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
 …
 GLUTprovide 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
application program
OpenGL Motif
widget or similar GLUT
GLX, AGL
or WGL GLU
X, Win32, Mac O/S OpenGL
software and/or hardware

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

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

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

21. init.c
black clear color
void init() opaque window
{
glClearColor (0.0, 0.0, 0.0, 1.0);
glColor3f(1.0, 1.0, 1.0); fill/draw with white
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);
}
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
z=0
into the image space.
 Any drawing outside the
region will be
automatically clipped
away.

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, GLfloat *b, GLfloat *c)
/* display one triangle */
{
glVertex2fv(a);
glVertex2fv(b);
glVertex2fv(c);
}

37. Triangle Subdivision
void divide_triangle(GLfloat *a, GLfloat *b, GLfloat *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);
}