TOPIC 3 INTRODUCTION TO OPENGL CGMB214: Introduction to Computer Graphics

TOPIC 3INTRODUCTION TO OPENGL

CGMB214: Introduction to Computer Graphics

Objectives

To demonstrate the basic usage of OpenGLTo write a simple OpenGL program in C

Introduction

A basic library of functions is provided in OpenGL It helps to specify

Graphics primitives Attributes Geometric transformation Viewing transformation Etc

OpenGL is designed to be hardware independent, therefore many operations, such as input and output routines are not included in the basic library

Basic OpenGL Syntax

Basic OpenGL Syntax

Function names in the OpenGL basic library are prefixed with gl

Each component word within a function name has its first letter capitalised.

E.g. glBegin, glClear, glCopyPixels, glPolygonMode

Basic OpenGL Syntax

Arguments, for example parameter name, a value for a parameter or a particular mode begin with uppercase letter GL.

The component name comprise of uppercase letter and underscore.

E.g. GL_2D, GL_RGB, GL_CCW, GL_POLYGON, GL_AMBIENT_AND_DIFFUSE

Basic OpenGL Syntax

Data type in OpenGL starts with capital letters and the remainder is written in lower case.

E.g. GLbyte, GLshort, GLint, GLfloat, GLdouble, GLboolean

Related Libraries

Related Libraries

OpenGL Utility (GLU) Provide routines for

Setting up viewing and projection matrices Describing complex object with lines and polygon

approximations Displaying quadrics and B-splines using linear

approximations Processing the surface rendering operations

Function names start with glu prefixOpen Inventor

Object oriented toolkit written in C++ for interactive 3D applications

OpenGL Extension to the X Window System (GLX) provides a set of routines that are prefixed with

the letter glX

Related Libraries

Apple GL (AGL) Interface for window-management operations in

Apple systems (prefix agl)Windows-to-OpenGL (WGL)

Prefixed with letter wglPresentation Manager to OpenGL (PGL)

An interface for IBM OS/2OpenGL Utility Toolkit (GLUT)

Provides library of functions for interacting with any screen-windowing system

Prefixed with letter glut

Header Files

Header Files

In our graphic program we will need to include the header file for the OpenGL core library

For most application we will also need GLUWe will also need to include the header file for

window system The header file that accesses WGL routines is

windows.h This header file must be listed before the OpenGL

and GLU header files because it contains macros needed by the Microsoft Windows version of OpenGL libraries

So the code should look like below# include <windows.h># include <GL/gl.h># include <GL/glu.h>

Header Files

However if we use GLUT to handle the window-managing operation we don’t need the gl.h and glu.h

This is because GLUT will ensure that these header files will be included correctly

So the code should look something like below

# include <GL/glut.h>

Header Files

Other than that we also need to include the header files required by C/C++

E.g.# include <stdio.h># include <stdlib.h># include <math.h>

Display-Window Management Using GLUT


Initialise GLUT

glutInit (&argc, argv);

Initialising function

Can be substitute with command line argument, but as for now, just leave it as it is

glutInit will initialize the GLUT library and negotiate a session with the window system. During this process, glutInit may cause the termination of the GLUT program with an error message to the user if GLUT cannot be properly initialized. glutInit must be called before others.


Initialise OpenGL on how to set up its frame buffer and colourglutInitDisplayMode (GL_SINGLE|GL_RGB);

Initialising display mode

function

Built in constant which are OR-ed together. Can be changed based on table

below Frame buffer is a special 2-dimensional array in main

memory where the graphical image is stored. OpenGL maintains an enhanced version of the frame buffer with additional information

The system also needs to know how we are representing colors of our general needs in order to determine the depth (number of bits) to assign for each pixel in the frame buffer

The argument to glutInitDisplayMode() is a logical-or (using the operator “|”) of a number of possible options


Display Mode Meaning

Colour Representation

GLUT_RGB Use RGB colour (default). 24-bit colour.

GLUT_RGBA Use RGB colour including α (for transparency)The α component indicates the opaqueness of the color(1 = fully opaque, 0 = fully transparent)

GLUT_INDEX For colourmapped colours (not recommended)


Display Mode Meaning

Buffering

GLUT_SINGLE Use single buffering(default)

GLUT_DOUBLE Use double buffering• two separates frame buffers front buffer and back

buffer.• front buffer for displaying back buffer for drawing• swapping to update the image

GLUT_DEPTH Use depth buffer• Is needed in 3-dimensional graphics for hidden

surface removal. • Use a special array called depth buffer. • It is a 2-dimensional array that stores the distance

(or depth) of each pixel from the viewer. This makes it possible to determine which surfaces are closest, thus visible, which are farther and thus hidden

• To understand the difference between these two options, we need to know how frame buffer works. In raster graphics systems, whatever is written to the frame buffer is immediately transferred to the display.

• This process is repeated frequently (30 – 60 times a second).

• To do this a typical approach is to first erase the old contents by setting all the pixels to some background color, i.e. black. After this, the new contents are drawn

• However, even though these processes might happen very fast, the process of setting the image to black and then redrawing everything produces a noticeable flicker in the image


Set the initial location of the window

glutInitWindowPosition (int x , int y);

Initialising Window Position function

Setting the position of the window from upper left corner of the screen


Initialise the size of the window

glutInitWindowSize(int width, int height);

Initialising window size

function

Setting the size of the window


Open and display a window

glutCreateWindow (“string”);

Function that open and display a

window

A character string that will appear on the title bar


Specify what the display window should containglutDisplayFunc (function name);

Function to specify what to be displayed on the

window

The name of the function that contains the lines to be executed


Make sure the window continue running. Should be at the last line

glutMainLoop();

Function to specify what to be displayed on the

window

This should be the last function in the program. It will display the initial graphics and puts the program into an infinite loop that checks for input from devices such as mouse. If the program is not an interactive program, this function will make sure the window is being displayed all the time until the user close the window.


300 px

400 px


To set the background color of the window

glClearColor (Red, Green, Blue, Opacity);

Function to specify the

background color of the window

The first 3 parameters determine the value of Red, Green and Blue,

respectively. The last parameter is for opaqueness


To set the color of the primitives

glColor3i (Red, Green, Blue);


background color of the window

The 3 parameters determine the value of Red, Green and Blue,

respectively.

The number 3 is for the number of color

component, Red, Green and Blue.

The letter ‘i’ is for the

data type. ‘i’ is for

integer


To display 2D picture (because 2D is a special case of 3D)glMatrixMode (Glenum mode);


current matrix

Can be set to either to define camera, perspective, measuring system etc.

glOrtho2D (start x, end x, start y, end y);

Function to defines the coordinate

reference frame

Specify the range of x and y

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TOPIC 3 INTRODUCTION TO OPENGL CGMB214: Introduction to Computer Graphics