GLUT Tutorial: Drawing Basic Shapes (Triangle and Rectangle)

This is the first tutorial on GLUT. In this tutorial I am going to show you how to draw basic 2D shapes like triangle and rectangles using OpenGL. Although OpenGL is basically made for 3D programming, drawing 2D shapes gives the basic outline and introduction to OpenGL and gives the idea about how to start drawing objects in OpenGL. The following example draws a triangle and a rectangle to a GLUT window. Drawing rectangle and triangle is very easy on OpenGL because it provides a function for it. glBegin(GL_TRIANGLES) and glBegins(GL_QUADS) are functions for drawing triangle and rectangle respectively. We have to give a 3D coordinates of each vertex and OpenGL automatically draws the object as specified by the attribute passed to function glBegin. For example if we call glBegin(GL_QUADS) and provide coordinates of four vertices then OpenGL will draw a rectangle for us.

#include<windows.h>
#include <iostream>
#include <stdlib.h>
#include <GL/glut.h>
using namespace std;
//Called when a key is pressed
void handleKeypress(unsigned char key, int x, int y) {
	switch (key) {
		case 27: //Escape key
			exit(0);
	}
}
//Initializes 3D rendering
void initRendering() {
	//Makes 3D drawing work when something is in front of something else
	glEnable(GL_DEPTH_TEST);
}
//Called when the window is resized
void handleResize(int w, int h) {
	//Tell OpenGL how to convert from coordinates to pixel values
	glViewport(0, 0, w, h);
	glMatrixMode(GL_PROJECTION); //Switch to setting the camera perspective
	//Set the camera perspective
	glLoadIdentity(); //Reset the camera
	gluPerspective(45.0,                  //The camera angle
				   (double)w / (double)h, //The width-to-height ratio
				   1.0,                   //The near z clipping coordinate
				   200.0);                //The far z clipping coordinate
}
//Draws the 3D scene
void drawScene() {
	//Clear information from last draw
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	glMatrixMode(GL_MODELVIEW); //Switch to the drawing perspective
	glLoadIdentity(); //Reset the drawing perspective
	glBegin(GL_QUADS); //Begin quadrilateral coordinates
	//Trapezoid
	glVertex3f(-0.7f, -1.5f, -5.0f);
	glVertex3f(0.7f, -1.5f, -5.0f);
	glVertex3f(0.7f, -0.5f, -5.0f);
	glVertex3f(-0.7f, -0.5f, -5.0f);
	glEnd(); //End quadrilateral coordinates
	glBegin(GL_TRIANGLES); //Begin triangle coordinates
	//Triangle
	glVertex3f(-0.5f, 0.5f, -5.0f);
	glVertex3f(-1.0f, 1.5f, -5.0f);
	glVertex3f(-1.5f, 0.5f, -5.0f);
	glEnd(); //End triangle coordinates
	glutSwapBuffers(); //Send the 3D scene to the screen
}
int main(int argc, char** argv) {
	//Initialize GLUT
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
	glutInitWindowSize(400, 400); //Set the window size
	//Create the window
	glutCreateWindow("Basic Shapes - programming-technique.blogspot.com");
	initRendering(); //Initialize rendering
	//Set handler functions for drawing, keypresses, and window resizes
	glutDisplayFunc(drawScene);
	glutKeyboardFunc(handleKeypress);
	glutReshapeFunc(handleResize);
	glutMainLoop(); //Start the main loop
	return 0;
}

The output of the above program is :