TOPIC 3 INTRODUCTION TO OPENGL CGMB214: Introduction to Computer Graphics.
Transcript of TOPIC 3 INTRODUCTION TO OPENGL CGMB214: Introduction to Computer Graphics.
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
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
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
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.
Display-Window Management Using GLUT
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-Window Management Using GLUT
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-Window Management Using GLUT
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
Display-Window Management Using GLUT
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
Display-Window Management Using GLUT
Initialise the size of the window
glutInitWindowSize(int width, int height);
Initialising window size
function
Setting the size of the window
Display-Window Management Using GLUT
Open and display a window
glutCreateWindow (“string”);
Function that open and display a
window
A character string that will appear on the title bar
Display-Window Management Using GLUT
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
Display-Window Management Using GLUT
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.
Display-Window Management Using GLUT
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
Display-Window Management Using GLUT
To set the color of the primitives
glColor3i (Red, Green, Blue);
Function to specify the
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
Display-Window Management Using GLUT
To display 2D picture (because 2D is a special case of 3D)glMatrixMode (Glenum mode);
Function to specify the
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