OpenGL object distortion in perspective view

Question

Do you guys have any idea what is the cause of this problem? It looks fine in orthographic view.

Orthographic View:

Perspective View:

Code that might be related:

//Global variable
float tx = 0, tz = 0, tSpeed = 1.0;         
bool isOrtho = true;                
float ONear = -20.0, OFar = 20.0;   
float PNear = 1.0, PFar = 41.0;     
float ptx = 0, pty = 0, ptSpeed = 0.1;  
float pry = 0, prSpeed = 1.0;           

void projection() {
    glMatrixMode(GL_PROJECTION);    //refer to projection matrix
    glLoadIdentity();               //reset projection matrix

    glTranslatef(ptx, pty, 0.0);    //translation for projection
    glRotatef(pry, 0.0, 1.0, 0.0);  //rotate for projection
    if (isOrtho) {
        //Ortho View
        glOrtho(-20.0, 20.0, -20.0, 20.0, ONear, OFar); //Ortho view
    }
    else {
        //Perspective view
        gluPerspective(45, 1.0, -1.0, 1.0);
        glFrustum(-20.0, 20.0, -20.0, 20.0, PNear, PFar);
    }
}

void display(){
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glEnable(GL_DEPTH_TEST);
    projection();

    glMatrixMode(GL_MODELVIEW);
    lighting();
    drawRobot();
}

Answer 1

The glu docs for gluPerspective state:

zNear
Specifies the distance from the viewer to the near clipping plane (always positive).

(emphasis mine).

Your code gluPerspective(45, 1.0, -1.0, 1.0); passes -1.0 for zNear, which is not in the allowed range of values.