Input and Interaction

Transcription

1 Input and Interaction 1

2 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 2

3 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 3

4 Graphical Input Devices can be described either by Physical properties Mouse Keyboard Trackball Logical Properties What is returned to program via API A position An object identifier Modes How and when input is obtained Request or event 4

5 Physical Devices mouse trackball light pen data tablet joy stick space ball 5

6 Incremental (Relative) Devices Devices such as data tablet return position directly to OS Devices such as mouse, trackball, and 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 6

7 Logical Devices Consider the C and C++ code C++: cin >> x; C: scanf ( %d, &x); What is the input device? Can t tell from the code Could be keyboard, file, output from another program Code provides logical input Number (int) returned to program regardless of physical device 7

8 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 a position Pick: return ID of an object Keyboard: return strings of characters Stroke: return array of positions Valuator: return floating point number Choice: return one of n items 8

9 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 9

10 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 10

11 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 11

12 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 12

13 Event Types Window: resize, expose, iconify Mouse: click one or more buttons Motion: move mouse Keyboard: press or release a key Idle: nonevent Define what should be done if no other event is in queue 13

14 Callbacks Programming interface for eventdriven 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 14

15 GLUT callbacks GLUT recognizes subset of events recognized by any particular window system (Windows, X, Macintosh) -glutdisplayfunc -glutmousefunc -glutreshapefunc -glutkeyboardfunc -glutidlefunc -glutmotionfunc, glutpassivemotionfunc 15

16 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 one is defined if no callback defined for event, event is ignored 16

17 The display callback Display callback executed whenever GLUT determines that window should be refreshed For example When window first opened When window reshaped When window exposed When user program decides it wants to change display In main.c - glutdisplayfunc(mydisplay) identifies function to be executed Every GLUT program must have a display callback 17

18 Posting redisplays Many events may invoke display callback function Can lead to multiple executions of display callback on single pass through event loop Can avoid this problem by instead using glutpostredisplay(); ==> sets a flag GLUT checks to see if flag set at end of event loop If set ==> display callback function is executed 18

19 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 is decoupled from display of its contents Graphics systems use dual ported memory Hence we can see partially drawn display See program single_double.c for example with rotating cube 19

20 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() } 20

21 Using the idle callback Idle callback executed whenever there are 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(); } 21

22 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 } 22