of the project we used glOrtho() and it looked really great.

glOrthoOBJ

But now the teacher said that we have to use glFrustum() for perspective and if I use that function, the object is drawn like this and I really don't know why does this happens:

glFrustumOBJ

This is my code from the init() function where everything changes: 

void init (void)
{
    /*  select clearing (background) color       */
    glClearColor (0.0, 0.0, 0.0, 0.0);
    glEnable(GL_CULL_FACE);
    glCullFace(GL_BACK);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(-100.0, 100.0, -60.0, 160.0, -100.0, 100.0);
    //glFrustum(-100, 100 ,-100 ,100 ,1 , 40);


    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glRotatef(90,0,1,0);
}

I'd appreciate your help.

EDIT: If I use glFrustum(-100, 100, -100, 100, 20, 200) it looks like this, like I'm getting closer but what about the left, right, top and bottom parameters? Are they okay with that values?

enter image description here

Hernán Casillas
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  • Try for closer near to far ratio, you'll get closer result to orthographic projection. Try for example glFrustum(-100, 100 ,-100 ,100 , 20, 150). 40 to 1 is extreme. – Victor Oct 30 '17 at 15:33
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    @Victor: the `far/near` ratio does in no way make a perspective projection closer to an orthonognal one, nor is a 40:1 ratio "extreme" in any way. – derhass Oct 30 '17 at 18:53

1 Answers1

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It's hard to be certain without more information. Perhaps the model could give some insight. But I suspect it may have to do with your clipping planes (nearVal and farVal as described here) arguments passed to glFrustum (1, 40). Perhaps try setting them to a broader range like your glOrtho call: 1, 150 (Note: neither nearVal or farVal can be negative when passed to glFrustum).

This all depends on the scale of the model and how it is positioned relative to the camera. If part of the model falls outside of the clipping planes, then it will be, well... clipped.

Sparky
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