Question

void init (void)
{
    glClearColor (1.0, 1.0, 1.0, 0.0);

    //glMatrixMode(GL_MODELVIEW);
    //gluLookAt(x0, y0, z0, xref, yref, zref, Vx, Vy, Vz);

    glMatrixMode(GL_PROJECTION);
    gluPerspective(45, 2, -1, 1);
    //glFrustum(xwMin, xwMax, ywMin, ywMax, dnear, dfar);
    //gluPerspective(45.0, 45, -1, 1);
}

void displayFcn (void)
{
    glClear(GL_COLOR_BUFFER_BIT);

    glColor3f(0.0, 1.0, 0.0);
    glPolygonMode(GL_FRONT, GL_FILL);
    glPolygonMode(GL_BACK, GL_LINE);

    glBegin(GL_TRIANGLES);
        glVertex3f(0.0, 0.0, 0.0);
        glVertex3f(0.1, 0.0, 0.0);
        glVertex3f(0.50, 0.866025, 0.0);    
    glEnd();

    glFlush();
}

void reshapeFcn(GLint newWidth, GLint newHeight)
{
    glViewport(0,0,newWidth, newHeight);

    winWidth = newWidth;
    winHeight = newHeight;

}
void main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
    glutInitWindowPosition(400,200);
    glutInitWindowSize(winWidth, winHeight);
    glutCreateWindow("Test");

    init();
    glutDisplayFunc(displayFcn);
    glutReshapeFunc(reshapeFcn);
    glutMainLoop();
}

Could someone explain a little bit and give suggestions how to make that triangle visible.

Answer 1

I don’t think that you’re allowed to set ‘zNear’ negative. That will do weird things to the depth buffer and mean that you see things behind the eye. Docs say it’s always positive. You’re also liable to get somewhat strange results if you fix the aspect ratio to 2, regardless of the aspect ratio of the window.

You also need to set the current matrix back to MODEL_VIEW as datenwolf has shown. You don’t need to use gluLookAt, but you need to do something to move the triangle away from the origin where the eye is located. You could do that by setting the z component to some negative value, or by applying a translation before the vertices:

glPushMatrix();
glTranslated(0,0,-5);
glBegin(GL_TRIANGLES);
    glVertex3f(0.0, 0.0, 0.0);
    glVertex3f(0.1, 0.0, 0.0);
    glVertex3f(0.50, 0.866025, 0.0);    
glEnd();
glPopMatrix();