1. OpenGL
A Brief Overview

2. What is OpenGL? It is NOT a programming language.
It is a Graphics Rendering API consisting of a set of functions with a well defined interface.
Whenever we say that a program is OpenGL-based or OpenGL applications, we mean that it is written in some programming language (such as C/C++ or Java) that makes calls to one or more of OpenGL libraries.

3. OpenGL & Alternative
Based on GL (graphics library) by Silicon Graphics Inc. (SGI) (
Advantages:
Runs on everything, including smart phones (OpenGL/ES)
Alternatives:
Microsoft’s Direct3D – limited to MS-Windows (
Sun’s Java3D – slower, implemented on top of OpenGL (

4. Useful Websites and Books
Official Site
Non official sites
BOOKS
OpenGL Red Book (
&
OpenGL Blue Book (

5. Library Layers OpenGL = GL + GLU
APP
EXT
GLUT
OpenGL = GL + GLU
Basic low-level GL routines implemented using OS graphics routines
Time-saving higher-level GLU routines implemented using GL routines
GLUT opens and manages OpenGL windows and adds helper functions
OpenGL Extensions provide direct device-dependent access to hardware
GLU GL OS
Driver HW

6. OpenGL API Functions OpenGL contains over 200 functions
Primitive functions: define the elements (e.g. points, lines, polygons, etc.)
Attribute functions: control the appearance of primitives (e.g. colors, line types, light source, textures, etc.)
Viewing functions: determine the properties of camera. Handle transformations.
Windowing functions: not part of core OpenGL (in GLUT)
Other functions

7. Libraries and Headers Library Name Library File Header File Note
OpenGL
opengl32.lib (Win)
-lgl (UNIX)
gl.h
“core” library
Auxiliary library
glu32.lib (Win)
-lglu (UNIX)
glu.h
handles a variety of accessory functions
Utility toolkit
glut32.lib (Win)
-lglut (UNIX)
glut.h
glaux.h
window management & others

8. Function Naming Conventions
glColor3f(…)
library name, command name, # of arguments, argument type
gl: OpenGL
glu: GLU
glut: GLUT
f: the argument is float type
i: the argument is integer type
v: the argument requires a vector

9. A Sample Program 1 2 3 4 #include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
void main (int argc, char **argv) {
glutInit (&argc, argv);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize (500, 500);
glutCreateWindow (“My First Program");
myinit ();
glutDisplayFunc ( display );
glutReshapeFunc ( resize );
glutKeyboardFunc ( key );
glutMainLoop (); } 1 2 3 4

10. 1. Initialize & Create Window
Initiate a GLUT window…
void main (int argc, char **argv) {
glutInit (&argc, argv); // GLUT initialization
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB); // display model
glutInitWindowSize (500, 500); // set window size
glutCreateWindow (“My First Program”); // create window …… }

11. 2. Initialize OpenGL State
Set up the canvas for drawing, including the background color and view
void myinit(void) {
glClearColor(1.0, 1.0, 1.0, 1.0); // background color
glColor3f(1.0, 0.0, 0.0); // line color
glMatrixMode(GL_PROJECTION); // set up viewing:
glLoadIdentity(); // load identity matrix
gluOrtho2D(0.0, 500.0, 0.0, 500.0); // specify Orthographic view
glMatrixMode(GL_MODELVIEW); // go back to MV matrix }

12. 3. Register Callback Functions
Event handling
void main (int argc, char **argv) { ……
glutDisplayFunc ( display ); // display callback
glutReshapeFunc ( resize ); // window resize callback
glutKeyboardFunc ( key ); // keyboard callback }
Event Loops and Callback Functions
Interactive programs react to the events such as mouse or keyboard events and window events.
Callback Function - Routine to call when something happens (eg. window resize, redraw, user input, etc)
GLUT uses a callback mechanism to do its event processing

13. (side note) GLUT Callback Functions
Contents of window need to be refreshed
glutDisplayFunc()
Window is resized or moved
glutReshapeFunc()
Key action
glutKeyboardFunc()
Mouse button action
glutMouseFunc()
Mouse moves while a button is pressed
glutMotionFunc()
Mouse moves regardless of mouse button state
glutPassiveMouseFunc()
Called when nothing else is going on
glutIdleFunc()

14. 3.1 Rendering Callback Here is the place you do the drawing!!!
void display( void ) {
int k;
glClear(GL_COLOR_BUFFER_BIT);
for( k=0; k<5000; k++) …… }
One of the most important callbacks is the glutDisplayFunc() callback.
This callback is called when the window needs to be refreshed. It’s here that you’d do all of your OpenGL rendering.

