Objectives
•Introduce basic input devices
Physical Devices
Logical Devices
Input Modes
•Event-driven input
•Introduce double buffering for smooth animations
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)
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
Physical Devices
mouse trackball light pen
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
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
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
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
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
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
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
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
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:
GLUT callbacks
GLUT recognizes subset of events
recognized by any particular window system (Windows, X, Macintosh)
-glutDisplayFunc -glutMouseFunc -glutReshapeFunc -glutKeyboardFunc -glutIdleFunc -glutMotionFunc, glutPassiveMotionFunc
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
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
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
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
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 */ .
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();
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 }