Open gl

Open gl

1. Introduction to
OpenGL Programming

2. What is OpenGL
OpenGL is a software API to graphics hardware.
designed as a streamlined, hardware-independent
interface to be implemented on many different hardware
platforms
Intuitive, procedural interface with c binding
No windowing commands !
No high-level commands for describing models of three-
dimensional objects
The OpenGL Utility Library (GLU) provides many of the
modeling features, such as quadric surfaces and NURBS
curves and surfaces

3. SGI and GL
Silicon Graphics (SGI) revolutionized the
graphics workstation by implementing the
pipeline in hardware (1982)
To access the system, application
programmers used a library called GL
With GL, it was relatively simple to program
three dimensional interactive applications

4. OpenGL
The success of GL lead to OpenGL (1992), a
platform-independent API that was
Easy to use
Close enough to the hardware to get excellent
performance
Focus on rendering
Omitted windowing and input to avoid window
system dependencies

5. OpenGL Evolution
Controlled by an Architectural Review Board
(ARB)
Members include SGI, Microsoft, Nvidia, HP,
3DLabs, IBM,…….
Relatively stable (present version 2.0)
Evolution reflects new hardware capabilities
3D texture mapping and texture objects
Vertex and fragment programs
Allows for platform specific features through
extensions

6. OpenGL Libraries
OpenGL core library
OpenGL32 on Windows
GL on most unix/linux systems (libGL.a)
OpenGL Utility Library (GLU)
Provides functionality in OpenGL core but avoids
having to rewrite code
Links with window system
GLX for X window systems, WGL for Windows
Cross-platform GUI libraries: GLUT, SDL, FLTK, QT,
…

7. Windowing with OpenGL
OpenGL is independent of any specific
window system
OpenGL can be used with different window
systems
X windows (GLX)
MFC
…
GLUT provide a portable API for creating
window and interacting with I/O devices

8. GLUT
OpenGL Utility Toolkit (GLUT)
Provides functionality common to all window systems
Open a window
Get input from mouse and keyboard
Menus
Event-driven
Code is portable but GLUT lacks the functionality of a
good toolkit for a specific platform
No slide bars, buttons, …

9. Software Organization
GLUT
GLU
OpenGL
GLX, AGL
or WGL
X, Win32, Mac O/S
software and/or hardware
application program
OpenGL Motif
widget or similar

10. OpenGL Architecture
Immediate Mode
Display
List
Polynomial
Evaluator
Per Vertex
Operations &
Primitive
Assembly
Rasterization
Per Fragment
Operations
Texture
Memory
CPU
Pixel
Operations
Frame
Buffer
geometry
pipeline

11. OpenGL as a state machine
GL State Variables- can be set and queried by OpenGL. Remains
unchanged until the next change.
Projection and viewing matrix
Color and material properties
Lights and shading
Line and polygon drawing modes
…
OpenGL functions are of two types
Primitive generating
Can cause output if primitive is visible
How vertices are processed and appearance of primitive are controlled by the
state
State changing
Transformation functions
Attribute functions

12. OpenGL Syntax
Functions have prefix gl and initial capital letters for each word

glClearColor(), glEnable(), glPushMatrix() …
glu for GLU functions

gluLookAt(), gluPerspective() …
Constants begin with GL_, use all capital letters
GL_COLOR_BUFFER_BIT, GL_PROJECTION, GL_MODELVIEW …
Extra letters in some commands indicate the number and type of
variables

glColor3f(), glVertex3f() …
OpenGL data types

GLfloat, GLdouble, GLint, GLenum, …
Underlying storage mode is the same
Easy to create overloaded functions in C++ but issue is efficiency

13. OpenGL function format
glVertex3f(x,y,z)
belongs to GL library
function name
x,y,z are floats
glVertex3fv(p)
p is a pointer to an array
dimensions

14. OpenGL #defines
Most constants are defined in the include files
gl.h, glu.h and glut.h
Note #include <GL/glut.h> should
automatically include the others
Examples
glBegin(GL_POLYGON)
glClear(GL_COLOR_BUFFER_BIT)
include files also define OpenGL data types:
GLfloat, GLdouble,….

