GLFW (OpenGL Framework) tutorial 2: Drawing Basic Shapes

Lets first discuss how OpenGL does. Describing co-ordinates system of OpenGL, center of co-ordinate is middle of widow, positive x-axis is pointed towards right and negative x-axis toward left ,positive y-axis is pointed towards up and negative y-axis towards bottom , positive z-axis is pointed towards outside of screen and negative z-axis towards inside the screen.

• 

OpenGL draw object similar to our eye . By default eye is in origin and we are looking toward negative z-axis.


Basic function of GLFW is describe below.

1.Including Header file:

At first we have to include header file of GLFW.

//include header file for glfw library so that we can use OpenGL

#include <GL/glfw.h>

2.Initialize GLFW:

Before using GLFW any function we have to initialize GLFW by calling

int glfwInit(void)

It initializes the parts of GLFW that are not dependent on a window, such as threading, timer and joystick input. glfwInit returns GL_TRUE if initialization succeeded, or GL_FALSE if it failed.

3.Opening An OpenGLWindow:

Opening an OpenGL window is done with the glfwOpenWindow function. The function takes nine
arguments, which are used to describe the following properties of the requested window:

• Window dimensions (width and height) in pixels.
• Color and alpha buffer bit depth.
• Depth buffer (Z-buffer) bit depth.
• Stencil buffer bit depth.
• Whether to use fullscreen or windowed mode.

It can be call like this

int glfwOpenWindow( int width, int height,

int redbits, int greenbits, int bluebits,

int alphabits, int depthbits, int stencilbits,

int mode );

4.Using Keyboard Input

One of the simplest ways of checking for keyboard input is to use the function glfwGetKey:

c syntax is given below:

int glfwGetKey( int key )

5.Drawing Geometric shape

Geometric shape in OpenGL is drawn by providing vertices command between primitives.Primitives are specifies like this

glBegin( primitivesType );
…
glEnd();
The vertices command is shown below

PrimitivesType determines how vertices are combined.

6.Vertices and primitives

A.Lines, GL_LINES

Pairs of vertices interpreted as individual line segments
Can specify line width using:
glLineWidth (float width)
glBegin(GL_LINES);
glColor3fv( color );
glVertex2f( P0.x, P0.y );
glVertex2f( P1.x, P1.y );
glVertex2f( P2.x, P2.y );
glVertex2f( P3.x, P3.y );
glVertex2f( P4.x, P4.y );
glVertex2f( P5.x, P5.y );
glVertex2f( P6.x, P6.y );
glVertex2f( P7.x, P7.y );
glEnd();

B.Line Strip, GL_LINE_STRIP

 series of connected line segments

C.Polygon,GL_POLYGON

D.Triangles,GL_TRIANGLES

E.Triangles strips,GL_TRIANGLE_STRIP

F.Triangles Fan,GL_TRIANGLE_STRIP

G.Quads,GL_QUADS

Now lets take a look to the sample source code to draw Basic shapes.

//include header file for glfw library so that we can use OpenGL

#include <GL/glfw.h>

#include <stdlib.h>  //needed for exit function

#include <iostream>

using namespace std;

//Initializes 3D rendering

void initializeRendering()

{

    glfwInit();

    //Makes 3D drawing work when something is in front of something else

    glEnable(GL_DEPTH_TEST);

}

//Called when a key is pressed

void GLFWCALL handleKeypress(int key,int press) //The key that was pressed

{

    switch (key) {

        case GLFW_KEY_ESC: //Escape key

            exit(0); //Exit the program

    }

}

//Called when the window is resized

void GLFWCALL handleResize(int width,int height)

{

    //Tell OpenGL how to convert from coordinates to pixel values

    glViewport( 0, 0, width, height );

    glMatrixMode( GL_PROJECTION ); //Switch to setting the camera perspective

    //Set the camera perspective

    glLoadIdentity(); //reset the camera

    gluPerspective( 45.0f,                      //camera angle

                (GLfloat)width/(GLfloat)height, //The width to height ratio

                 1.0f,                          //The near z clipping coordinate

                100.0f );                       //The far z clipping coordinate

}

void display()

{

    glClearColor( 0.0f, 0.0f, 0.0f, 0.0f ); //clear background screen to black

    //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(0.4f, -0.5f, -5.0f);

    glVertex3f(-0.4f, -0.5f, -5.0f);

    glEnd(); //End quadrilateral coordinates

    glBegin(GL_TRIANGLES); //Begin triangle coordinates

    //Pentagon

    glVertex3f(0.5f, 0.5f, -5.0f);

    glVertex3f(1.5f, 0.5f, -5.0f);

    glVertex3f(0.5f, 1.0f, -5.0f);

    glVertex3f(0.5f, 1.0f, -5.0f);

    glVertex3f(1.5f, 0.5f, -5.0f);

    glVertex3f(1.5f, 1.0f, -5.0f);

    glVertex3f(0.5f, 1.0f, -5.0f);

    glVertex3f(1.5f, 1.0f, -5.0f);

    glVertex3f(1.0f, 1.5f, -5.0f);

    //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

    glfwSwapBuffers();

}

int main()

{

   // int     width, height;

    //int     frame = 0;

    bool    running = true;

    initializeRendering();

    if( !glfwOpenWindow( 512, // width of window

                         512, //height of window

                          1,  //redbits

                          0,  //greenbits

                          0,  //bluebits

                          0,  //alphabits

                          0,  //depthbits

                          0, //stencilbits

                          GLFW_WINDOW ) //mode

        ) //return false if window is not created

    {

        glfwTerminate(); //terminating glfw window

        return 0;

    }

    glfwSetWindowTitle("codeincodeblock.blogspot.com - basic shape");

    glfwSetWindowSizeCallback(handleResize); //callback function of GLFW to handle window resize

    glfwSetKeyCallback(handleKeypress); //callback function to handle keypress

    while(running) // infinite loop to draw object again and again

    {              // because once object is draw then window is terminated

        display();

        running = glfwGetWindowParam( GLFW_OPENED ); //when glfw window is opened then it return true

                                                     //if closed then return false

    }

    return 0;

}

The output of the program is show below:



You can also download the project file From here Download