1. 1 91.427 Computer Graphics I, Fall 2010 Input and Interaction

2. 2 91.427 Computer Graphics I, Fall 2010 Objectives Introduce basic input devices ­Physical Devices ­Logical Devices ­Input Modes Event-driven input Introduce double buffering for smooth animations Programming event input with GLUT

3. 3 91.427 Computer Graphics I, Fall 2010 Project Sketchpad Ivan Sutherland (MIT 1963) Established basic interactive paradigm Characterizes interactive computer graphics: ­User sees object on display ­User points to (picks) object with input device (light pen, mouse, trackball) ­Object changes (moves, rotates, morphs) ­Repeat

4. 4 91.427 Computer Graphics I, Fall 2010 Graphical Input Devices can be described either by ­Physical properties Mouse Keyboard Trackball ­Logical Properties What returned to program via API –position –object identifier Modes ­How & when input obtained Request or event

5. 5 91.427 Computer Graphics I, Fall 2010 Physical Devices mousetrackball light pen data tablet joy stick space ball

6. 6 91.427 Computer Graphics I, Fall 2010 Incremental (Relative) Devices data tablet ­return position directly to OS mouse, trackball, joy stick ­return incremental inputs (or velocities) to OS ­Must integrate these inputs to obtain absolute position Rotation of cylinders in mouse Roll of trackball Difficult to obtain absolute position Can get variable sensitivity

7. 7 91.427 Computer Graphics I, Fall 2010 Logical Devices Consider the C and C++ code ­C++: cin >> x; ­C: scanf (“%d”, &x); What is the input device? ­Can’t tell from code ­Could be keyboard, file, output from another program Code provides logical input ­Number ( int ) returned to program regardless of physical device

8. 8 91.427 Computer Graphics I, Fall 2010 Graphical Logical Devices Graphical input more varied than input to standard programs ­Usually numbers, characters, or bits Two older APIs (GKS, PHIGS) defined six types of logical input ­Locator: return position ­Pick: return ID of object ­Keyboard: return strings of characters ­Stroke: return array of positions ­Valuator: return floating point number ­Choice: return one of n items

9. 9 91.427 Computer Graphics I, Fall 2010 X Window Input X Window System introduced client-server model for network of workstations ­Client: OpenGL program ­Graphics Server: bitmap display with pointing device and keyboard

10. 10 91.427 Computer Graphics I, Fall 2010 Input Modes Input devices contain trigger ­Can be used to send signal to OS ­Button on mouse ­Press or release key When triggered, input devices return information (their measure) to system ­Mouse returns position information ­Keyboard returns ASCII code

11. 11 91.427 Computer Graphics I, Fall 2010 Request Mode Input provided to program only when user triggers device Typical of keyboard input ­Can erase (backspace), edit, correct until enter (return) key (the trigger) depressed

12. 12 91.427 Computer Graphics I, Fall 2010 Event Mode Most systems have > 1 input device Each can be triggered at arbitrary time by user Each trigger generates event ==> measure put in event queue ==> can be examined by user program

13. 13 91.427 Computer Graphics I, Fall 2010 Event Types Window: resize, expose, iconify Mouse: click one or more buttons Motion: move mouse Keyboard: press / release key Idle: nonevent ­Define what should be done if no event in queue

14. 14 91.427 Computer Graphics I, Fall 2010 Callbacks Programming interface for event- driven input Define callback function for each type of event graphics system recognizes This user-supplied function executed when event occurs GLUT example: glutMouseFunc(mymouse) mouse callback function

15. 15 91.427 Computer Graphics I, Fall 2010 GLUT callbacks GLUT recognizes subset of events recognized by any particular window system (Windows, X, Macintosh) ­glutDisplayFunc ­glutMouseFunc ­glutReshapeFunc ­glutKeyboardFunc ­glutIdleFunc ­glutMotionFunc, glutPassiveMotionFunc

16. 16 91.427 Computer Graphics I, Fall 2010 GLUT Event Loop Recall that last line in main.c for program using GLUT must be glutMainLoop(); ==> put program in infinite event loop In each pass through event loop, GLUT ­looks at events in queue ­for each event in queue, GLUT executes appropriate callback function if defined if no callback defined for event –event ignored

17. 17 91.427 Computer Graphics I, Fall 2010 The display callback Display callback executed whenever GLUT determines that window should be refreshed For example, when ­window first opened window reshaped exposed ­user program decides it wants to change display In main.c ­glutDisplayFunc(mydisplay) identifies function to be executed ­Every GLUT program must have display callback

18. 18 91.427 Computer Graphics I, Fall 2010 Posting redisplays Many events invoke display callback function ­  multiple executions of display callback on single pass through event loop Can avoid this problem by using glutPostRedisplay(); ==> sets flag GLUT checks to see if flag set at end of event loop If set ==> display callback function executed

19. 19 91.427 Computer Graphics I, Fall 2010 Animating a Display When redraw display through display callback, usually start by clearing window ­glClear() then draw altered display Problem: drawing of information in frame buffer decoupled from display of contents ­Graphics systems use dual ported memory Hence can see partially drawn display ­See program single_double.c for example with rotating cube

20. 20 91.427 Computer Graphics I, Fall 2010 Double Buffering Instead of one color buffer, use two ­Front Buffer: displayed but not written to ­Back Buffer: written to but not displayed Program then requests double buffer in main.c ­glutInitDisplayMode(GL_RGB | GL_DOUBLE) ­At end of display callback buffers are swapped void mydisplay() { glClear(GL_COLOR_BUFFER_BIT|….). /* draw graphics here */. glutSwapBuffers() }

21. 21 91.427 Computer Graphics I, Fall 2010 Using the idle callback Idle callback executed whenr no events in event queue ­glutIdleFunc(myidle) ­Useful for animations void myidle() { /* change something */ t += dt glutPostRedisplay(); } Void mydisplay() { glClear(); /* draw something that depends on t */ glutSwapBuffers(); }

22. 22 91.427 Computer Graphics I, Fall 2010 Using globals Form of all GLUT callbacks fixed ­void mydisplay() ­void mymouse(GLint button, GLint state, GLint x, GLint y) Must use globals to pass information to callbacks float t; /*global */ void mydisplay() { /* draw something that depends on t }