OpenGL polygon z buffer problem - opengl

Question

Here is a quick and dirty GLUT based C++ program for windows that draws two rectangles, blue and green on a flat red background. Pressing 'a' or 'z' makes them orbit along the X axis in either direction. My problem is that if I enable GL_DEPTH_TEST, it sometimes draws one rectangle, sometimes two or sometimes just the background but never correctly with the nearer polygon obscuring some or all parts of the farther one. Not setting GL_DEPTH_TEST just makes the polygons appear in the drawing order.
What's wrong with the code below?
#include <windows.h>
#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
#include <cmath>
#pragma comment(lib, "opengl32.lib")
#pragma comment(lib, "glu32.lib")
int angle = 0;
void oglDraw()
{
angle += 360;
angle %= 360;
float fAngle = angle / (180 / 3.14159);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(90, 1, 0, 10);
gluLookAt(0, 0, -1, 0, 0, 1, 0, 1, 0);
float yFactor = 1;
float zFactor = 1;
float y = yFactor * sin(fAngle);
float z = 1 + zFactor - cos(fAngle) * zFactor;
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glClearColor(1, 0, 0, 1);
glPolygonMode(GL_FRONT, GL_FILL);
glBegin(GL_POLYGON);
glColor4f(0, 0, 1, 1);
glVertex3f(-1.0, y-1.0, z);
glVertex3f(+1.0, y-1.0, z);
glVertex3f(+1.0, y+1.0, z);
glVertex3f(-1.0, y+1.0, z);
glEnd();
fAngle = (180 - angle) / (180 / 3.14159);
y = -yFactor * sin(fAngle);
z = 1 + zFactor - cos(fAngle) * zFactor;
glBegin(GL_POLYGON);
glColor4f(0, 1, 0, 1);
glVertex3f(-1.0, y-1.0, z);
glVertex3f(+1.0, y-1.0, z);
glVertex3f(+1.0, y+1.0, z);
glVertex3f(-1.0, y+1.0, z);
glEnd();
glFlush();
glutSwapBuffers();
}
//////////////////////////////////////////////////////////////////////////
void oglKeyboard(byte ch, int x, int y)
{
if(ch == 'z')
{
angle++;
glutPostRedisplay();
}
else
if(ch == 'a')
{
angle--;
glutPostRedisplay();
}
}
//////////////////////////////////////////////////////////////////////////
int main(int argc, char **argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH );
glutInitWindowSize(1024, 768);
glutCreateWindow("OGL test");
gluOrtho2D(0, 1024, 768, 0);
glEnable(GL_DEPTH_TEST);
glutDisplayFunc(oglDraw);
glutKeyboardFunc(oglKeyboard);
glutMainLoop();
}

Answer 1

Pass something greater than zero for gluPerspective()'s zNear:
gluPerspective(90, 1, 0.1, 10);