glFrustum

Hey iv'e started learning OpenGL (again) and I ran into this...
when i use glFrustum() the output I get is all distored but if i use gluPerspective() the program works perfect, it's my understanding is that both the functions do the same thing, but with differernt arguments, so why does one work and the other not??
The program changes the viewport so it can be rotated around a wirecube always

heres the code:

Code:

#include <GL/glut.h>
#include <iostream>
using namespace std;

float a,b;


/********** DISPLAY FUNCTION **********/
void display(void)
{
glPushMatrix();

gluLookAt(b, 0.0, a , 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
glClear(GL_COLOR_BUFFER_BIT);

glutWireCube(25.f);// make a cube the size one quater of the viewing volume

glutSwapBuffers();     //for double duffer

glPopMatrix();
}


/********** CHANGESIZE FUNCTION **********/
void ChangeSize(GLsizei w, GLsizei h) //changesize prototype
{
glViewport(0,0,w,h);


glMatrixMode(GL_PROJECTION);
glLoadIdentity();
//glFrustum(-100.0 , 100.0 , -100.0, 100.0, 1.0, 100.0);// this defines the viewing area\volume
gluPerspective(90.0, 1.0 ,1.0,100.0);


glMatrixMode(GL_MODELVIEW);
glLoadIdentity();    

	glutPostRedisplay();
}



/********** KEYS FUNCTION **********/


void Keys(int key, int x, int y)
	{
	if(key == GLUT_KEY_UP)
{a++;
cout << "Z = " << a <<endl;
}

	if(key == GLUT_KEY_DOWN)
{a--;
cout << "Z = " << a <<endl;
}

	if(key == GLUT_KEY_LEFT)
{b++;
cout << "X = " << b <<endl;
}

	if(key == GLUT_KEY_RIGHT)
{b--;
cout << "X = " << b <<endl;
}

	glutPostRedisplay();	// Refresh the Window... APPARENTLLY
	}


/********** MAIN FUNCTION **********/

int main(int argc, char *argv[])
{
    a = -30.0;
    b = 0.0;
    
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH); // SETS DISPLAY MODE
    glutInitWindowSize(300,300);
    glutCreateWindow("Mattys Test"); //Creates a window
    
    glutDisplayFunc(display);        //call back functions
    glutReshapeFunc(ChangeSize);
	glutSpecialFunc(Keys);    //KEYBOARD FUNCTIONS


    glClearColor(0.0f, 0.0f, 1.0f, 1.0f); //CLEAR COLOUR ONLY NEEDS TO BE SET ONCE!

    glutMainLoop();
    
return 0;
}

Probably because if you do the math, the glFrustum call you have is equivalent to

Code:

gluPerspective(178.854, 1.0, 1.0, 100.0);

If you really want a 90 degree field of view angle, then your near plane would have to start at 100. (Or alternatively, your other directions go from -1 to 1.)

whats the relationship between the two i'm not quite sure how to calculate one into the other(well im guessing near and far is the same but the fovy and aspect confuses me a little)

Originally Posted by Matty_Alan

whats the relationship between the two i'm not quite sure how to calculate one into the other(well im guessing near and far is the same but the fovy and aspect confuses me a little)

Check here, it's explained a number of different ways.
glFrustum and glPerspective difference? - GameDev.Net Discussion Forums