Input and Interaction
91.427 Computer Graphics I, Fall 2008
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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
91.427 Computer Graphics I, Fall 2008
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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
91.427 Computer Graphics I, Fall 2008
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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
91.427 Computer Graphics I, Fall 2008
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Physical Devices
mouse
data tablet
91.427 Computer Graphics I, Fall 2008
trackball
joy stick
light pen
space ball
5
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
91.427 Computer Graphics I, Fall 2008
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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
91.427 Computer Graphics I, Fall 2008
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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
91.427 Computer Graphics I, Fall 2008
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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
91.427 Computer Graphics I, Fall 2008
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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
91.427 Computer Graphics I, Fall 2008
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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
91.427 Computer Graphics I, Fall 2008
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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
91.427 Computer Graphics I, Fall 2008
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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
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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
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GLUT callbacks
GLUT recognizes subset of events
recognized by any particular window
system (Windows, X, Macintosh)
-glutDisplayFunc
-glutMouseFunc
-glutReshapeFunc
-glutKeyboardFunc
-glutIdleFunc
-glutMotionFunc,
glutPassiveMotionFunc
91.427 Computer Graphics I, Fall 2008
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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
91.427 Computer Graphics I, Fall 2008
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The display callback
•Display callback executed whenever GLUT
determines that window should be refreshed
•For example
­
­
­
­
When
When
When
When
window first opened
window reshaped
window exposed
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
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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
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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
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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()
}
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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();
}
91.427 Computer Graphics I, Fall 2008
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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
}
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