15. 3.2 Window Resize Callback
void resize(int w, int h) { ……
display(); }
It’s called when the window is resized or moved

16. 3.3 Keyboard Input Callback
void key( unsigned char mkey, int x, int y ) {
switch( mkey )
case ‘q’ :
exit( EXIT_SUCCESS );
break; …… }
Above is a simple example of a user input callback. In this case, the routine was registered to receive keyboard input. GLUT supports user input through a number of devices including the keyboard, mouse, dial and button boxes and spaceballs.

17. 4. Event Process Loop
This is where your application receives events, and schedules when callback functions are called
void main (int argc, char **argv) { ……
glutMainLoop(); }
Enter the main event processing loop. This is where your application receives events, and schedules when callback functions are called.

18. Viewport Coordinates Physical per-pixel integer coordinates
(HRES-1,
VRES-1)
(0,VRES-1)
Physical per-pixel integer coordinates
Also called screen or device coordinates
glViewport(x,y,w,h)
x,y – lower left pixel (integers)
w – width
h – height
Sometimes (0,0) is in the upper left corner (e.g. for mouse input)
(0,0)
(HRES-1,0)

19. Window Coordinates Logical, mathematical floating-point coordinates
(1,1)
(-1,1)
Logical, mathematical floating-point coordinates
glOrtho(l,r,b,t,n,f) (for 3D)
left, right, bottom, top
near, far: limits depth
For 2D use
gluOrtho2D(l,r,b,t) calls glOrtho(l,r,b,t,-1,1)
To use per-pixel coordinates, call:
gluOrtho2D(-.5,-.5,w-.5,h-.5);
(-1,-1)
(1,-1)

20. 2D Geometric Primitives
Primitives – fundamental entities such as point and polygons
Basic types of geometric primitives
Points
Line segments
Polygons

21. 2D Geometric Primitives
GL_POINTS
GL_LINES
GL_LINE_STRIP
GL_LINE_LOOP
GL_TRIANGLES
GL_TRIANGLE_FAN
GL_POLYGON
GL_QUADS
All geometric primitives are specified by vertices

22. Specifying Geometric Primitives
glBegin( type ); glVertex*(…); …… glEnd();
OpenGL organizes vertices into primitives based upon which type is passed into glBegin(). The possible types are:
GL_POINTS
GL_LINE_STRIP
GL_LINES
GL_LINE_LOOP
GL_POLYGON
GL_TRIANGLE_STRIP
GL_TRIANGLES
GL_TRIANGLE_FAN
GL_QUADS
GL_QUAD_STRIP
type determines how vertices are combined

23. An Example void drawSquare (GLfloat *color) {
glColor3fv( color ); // sets the color of the square
glBegin(GL_POLYGON);
glVertex2f ( 0.0, 0.0 );
glVertex2f ( 1.0, 0.0 );
glVertex2f ( 1.1, 1.1 );
glVertex2f ( 0.0, 1.0 );
glEnd();
glFlush(); // force the renderer to output the results }

24. Points (1,1) (-1,1) glBegin(GL_POINTS); glVertex2f(-.6,1.);
glEnd();
(-1,-1)
(1,-1)

25. Lines (1,1) (-1,1) glBegin(GL_LINES); glVertex2f(-.6,1.);
glEnd();
(-1,-1)
(1,-1)

26. Line Strip (1,1) (-1,1) glBegin(GL_LINE_STRIP); glVertex2f(-.6,1.);
glEnd();
(-1,-1)
(1,-1)

27. Line Loop (1,1) (-1,1) glBegin(GL_LINE_LOOP); glVertex2f(-.6,1.);
glEnd();
(-1,-1)
(1,-1)

28. Polygon OpenGL only supports convex polygons
(and really only triangles)
(1,1)
(-1,1)
glBegin(GL_POLYGON);
glVertex2f(-.6,1.);
glVertex2f(-.6,.6);
glVertex2f(-.2,.6);
glVertex2f(-.2,-.6);
glVertex2f(-.6,-.6);
glVertex2f(-.6,-1.);
glVertex2f(.6,-1.);
glVertex2f(.6,-.6);
glVertex2f(.2,-.6);
glVertex2f(.2,.6);
glVertex2f(.6,.6);
glVertex2f(.6,1.);
glEnd();
(-1,-1)
(1,-1)