15. GLUT
Developed by Mark Kilgard
Hides the complexities of differing window
system APIs
Default user interface for class projects
Glut routines have prefix glut
glutCreateWindow() …
Has very limited GUI interface
Glui is the C++ extension of glut

16. Glut Routines
Initialization: glutInit() processes (and removes) command-line
arguments that may be of interest to glut and the window system and
does general initialization of Glut and OpenGL
Must be called before any other glut routines
Display Mode: The next procedure, glutInitDisplayMode(),
performs initializations informing OpenGL how to set up the frame
buffer.
Display Mode Meaning
GLUT_RGB Use RGB colors
GLUT_RGBA Use RGB plus alpha (for transparency)
GLUT_INDEX Use indexed colors (not recommended)
GLUT_DOUBLE Use double buffering (recommended)
GLUT_SINGLE Use single buffering (not recommended)
GLUT_DEPTH Use depth-buffer (for hidden surface removal.)

17. Glut Routines
Window Setup
glutInitWindowSize(int width, int height)
glutInitWindowPosition(int x, int y)
glutCreateWindow(char* title)

18. A Simple Program
Generate a square on a solid background

19. simple.c
#include <GL/glut.h>
void mydisplay(){
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_POLYGON);
glVertex2f(-0.5, -0.5);
glVertex2f(-0.5, 0.5);
glVertex2f(0.5, 0.5);
glVertex2f(0.5, -0.5);
glEnd();
glFlush();
}
int main(int argc, char** argv){
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(500,500);
glutInitWindowPosition(0,0);
glutCreateWindow("simple");
glutDisplayFunc(mydisplay);
init();
glutMainLoop();
}

20. Closer Look at the main()
#include <GL/glut.h>
int main(int argc, char** argv)
{
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowSize(500,500);
glutInitWindowPosition(0,0);
glutCreateWindow("simple");
glutDisplayFunc(mydisplay);

init();

glutMainLoop();
}
includes gl.h
define window properties
set OpenGL state
enter event loop
display callback

21. init.c
void init()
{
glClearColor (0.0, 0.0, 0.0, 1.0);
glColor3f(1.0, 1.0, 1.0);
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);
}
black clear color
opaque window
fill/draw with white
viewing volume

22. Event Handling
Virtually all interactive graphics programs are even
driven
GLUT uses callbacks to handle events
Windows system invokes a particular procedure when an
event of particular type occurs.
MOST IMPORTANT: display event
Signaled when window first displays and whenever portions
of the window reveals from blocking window
glutDisplayFunc(void (* func)(void)) registers
the display callback function
Running the program: glutMainLoop()
Main event loop. Never exit()

23. More Callbacks
glutReshapeFunc(void (* func)(int w, int h)) indicates
what action should be taken when the window is resized.
glutKeyboardFunc(void (*func)(unsigned char key, int
x, int y)) and glutMouseFunc(void (*func)(int button,
int state, int x, int y)) allow you to link a keyboard key or a
mouse button with a routine that's invoked when the key or mouse
button is pressed or released.
glutMotionFunc(void (*func)(int x, int y)) registers a
routine to call back when the mouse is moved while a mouse button is
also pressed.
glutIdleFunc(void (*func)(void)) registers a function that's to
be executed if no other events are pending - for example, when the
event loop would otherwise be idle

24. Compilation on Windows
See class web site on how to set up a project
with OpenGL
Visual C++
Get glut.h, glut32.lib and glut32.dll from web
Create a console application
Add opengl32.lib, glut32.lib, glut32.lib to project
settings (under link tab)

25. Simple Animation
Animation
Redraw + swap buffers
What looks like if using single buffer
Example program
More on the glut documentation
Chapter 2 of textbook
OpenGL redbook
Links in the class resources page

26. OpenGL Drawing
We have learned how to create a window
Steps in the display function
Clear the window
Set drawing attributes
Send drawing commands
Swap the buffers
OpenGL coordinate system has different origin from
the window system
Uses lower left corner instead of upper left corner as origin

27. Clear the Window
glClear(GL_COLOR_BUFFER_BIT)
clears the frame buffer by overwriting it with the
background color.
Background color is a state set by
glClearColor(GLfloat r, GLfloat g,
GLfloat b, GLfloat a) in the init().

