1. Fundamentals of Computer Graphics Part 3
prof.ing.Václav Skala, CSc.
University of West Bohemia
Plzeň, Czech Republic
©2002
Prepared with Angel,E.: Interactive Computer Graphics – A Top Down Approach with OpenGL, Addison Wesley, 2001

2. Fundamentals of Computer Graphics
Input & Interaction
Till now – no interaction
Introduction of devices for interaction
how devices “appear” in your program
client-server network & client-server graphics
development of a painting program
First attempt: - 37 years! Project Sketchpad – Sutherland
Different approach will be taken – we will use API, but OpenGL does not support it directly – due to portability of the renderer, interaction with OS etc.
Fundamentals of Computer Graphics

3. Fundamentals of Computer Graphics
Input Devices
Two possible ways to see input devices:
as a physical device – keyboard, mouse, trackball, etc.
as a logical device – from a programmer perspective – with specified functionality, in graphics more complex
the separation of physical and logical levels enable us to make programs more flexible, independent from the actual physical device
Fundamentals of Computer Graphics

4. Physical Input Devices
pointing device – allows to indicate position & send signals/interrupts to the computer – relative/absolute positioning
keyboard device – almost physical keyboard – returns character codes to a program
Fundamentals of Computer Graphics

5. Physical Input Devices
Absolute positioning:
data tablets
light pen
joystick – variable-sensitivity device & haptic device
spaceball – up-down, left-right, front-back & 3 independent twists
Fundamentals of Computer Graphics

6. Fundamentals of Computer Graphics
Logical Input Devices
Some APIs (PHIGS, GKS, Direct xx) supports 6 classes of logical input devices – OpenGL does not take this approach
String – logical device providing ASCII strings – keyboard
Locator – provides a position in world coordinates – usually implemented via pointing device – mouse, trackball. OpenGL provides similar but conversion from screen coordinates to world coordinates must be made by a user
Pick – returns identifier of an object – in OpenGL process called selection can be used to accomplish picking
Fundamentals of Computer Graphics

7. Fundamentals of Computer Graphics
Logical Input Devices
Choice – allows the user to select on of a discrete number of options – in OpenGL various widgets provided by the window system can be used; widget is a graphical interactive device provided by window system or a toolkit (menu with n selections etc.)
Dial – provides analog input to the user program – slidebars etc.
Stroke – device returns an array of locations – different implementations – usually: mouse button down, transfer data to an array with different positions, release button – ends the transfer
Fundamentals of Computer Graphics

8. Fundamentals of Computer Graphics
Input Devices & Modes
Two entities:
a measure process – is what the device returns to the user program (string from a keyboard)
a device trigger – is a physical input on the device which user can signal the computer (return – end of the process)
Modes:
request – value is returned on a request
sample mode – actual value is given (no buffering)
request_locator ( device_id, &measure); /* usual form */
sample_locator (device_id, &measure);
Fundamentals of Computer Graphics

9. Fundamentals of Computer Graphics
Input Devices & Modes
Request versus Sample modes
Generally sample & request modes are not sufficient for Human-Computer-Interface (HCI)
Fundamentals of Computer Graphics

10. Fundamentals of Computer Graphics
Input Devices & Modes
Event mode
working in environment with multiple input devices – each has its own trigger and running measure process
each time when the device is triggered – the event is generated, the measure with the process identifier is placed in an event queue. The program can examine the front event in the queue and decides what to do – this is used for GKS, PHIGS etc.
Fundamentals of Computer Graphics

11. Input Devices & Modes - Callback
another approach – association of a callback function with a specific type of event – mostly used in windowing system and client/server environments
int main (...) { ....glutMouseFunc(mouse); glutDisplayFunc(...);..}
void mouse_callback_func(int button, int state, int x, int y)
{ if button ==GLUT_LEFT_BUTTON && state==GLUT_DOWN) drawSquare(x,y); /*users function*/
if button ==GLUT_MIDDLE_BUTTON && state==GLUT_DOWN) exit ();
} /* window reshape principle */
Fundamentals of Computer Graphics

12. Fundamentals of Computer Graphics
Clients-Servers
Our programs worked on single & isolated system so far, but it should also work in distributed computing and networks
Distributed graphics, projection walls etc.
Fundamentals of Computer Graphics

13. Fundamentals of Computer Graphics
Display Lists
Instructions are stored in a display memory – display file – display list
Modes:
- immediate – each element is processed and displayed
- retained – objects are defined & stored in a display list on a server & redisplayed on the client request
Early graphics systems
Display processor architecture
Fundamentals of Computer Graphics