29. Quads (1,1) (-1,1) glBegin(GL_QUADS); glVertex2f(-.6,1.);
glEnd();
(-1,-1)
(1,-1)

30. Quads glBegin(GL_QUADS); glVertex2f(-.6,1.); glVertex2f(-.6,.6);
glEnd();
glVertex2f(-.2,.6); glVertex2f(-.2,-.6);
glVertex2f(.2,-.6);
glVertex2f(.2,.6);
glVertex2f(.6,.6);
glVertex2f(.6,1.);

31. Quads (1,1) (-1,1) (-1,1) glBegin(GL_QUADS); glVertex2f(-.6,1.);
glEnd();
(-1,-1)
(1,-1)

32. Triangles (1,1) (-1,1) glBegin(GL_TRIANGLES); glVertex2f(-.6,1.);
glEnd();
(-1,-1)
(1,-1)

33. Triangles (1,1) (-1,1) glBegin(GL_TRIANGLES); glVertex2f(-.6,1.);…
glEnd();
(-1,-1)
(1,-1)

34. Triangle Strip (1,1) (-1,1) glBegin(GL_TRIANGLE_STRIP);
glVertex2f(-.6,1.);
glVertex2f(-.6,.6);
glVertex2f(-.2,.6);
glVertex2f(-.2,-.6);
glVertex2f(-.6,-.6);
glVertex2f(-.6,-1.);
glVertex2f(.6,-1.);
glVertex2f(.6,-.6);
glVertex2f(.2,-.6);
glVertex2f(.2,.6);
glVertex2f(.6,.6);
glVertex2f(.6,1.);
glEnd();
(-1,-1)
(1,-1)

35. Triangle Strip
First two vertices prime the pump, then every new vertex creates a triangle connecting it to the previous two vertices
(1,1)
(-1,1)
glBegin(GL_TRIANGLE_STRIP);
glVertex2f(-.6,1.);
glVertex2f(.6,1.);
glVertex2f(-.2,.6);
glVertex2f(.2,.6);
glVertex2f(-.2,-.6);
glVertex2f(.2,-.6);
glVertex2f(.6,-1.);
glVertex2f(.6,-.6);
glEnd(); …
(-1,-1)
(1,-1)

36. First two vertices prime the pump, then every new vertex creates a triangle connecting it to the previous vertex and the first vertex
Triangle Fan
(1,1)
(-1,1)
glBegin(GL_TRIANGLE_FAN);
glVertex2f(-.2,.6);
glVertex2f(-.6,.6);
glVertex2f(-.6,1.);
glVertex2f(.6,1.);
glVertex2f(.2,.6);
glVertex2f(.2,-.6);
glVertex2f(-.2,-.6);
glEnd(); …
(-1,-1)
(1,-1)

37. Assigning Color glColor3f(0,0,1); glBegin(GL_POLYGON);
glVertex2f(-1,1);
glVertex2f(-1,-1);
glVertex2f(1,-1);
glEnd();
glBegin(GL_POLYGON);
glColor3f(0,1,0);
glVertex2f(-1,1);
glColor3f(0,0,1);
glVertex2f(-1,-1);
glColor3f(1,0,0);
glVertex2f(1,-1);
glEnd();
glColor3f(1,0,0);
glBegin(GL_POLYGON);
glVertex2f(-1,1);
glVertex2f(-1,-1);
glVertex2f(1,-1);
glEnd();
glColor3f(0,0,0);
glBegin(GL_LINE_LOOP);

38. Graphics Pipeline of OpenGL
Model Coords
Model Xform
World Coords
Viewing Xform
Viewing Coords
Perspective Distortion
Still Clip Coords
Clipping
Clip Coords
Homogeneous Divide
Window
Coordinates
Window to
Viewport
Viewport
Coordinates

39. Coordinate Systems and Transformation
Identify which matrix we wish to alter
Set the matrix to an identity matrix
Alter the matrix to form the desired matrix
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-1.0, 1.0, -1.0, 1.0);

40. Coordinate Systems and Transformation
glMatrixMode(GLenum mode);
Identify which matrix we wish to alter – The mode is usually GL_MODELVIEW, GL_PROJECTION
glLoadIdentity();
Set the current matrix to an identity matrix

41. Transformations and Camera Analogy
Modeling transformation
Positioning and moving the model.
Viewing transformation
Positioning and aiming camera in the world.
Projection transformation
Adjusting the lens of the camera.
Viewport transformation
Enlarging or reducing the physical photograph. .