28. Drawing Attributes: Color

glColor3f(GLfloat r, GLfloat g, GLfloat b) sets the
drawing color
glColor3d(), glColor3ui() can also be used
Remember OpenGL is a state machine
Once set, the attribute applies to all subsequent defined
objects until it is set to some other value
glColor3fv() takes a flat array as input
There are more drawing attributes than color
Point size: glPointSize()
Line width: glLinewidth()
Dash or dotted line: glLineStipple()
Polygon pattern: glPolygonStipple()
…

29. Drawing Commands
Simple Objects glRectf()
Complex Objects
Use construct
glBegin(mode) and
glEnd() and a list of
vertices in between
glBegin(mode)
glVertex(v0);
glVertex(v1);
...
glEnd();
Some other commands can
also be used between
glBegin() and glEnd(), e.g.
glColor3f().
Example

30. Projection and Viewport

31. Orthographic projection
Orthographic View
glOrtho(left,
right, bottom,
top, front, back)
Specifies the
coordinates of 3D
region to be projected
into the image space.
Any drawing outside the
region will be
automatically clipped
away.
z=0

32. Viewports
Do not have use the entire window for the
image: glViewport(x,y,w,h)
Values in pixels (screen coordinates)

33. Window to Viewport mapping
Aspect Ratio: Height/Width
If the aspect ratio of the window
Is different from that of the
viewport, the picture will be
distorted.

34. Sierpinski Gasket (2D)
Start with a triangle
Connect bisectors of sides and remove central
triangle
Repeat

35. Example
Five subdivisions

36. Gasket Program
#include <GL/glut.h>
/* initial triangle */
GLfloat v[3][2]={{-1.0, -0.58},
{1.0, -0.58}, {0.0, 1.15}};
int n; /* number of recursive steps */
void triangle( GLfloat *a, G Lfloat *b, GLfloat *c)
/* display one triangle */
{
glVertex2fv(a);
glVertex2fv(b);
glVertex2fv(c);
}

37. Triangle Subdivision
void divide_triangle(GLfloat *a, GLfloat *b, G Lfloat *c, int
m)
{
/* triangle subdivision using vertex numbers */
point2 v0, v1, v2;
int j;
if(m>0)
{
for(j=0; j<2; j++) v0[j]=(a[j]+b[j])/2;
for(j=0; j<2; j++) v1[j]=(a[j]+c[j])/2;
for(j=0; j<2; j++) v2[j]=(b[j]+c[j])/2;
divide_triangle(a, v0, v1, m-1);
divide_triangle(c, v1, v2, m-1);
divide_triangle(b, v2, v0, m-1);
}
else(triangle(a,b,c));
/* draw triangle at end of recursion */
}

38. Gasket Display Functions
void display()
{
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_TRIANGLES);
divide_triangle(v[0], v[1], v[2], n);
glEnd();
glFlush();
}
•By having the glBegin and glEnd in the display callback rather
than in the function triangle and using GL_TRIANGLES rather
than GL_POLYGON in glBegin, we call glBegin and glEnd
only once for the entire gasket rather than once for each triangle

39. Example: 3D Gasket
after 5 iterations

40. Tetrahedron Code
void tetrahedron( int m)
{
glColor3f(1.0,0.0,0.0);
divide_triangle(v[0], v[1], v[2], m);
glColor3f(0.0,1.0,0.0);
divide_triangle(v[3], v[2], v[1], m);
glColor3f(0.0,0.0,1.0);
divide_triangle(v[0], v[3], v[1], m);
glColor3f(0.0,0.0,0.0);
divide_triangle(v[0], v[2], v[3], m);
}