Built-in polygon clipping in OpenGL - opengl

Question

Why does the below code is showing a triangle in front of the quad for:
gluLookAt(10, 10, 60, 0, 0, 0, 0, 1, 0)
and
glOrtho(-30, 30, -30, 30, 0, 90)
Expected Output: Because the 'z' value of the triangle is less than the quad and the observer is at 60 unit in the positive 'z' axis, the rectangle should have been visible as a whole and only a few parts of the triangle would had been visible.
glBegin(GL_QUADS);
glColor3f(1, 1, 1);
glVertex3f(10, 10, 15); glVertex3f(20, 10, 15); glVertex3f(20, 20, 15); glVertex3f(10, 20, 15);
glEnd();
glBegin(GL_TRIANGLES);
glColor3f(0, 1, 0);
glVertex3f(0, 0, 0);
glVertex3f(22, 14, 0);
glVertex3f(16, 22, 0);
glEnd();

Answer 1

First of all, your question is totally unrelated to clipping - Clipping means calculating the intersection of the primitves against some clip area or clip volume.
The triangle does appear because you draw it after the quad. By default, OpenGL will not apply any visibility algorithms. However, OpenGL does support the Z buffer algorithm in form of the depth test.

Answer 2

So finally I have got answer to this.
It required few lines of code to be added.
Adding below 3 lines in the initialization step
glClearDepth(1.0f);
glDepthFunc(GL_LESS);
glEnable(GL_DEPTH_TEST);
Initializing display mode to add depth information (GLUT_DEPTH ) as below
glutInitDisplayMode(GLUT_DEPTH | GLUT_DOUBLE | GLUT_RGB);
Below is the complete code for displaying a rectangle behind which is a traingle.
#include <GL/glut.h>
#include <stdlib.h>
#include <math.h>
#include<iostream>
using namespace std;
void FPS(void);
GLint gFramesPerSecond = 0;
int TIMER_SPEED = 10000;
float i = 40;
void display(void)
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
gluLookAt(10, 10, i-=.2, 10, 10, -10, 0, 1, 0);
glBegin(GL_LINES);
glColor3f(1, 0, 0);
glVertex3f(0, 0, 0); glVertex3f(20, 0, 0);
glColor3f(0, 1, 0);
glVertex3f(0, 0, 0); glVertex3f(0, 20, 0);
glColor3f(0, 0, 1);
glVertex3f(0, 0, 0); glVertex3f(0, 0, 50);
glEnd();
glBegin(GL_QUADS);
glColor3f(1, 1, 1);
glVertex3f(10, 10, 10); glVertex3f(20, 10, 15); glVertex3f(20, 20, 15); glVertex3f(10, 20, 15);
glEnd();
glBegin(GL_TRIANGLES);
glColor3f(0, 1, 0);
glVertex3f(12, 12, 20);
glVertex3f(22, 14, 12);
glVertex3f(16, 22, 12);
glEnd();
glutSwapBuffers();
}
void timer(int v)
{
static GLfloat u = 0.0;
u += 0.01;
glutTimerFunc(TIMER_SPEED/60.0, timer, ++v);
display();
}
void Init(void)
{
glClearColor(0.0, 0.0, 0.0, 0.0);
glClearDepth(1.0f);
glDepthFunc(GL_LESS);
glEnable(GL_DEPTH_TEST);
}
void Resize(int width, int height)
{
glViewport(0, 0, width, height);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glFrustum(-15, 30, -15, 30, 15, 40);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
int main(int argc, char **argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DEPTH | GLUT_DOUBLE | GLUT_RGB);
glutInitWindowSize(640, 480);
glutInitWindowPosition(200, 200);
glutCreateWindow("3D Object in OpenGL");
glutKeyboardFunc(myKeyboard);
Init();
glutDisplayFunc(display);
glutReshapeFunc(Resize);
glutTimerFunc(100, timer, 0);
glutMainLoop();
return 0;
}