OpenGL Perspective

Question

I'm trying to depict a cube using a perspective projection, but all I get is the corner of a square. The face of the square is set at the origin and expands in the positive direction. Using glOrtho I can set the coordinate system, but I'm having trouble doing the same thing using glPerspective.

#include <gl/glut.h>

void mesh(void) {
float v[8][3] = { /* Vertices for 8 corners of a cube. */
{0.0, 0.0, 0.0}, {100.0, 0.0, 0.0}, {100.0, 100.0, 0.0}, {0.0, 100.0, 0.0},
{0.0, 0.0, -100.0}, {100.0, 0.0, -100.0}, {100.0, 100.0, -100.0}, {0.0, 100.0, -100.0} };
float n[6][3] = { /* Normals for the 6 faces of a cube. */
{0.0, 0.0, 1.0}, {1.0, 0.0, 0.0}, {0.0, 1.0, 0.0},
{-1.0, 0.0, 0.0}, {0.0, -1.0, 0.0}, {0.0, 0.0, -1.0} };
int f[6][4] = { /* Indexes of the vertices in v that make the 6 faces of a cube. */
{0, 1, 2, 3}, {1, 5, 6, 2}, {3, 2, 6, 7}, 
{0, 4, 7, 3}, {0, 1, 5, 4}, {4, 5, 6, 7} };



for (int j = 0; j < 6; j++) {
    glBegin(GL_QUADS);
    glNormal3fv(&n[j][0]);
    glVertex3fv(&v[f[j][0]][0]);
    glVertex3fv(&v[f[j][1]][0]);
    glVertex3fv(&v[f[j][2]][0]);
    glVertex3fv(&v[f[j][3]][0]);
    glEnd();
    glFlush();
}
}

void display(void) {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glColor3f(1.0, 0.0, 0.0);
mesh();
}

void main(int argc, char** argv) {
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGB |GLUT_DEPTH |GLUT_SINGLE);
glutInitWindowSize(400, 300);
glutInitWindowPosition(200, 200);
glutCreateWindow("Mesh");
glClearColor(0.0, 0.0, 0.0, 1.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
//glRotatef(15, 0.0, 0.0, 1.0);
//glOrtho(-400.0, 400.0, -300.0, 300.0, 200.0, -200.0);
gluPerspective(120,1,0,600);
glEnable(GL_DEPTH_TEST);
glEnable(GL_NORMALIZE);
glutDisplayFunc(display);
glutMainLoop();
}

why are you hanging yourself? that has got to be one of the most complicated methods to draw anything i have ever seen.. a 2d dimensional array? — iKlsR, Jul 19 '12 at 14:13
You mean it's easier than this?!! I think my professor is trying to get us to see how projection works. I agree this is hard so I'm hoping drawing 3D figures is not as cumbersome. — Gabriel Ortega, Jul 19 '12 at 15:08

iKlsR · Accepted Answer · 2012-07-19T14:41:29.563

4

You say you only see corners of the cube? Then your Field of view is too wide.. you are using gluPerspective() and providing your calculations are correct.. the values are a bit off imo, the function parameters are:

void gluPerspective(GLdouble fovy,
                    GLdouble aspect_ratio,
                    GLdouble zNear,
                    GLdouble zFar);

i propose changing that to something like

gluPerspective(45.0f,
               width_of_window / height_of_window,    //aspect ratio
               0.1f, 
               500.0f);

edited Jul 19 '12 at 14:41

answered Jul 19 '12 at 14:19

iKlsR

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Just read this: http://stackoverflow.com/questions/9458104/perspective-projection-with-opengl?rq=1 – Gabriel Ortega Jul 19 '12 at 15:41
I think since the camera is set on the z axis by default and the face of the cube was sitting on the x and y axis, no matter where the near and far was the cube was right up against the near plane. I offset all the z values by -100 and now I can see the entire cube. – Gabriel Ortega Jul 19 '12 at 15:50

OpenGL Perspective

1 Answers1