14. Definition & Execution of Display Lists
#define BOX 1
glNewList(Box, GL_COMPILE); /* sends to the server*/
glBegin(GL_POLYGON);
glColor3f(1.0, 1.0, 1.0);
glVertex2f(-1.0, -1.0);
glVertex2f( 1.0, -1.0);
glVertex2f( 1.0, 1.0);
glVertex2f(-1.0, 1.0);
glEnd( );
glEndList ( );
/* GL_COMPILE_AND_EXECUTE – immediate display */
Drawing – execution
glCallList(BOX);
Fundamentals of Computer Graphics

15. Display Lists & Transformations
If model-view or projection matrices changed between execution of the display list – the drawn model will appear at different positions
glMatrixModel(GL_PROJECTION);
for (i=1; i<5; i++); {
glLoadIdentity( );
gluOrtho2D(-2.0*i, 2.0*i, -2.0*i, 2.0*i );
glCallList(BOX); }
Fundamentals of Computer Graphics

16. Display Lists & Push/Pop
! Color is changed whenever the list is executed
Execution of the display list changes the state and attributes in general – may cause unexpected effects
it is possible and recommended to store them in the stack
at the beginning of the display list specification
glPushAttrib(GL_ALL_ATTRIB_BITS);
glPushMatrix( );
at the end of the display list specification
glPopAttrib( );
glPopMatrix ( );
Fundamentals of Computer Graphics

17. Characters, Fonts & Strings
Study chapter on your own
Fundamentals of Computer Graphics

18. Programming Event Driven Input
Pointing device:
passive move event – mouse moved without pressing a button
move event - mouse moved with a button pressed, or button released (some systems counts the pushing & releasing of a button as only a single event)
glutMouseFunc(mouse_callback_func) /* registration */
void mouse_callback_func(int button, int state, int x, int y);
{ if (button==GLUT_LEFT_BUTTON && state==GLUT_DOWN) exit ( );
} /* the left button pressed will be omitted no action will be taken as no corresponding action is specified */
Fundamentals of Computer Graphics

19. Programming Event Driven Input
int main (int argc, char **argv); {
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutCreateWindow(“square”); /* create a window with name square */
myinit( );
glutReshapeFunc(myReshape); /* generated if window size changed*/
glutMouseFunc(mouse);
/* activated if status or position of a mouse changed */
glutDisplayFunc(display);
/* GLUT call back requires it strictly – in case of no action */
/* void display ( ) { } must be specified – empty function */
glutMainLoop( ); }
Fundamentals of Computer Graphics

20. Programming Event Driven Input
Keyboards events:
glutKeyBoardFunc ( keyboard); /* registration */
void keyboard (unsigned char key, int x, int y);
{ if (key == ‘q’ || key == ‘Q’) exit ( ); /* exits the program */ }
Special functions:
glutPostDisplay ( ) /* if window iconified redrawing is postoned */
Multiple Window management:
id = glutCreateWindow(“second window”); /* int id*/
glutSetWindow (id) ; /* sets the window into which object will be rendered */
Fundamentals of Computer Graphics

21. Menus & Picking Study chapter 3.6 – 3.7 on your own
Fundamentals of Computer Graphics

22. Fundamentals of Computer Graphics
Menu
Pop-up menus
glutCreateMenu(demo_menu);
glutAddmenuEntry(“quit”,1);
glutAddmenuEntry(“increase square size”,2);
glutAddmenuEntry(“decrease square size”,3);
glutAttachmenu(GLUT_RIGHT_BUTTON);
void demo_menu(int id);
{ if (id ==1) exit ( );
else if (id ==2) size = size * 2;
else if (id ==3) size = size / 2;
glutPostRedisplay( );
/* glutDisplayFunc is called-redisplay without menu */ }
Fundamentals of Computer Graphics

23. Animation Study chapter 3.9 on your own
Fundamentals of Computer Graphics

24. Fundamentals of Computer Graphics
Animation
For animation – double buffering
glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE);
Buffers:
front – content visible on the display
back – where the rendering is made
to the display function must be added
glutSwapBuffers ( );
Fundamentals of Computer Graphics

25. Fundamentals of Computer Graphics
Conclusion
Study on your own:
Chapter – 3, text, strings & character drawing
Chapter 3.8 – simple Paint Program,
Chapter 3.9 – Animating Interactive Program
Chapter 3.10 – Design of Interactive Programs
Make some experiments with display list construction and discuss efficiency of its use for complex objects
Fundamentals of Computer Graphics