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An Introduction to OpenGL Programming

Nov 12, 2014 By MihalisTsoukalos

in HOW-TOs OpenGL Programming

207

OpenGL is awell-known standard for generating 3-D as well as 2-D graphics that is

extremely powerful and has many capabilities. OpenGL is defined and released by

the OpenGL Architecture Review Board (ARB).

This article is a gentle introduction to OpenGL that will help you understand drawing

using OpenGL.

The latest version of OpenGL at the time of this writing is 4.4, which uses a different

technique for drawing from the one presented here. Nevertheless, the purpose of this

article is to help you understand OpenGL philosophy, not teach you how to code using

the latest OpenGL version. Therefore, the presented source code can be run on Linux

machines that have an older OpenGL version installed.

Installing OpenGL

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2/12


I you run t e o ow ng comman on a De an 7 system to n a pac ages t at

contain the word "opengl", you'll get plenty of output (Figure 1):

$ apt-cache search opengl

Figure 1. Running apt-cache search opengl

There are many free OpenGL implementations for Linux, but you need only one. I

installed FreeGLUT, because it is the most up to date. FreeGLUT is an open-source

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3/12


alternative to the OpenGL Utility Toolkit (GLUT) library:

root@mail:~# apt-get install freeglut3 freeglut3-dev libglew-dev

Reading package lists... Done

Building dependency tree

Reading state information... Done

The following package was automatically installed

and is no longer required: fonts-liberation

Use 'apt-get autoremove' to remove it.

The following extra packages will be installed:

libgl1-mesa-dev libglu1-mesa-dev libice-dev libpthread-stubs0

libpthread-stubs0-dev libsm-dev libx11-dev libx11-doc

libxau-dev libxcb1-dev libxdmcp-dev libxext-dev libxt-dev

mesa-common-dev x11proto-core-dev x11proto-input-dev

x11proto-kb-dev x11proto-xext-dev xorg-sgml-doctools xtrans-dev

Suggested packages: libice-doc libsm-doc libxcb-doc libxext-doc libxt-doc

The following NEW packages will be installed:

freeglut3 freeglut3-dev libgl1-mesa-dev libglu1-mesa-dev

libice-dev libpthread-stubs0 libpthread-stubs0-dev libsm-dev

libx11-dev libx11-doc libxau-dev libxcb1-dev libxdmcp-dev

libxext-dev libxt-dev mesa-common-dev x11proto-core-dev

x11proto-input-dev x11proto-kb-dev x11proto-xext-dev

xorg-sgml-doctools xtrans-dev

0 upgraded, 22 newly installed, 0 to remove and 0 not upgraded.Need to get 7,651 kB of archives.

After this operation, 24.9 MB of additional disk space

will be used.

Do you want to continue [Y/n]?

You also will need a C++ compiler to compile the code.

Finally, y ou may need to install the mesa-utils package in order to be able to use the

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4/12


glxinfo command:

# apt-get install mesa-utils

The glxinfo command displays useful information about your OpenGL installation, as

you can see in the following output:

...

GLX version: 1.4

GLX extensions:

GLX_ARB_get_proc_address, GLX_ARB_multisample,

GLX_EXT_import_context, GLX_EXT_texture_from_pixmap,

GLX_EXT_visual_info, GLX_EXT_visual_rating,

GLX_MESA_copy_sub_buffer, GLX_MESA_multithread_makecurrent,

GLX_OML_swap_method, GLX_SGIS_multisample, GLX_SGIX_fbconfig,

GLX_SGIX_pbuffer, GLX_SGI_make_current_read

OpenGL vendor string: VMware, Inc.

OpenGL renderer string: Gallium 0.4 on llvmpipe

(LLVM 3.4, 128 bits)

OpenGL version string: 2.1 Mesa 10.1.3

OpenGL shading language version string: 1.30

OpenGL extensions:

...

Mesa is a 3-D graphics library with an API that is so very similar to OpenGL's, it is

pretty much indistinguishable.

OpenGL Pipeline

Figure 2taken from the OpenGL Shading Language book (aka "T he Orange Book")

shows the programmable OpenGL pipeline with the vertex and fragment

processors. As you can imagine, the OpenGL pipeline is complex, but y ou do not have

to understand it fully in order to be able to use OpenGL. T he Pipeline shows how

OpenGL operates in the background. Newer versions of the OpenGL Pipeline are

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5/12


even more complex!

