I assume you now know the basic structure of an OpenGL glut program so I woun't explain that again. If you want to remember those things visit the previous tutorial.

Normally we are going to draw things in the display function. All the drawing in OpenGL is done vertex by vertex. If you want to draw a triangle we do a glBegin(GL_TRIANGLES) and then we give him 3 vertexes (or multiple of 3 if you intend to draw more then one triangles at once). These vertexes are given by there coordinates. Now you are used to coordinates in 2D where the screen would start in the upper left corner (0,0) and end in the lower left corner (800,600) for example. In OpenGL things are totaly different. Now you work in 3D. You have (x,y,z) coordinates and they are infinite in both ways! You must tell OpenGL at part of the 3D world you are looking at and then you can draw in relative coordinates. A common way is to start by loading the Identity Matrix. That will put the axes in the middle of your screen. The origin (0,0) is in the middle, the X axis is horizontal and starts from -1 in the left and ends with 1 in the right, the Y axis is vertical and start with -1 in the bottom and 1 at the top, and the Z axis goes through the screen. It's negative into the screen and positive outside.

If you want something to be visible you will have to draw it with a negative Z value (either that or you change the view point to look at some other area of your 3D world).

You may have noticed that openGL functions end with either 3f, 3v, 3i, 2f, 2v, 2i like in glVertex3f(x,y,z); The 3 means it's expecting 3 parameters, f means they are floats, i = integers and v that the parameters are a float array.

You can get the Example Visual Studio Project here

or you can download just the triangles.cpp file

// if you are not using Visual Studio to compile this then remove stdafx.h

#include <stdafx.h>
#include <stdlib.h>
#include <windows.h>
#include <GL/glut.h>

void init(void)
{

glClearColor (0.0, 0.0, 0.0, 0.0);
glEnable(GL_DEPTH_TEST);
glShadeModel (GL_SMOOTH);

}


void display(void)
{

/* First of all we clear both the screen (The color buffer) and the depth buffer on your video card
The depth buffer can tell us the depth of each pixel on the screen, so when we draw something object in the back will
no overwrite object in the front :)
We want this information gone too since we now render a new frame*/

glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
/* Loading the Identity matrix means we reset the screen coordinate system to XYZ axis of lenght 1:
The screen starts at z=0, x=-1 to x=1 and y=-1 to y=1 */
glLoadIdentity ();

/* everything we draw after this call is translated 6 units into the screen*/
glTranslatef(0,0.0f,-6.0f);
/* A glBegin call tells openGL we are drawing something. Since we will just feed the graphics card with vertex coordinates we need
to tell before we start drawing what those vertexes mean....as in what shape we are drawing
Even if we specify that we make triangles it doesn't mean we have to give 3 vertexes only....give it 6 vertexes and you get 2 triangles...9 vertexes and you get 3 triangles.*/
glBegin(GL_TRIANGLES);

/*After setting the drawing color everything that follows will have that color. Yes, normaly you would set one color for the whole object
but I'm going to show you that you can set individual colors for each vertex your draw. How is the object going to look like then?
Well OpenGL will do a gradient between the 2 colors you set for neighbouring vertexes*/
glColor3f(1.0f,0.0f,0.0f);
/* This function will put a vertex at the given coordinates */
glVertex3f( 0.0f, 1.0f, 0.0f);
/* we change the color again - note if we didn't change it then openGL would simply use the last color we gave*/
glColor3f(0.0f,1.0f,0.0f); // Set The Color To Green
glVertex3f(-1.0f,-1.0f, 0.0f);
glColor3f(0.0f,0.0f,1.0f); // Set The Color To Blue
glVertex3f( 1.0f,-1.0f, 0.0f);

glEnd(); // Done Drawing A Triangle

Sleep(5);
glutSwapBuffers();

glViewport (0, 0, (GLsizei) w, (GLsizei) h);
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
gluPerspective(60.0, (GLfloat) w/(GLfloat) h, 1.0, 20.0);
glMatrixMode (GL_MODELVIEW);

}

void keyboard(unsigned char key, int x, int y)
{

switch (key) {

case 27: // ESC key
exit(0);
break;

}

}


int main(int argc, char** argv)
{

glutInit(&argc, argv);
glutInitDisplayMode (GLUT_RGB | GLUT_DEPTH | GLUT_DOUBLE);
glutInitWindowSize (500, 500);
glutInitWindowPosition (100, 100);
glutCreateWindow (argv[0]);
init ();
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutMainLoop();
return 0;

}