42. Transformations in OpenGL
Transformations are specified by matrix operations. Desired transformation can be obtained by a sequence of simple transformations that can be concatenated together.
Transformation matrix is usually represented by 4x4 matrix (homogeneous coordinates).
Provides matrix stacks for each type of supported matrix to store matrices.

43. Specifying Operations (1)
Translation
glTranslate {fd} (TYPE x, TYPE y, TYPE z)
Multiplies the current matrix by a matrix that translates an object by the given x, y, z.

44. Specifying Operations (2)
Scaling
glScale {fd} (TYPE x, TYPE y, TYPE z)
Multiplies the current matrix by a matrix that scales an object by the given x, y, z.

45. Specifying Operations (3)
Rotation
glRotate {fd} (TYPE angle,
TYPE x, TYPE y, TYPE z)
Multiplies the current matrix by a matrix that rotates an object in a counterclockwise direction about the ray from origin through the point by the given x, y, z. The angle parameter specifies the angle of rotation in degrees.

46. Order of Transformations
The transformation matrices appear in reverse order to that in which the transformations are applied.
In OpenGL, the transformation specified most recently is the one applied first.

47. Viewing-Modeling Transformation
If given an object, and we want to render it from a viewpoint, what information do we have to have?
Viewing position
Which way I am looking at
Which way is “up”
…..

48. Default Viewing +X +Z +Y
By default, the camera is at the origin, looking down the negative z-axis
In the default position, the camera is at the origin, looking down the negative z-axis
If you get this wrong, you may see nothing in your image.

49. Where are we and what are we looking at?y y
Loot at
(atx, aty, atz)
View-up vector
(upx, upy, upz) x x
Model z z
Eyepoint
(eyex, eyey, eyez)

50. Viewing in OpenGL Look-At Function
gluLookAt (eyex, eyey, eyez, atx, aty, atz, upx, upy, upz )
Defines a viewing matrix and multiplies the current matrix by it. Alters the ModelView matrix.

51. Projection Transformation
Projection & Viewing Volume
Projection Transformation
Viewpoint Transformation

52. Perspective Projection Volume
Far-plane: zFar
Near-plane: zNear
Viewing volume h w
aspect ratio = w/h y z x
fovy

53. Perspective Projection Commands
glFrustum( left, right, bottom, top, zNear, zFar )
Creates a matrix for a perspective viewing frustum and multiplies the current matrix by it. Alters the Projection matrix.

54. Perspective Projection Commands
gluPerspective( fovy, aspect, zNear, zFar )
Alternative to glFrustum(..). Creates a matrix for an perspective viewing frustum and multiplies the current matrix by it. Alters the Projection matrix.
Note: fovy is the field of view (fov) angle between the top and bottom planes of the clipping volume. aspect is the aspect ratio

55. Enabling GL Features
Features – lighting, hidden-surface removal, texture mapping, etc…
Each feature will slow down the rendering process.
We can enable/disable each feature individually
void glEnable(GLenum feature)
void glDisable(GLenum feature)
E.g. glEnable(GL_LIGHTING);

56. Enabling GL Features GL_ALPHA_TEST GL_BLEND GL_CLIP_PLANE i
If enabled, do alpha testing.
GL_AUTO_NORMAL
If enabled, generate normal vectors when either GL_MAP2_VERTEX_3 or GL_MAP2_VERTEX_4 is used to generate vertices. See glMap2.
GL_BLEND
If enabled, blend the incoming RGBA color values with the values in the color buffers..
GL_CLIP_PLANE i
If enabled, clip geometry against user-defined clipping plane i..
GL_COLOR_LOGIC_OP
If enabled, apply the currently selected logical operation to the incoming RGBA color and color buffer values.
GL_COLOR_MATERIAL
If enabled, have one or more material parameters track the current color.
GL_COLOR_TABLE
If enabled, preform a color table lookup on the incoming RGBA color values.
GL_CONVOLUTION_1D
If enabled, perform a 1D convolution operation on incoming RGBA color values.
GL_CONVOLUTION_2D
If enabled, perform a 2D convolution operation on incoming RGBA color values. ……

57. Double Buffering
Use two buffers: front buffer and back buffer to guarantee the displaying of a fully redrawn buffer image
glutSwapBuffers();
Replace glFlush() by glutSwapBuffer() in the display callback if using double buffering

58. Advanced Topics Lighting Texture Mapping Plotting Implicit Functions
Shadows
Fog
Picking (object selection)
GUI (glut pop-up menus, glui library)
Reference links given on slide 3

59. Acknowledgment Thanks for materials from Dr. John C. Hart
Dr. Eugene Zhang
Dr. Mike Bailey