Figure 2. OpenGL Architecture

OpenGL is a big state machine. Most calls to OpenGL functions modify a global state

that you cannot access directly.

The OpenGL Shading Language code that is intended for ex ecution on one of the

OpenGL programmable processors is called a Shader. The OpenGL Shading

Language has its roots in C (presenting the OpenGL Shading Language is beyond the

scope of this article).

OpenGL does not define a windowing layer, because it tries to be platform-neutral

and leaves this functionality to the operating system. The OS must provide arendering context that accepts commands as well as a framebuffer that keeps the

results of the drawing commands.

Matrix Algebra is extensively used in 3-D graphics, so it is good for you to know how

to add, multiply, subtract and divide matrices, although you will not need to code such

operations yourself. It also is useful to become familiar with 3-D coordinates and be

able to sketch on paper the 3-D scene you are trying to draw.
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6/12


Drawing a Triangle

Now it's time for some real OpenGL code. The code in Listing 1, when ex ecuted,

draws a triangle on-screen using OpenGL.

Listing 1. triangle.cc

// Programmer: Mihalis Tsoukalos

// Date: Wednesday 04 June 2014

//

// A simple OpenGL program that draws a triangle.

#include "GL/freeglut.h"

#include "GL/gl.h"

void drawTriangle()

{

glClearColor(0.4, 0.4, 0.4, 0.4);

glClear(GL_COLOR_BUFFER_BIT);

glColor3f(1.0, 1.0, 1.0);

glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);

glBegin(GL_TRIANGLES);

glVertex3f(-0.7, 0.7, 0);

glVertex3f(0.7, 0.7, 0);

glVertex3f(0, -1, 0);

glEnd();

glFlush();

}

int main(int argc, char **argv)

{
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7/12


glutInit(&argc, argv);

glutInitDisplayMode(GLUT_SINGLE);

glutInitWindowSize(500, 500);

glutInitWindowPosition(100, 100);

glutCreateWindow("OpenGL - Creating a triangle");

glutDisplayFunc(drawTriangle);

glutMainLoop();

return 0;

}

The code in Listing 1 for setting up OpenGL is large, but y ou will have to learn it only

once.

On a Debian 7 system, the following command compiled the triangle.cc OpenGL

program without any error messages:

$ g++ triangle.cc -lglut -o triangle

On an Ubuntu Linux system, the same command produced the following error

messages:

/usr/bin/ld: /tmp/ccnBP5li.o: undefined reference to symbol

&rarrhk;'glOrtho'

//usr/lib/x86_64-linux-gnu/mesa/libGL.so.1: error adding

//symbols: DSO missing from command line

collect2: error: ld returned 1 exit status

The solution is to compile the triangle.cc program by linking the executable to an

additional library (-lGL):

mtsouk@mtsouk-VirtualBox:/media/sf_OpenGL.LJ/code$ g++

&rarrhk;triangle.cc -lglut -lGL -o triangle


8/12


The libGL.so library accepts OpenGL commands and makes sure that they are put on

the screen in some way. If y our graphics card does not have 3-D acceleration, libGL

contains a software renderer that gives a 2-D image as output to the X Window

System. This is the case with Mesa. libGL also can pass the OpenGL information to

the X Window System, if the GLX extension is present. Then, the X Window System

can do software rendering with the help of Mesa or use hardware acceleration.

The output from the executable will produce the triangle shown in Figure 3. The

correct compilation of triangle.cc is proof that your Linux system can be used fordeveloping OpenGL applications.


9/12

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Figure 3. Drawing a T riangle Using OpenGL

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Juan de Seven Wexford
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10/12

df di b PRO i Are you a developer? Try out the HTML to PDF API

our own OS, and unlike Apple we have basically no advertising budget, so maybe highligh

community more than free nudges and winks to Apple.

Shahbaz Youssefi

To anyone reading this article!

This is thecode style of OpenGL 1. It is virtually deprecated and no one codes like this an

have an idea how OpenGL works, don't waste your time much learning ancient stuff like t

You can learn on modern computer graphics (with OpenGL of course) for free here: http:

Sam

I get this on your last example:

g++ rotateCube.cc-lglut -lGL -o rotateCube

/tmp/ccwd9YdJ.o: In function handleResize(int, int)':

rotateCube.cc:(.text+0xa65): undefined reference to gluPerspective'

collect2: error: ld returned 1 exit status

Is there another library required?

Julin

I've found that you have to add the -lGLU flag to compile this in Ubuntu 13.04. Hop
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