Input and Interaction

Yuanfeng Zhou

Shandong University Software College

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Objectives

• Introduce the basic input devices Physical Devices

Logical Devices

Input Modes

•Event-driven input• Introduce double buffering for smooth animations

•Programming event input with GLUT

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Project Sketchpad

• Ivan Sutherland (MIT 1963)

established the basic interactive

paradigm that characterizes

interactive computer graphics: User sees an object on the display

User points to (picks) the object with an

input device (light pen, mouse, trackball)

Object changes

(moves, rotates, morphs)

Repeat

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Interaction with Graphics System

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ChangeImage

Reactto

Change

Graphics System User

InputDevice

Display

Graphical Input

•Devices can be described either by Physical properties

• Mouse• Keyboard• Trackball (Thinkpad)

Logical properties• What is returned to program via API (Application

Programming Interface)– A position– An object identifier

•Modes How and when input is obtained

• Request or event5

Logical Properties

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ApplicationModel

ApplicationProgram

GraphicsSystem

OutputDevices

InputDevices

API

Function Callsor Protocol

Data

WM_LBUTTONDOWN#define ON_BN_LBUTTONDOWN(id, memberFxn)void CMyProgram::OnLButtonDown(UINT nFlags, CPoint point) { processing code…

CButton::OnLButtonDown(nFlags, point); }

Physical Devices

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mouse trackball light pen

data tablet joy stick space ball

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

•The code provides logical input A number (an int) is returned to the program

regardless of the physical device

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Graphical Logical Devices

• Graphical input is more varied than input to standard programs which is usually numbers, characters, or bits

• Two older APIs (GKS77’, PHIGS90’) 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

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X Window Input

• The X Window System introduced a client-server model for a network of workstations

Client: OpenGL program

Graphics Server: bitmap display with a pointing device and a keyboard

Display List in OpenGL

•Matrix computation• Image display•Light, material•Texture•Polygonal objects display

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Display List in OpenGL

• void glNewList(GLuint list, GLenum mode);• mode: GL_COMPILE and GL_COMPILE_AND_EXECUTE

• void glEndList(void);• NOT included in Display List:• glDeleteLists() glIsEnable()• glFeedbackBuffer() glIsList()• glFinish() glPixelStore()• glGenLists() glRenderMode()• glGet*() glSelectBuffer()

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Display List in OpenGL

•Example:GLuint listName = 1;

void myinit (void)

{

glNewList (listName, GL_COMPILE);

glColor3f (1.0, 0.0, 0.0);

glBegin (GL_TRIANGLES);

glVertex2f (0.0, 0.0);

glVertex2f (1.0, 0.0);

glVertex2f (0.0, 1.0);

glEnd ();

glTranslatef (1.5, 0.0, 0.0);

glEndList ();

glShadeModel (GL_FLAT);

}

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void CALLBACK display(void) { GLuint i; glClear(GL_COLOR_BUFFER_BIT); glColor3f (0.0, 1.0, 0.0); glPushMatrix(); for (i = 0; i <5; i++) glCallList (listName); drawLine (); glPopMatrix(); glFlush (); }  

Input Mode

• Input devices contain a trigger which can be used to send a signal to the operating system

Button on mouse

Pressing or releasing a key

•When triggered, input devices return information (their measure) to the system

Mouse returns position information

Keyboard returns ASCII code

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Request Mode

• Input provided to program only when user triggers the device

•Typical of keyboard input Can erase (backspace), edit, correct until enter

(return) key (the trigger) is depressed

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1. Program requests measure of a device and blocks.2. Measure process maintains current measure.3. Trigger process signals measure process to return measure.request_locator(device_id, &measure);

Event Mode

•Most systems have more than one input device, each of which can be triggered at an arbitrary time by a user

•Each trigger generates an event whose measure is put in an event queue which can be examined by the user program

16glutMainLoop();

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 event-driven input

•Define a callback function for each type of event the graphics system recognizes

•This user-supplied function is executed when the event occurs

•GLUT example: glutMouseFunc(mymouse)

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mouse callback function

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GLUT callbacks

GLUT recognizes a subset of the events recognized by any particular window system (Windows, X, Macintosh) glutDisplayFunc glutMouseFunc glutReshapeFunc glutKeyboardFunc glutIdleFunc glutMotionFunc glutPassiveMotionFunc

GLUT Event Loop

• Recall that the last line in main.c for a program using GLUT must be

glutMainLoop();

which puts the program in an infinite event loop

• In each pass through the event loop, GLUT looks at the events in the queue

for each event in the queue, GLUT executes the appropriate callback function if one is defined

if no callback is defined for the event, the event is ignored

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Using globals

• The form of all GLUT callbacks is fixed void mydisplay() void mymouse(GLint button, GLint state, GLint x, GLint y)

• Must use globals to pass information to callbacks

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float t; /*global */

void mydisplay(){/* draw something that depends on t}

GLUT Display Event

Trigger: GLUT determines that the window needs to be redisplayed. A display event is generated when the window is first drawn.Callback function form: void display();

Registration: glutDisplayFunc(display);

The display callback is executed whenever GLUT determines that the window should be refreshed, for example

When the window is first openedWhen the window is reshapedWhen a window is exposedWhen the user program decides it wants to change the display

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Posting redisplays

• Many events may invoke the display callback function

Can lead to multiple executions of the display callback on a single pass through the event loop

• We can avoid this problem by instead usingglutPostRedisplay();

which sets a flag. • GLUT checks to see if the flag is set at the end of

the event loop• If set then the display callback function is

executed

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Animating a Display

• When we redraw the display through the display callback, we usually start by clearing the window glClear()

then draw the altered display• Problem: the drawing of information in the frame

buffer is decoupled from the display of its contents

Graphics systems use dual ported memory

• Hence we can see partially drawn display See the program single_double.c for an example

with a rotating cube

GLUT Mouse Event

Trigger: Any mouse button is depressed or released.

Callback function form:

void mouse_callback_func(int button, int state, int x, int y);

Registration:

glutMouseFunc(mouse_callback_function);

GLUT Defined Mouse Constants

GLUT_LEFT_BUTTONGLUT_MIDDLE_BUTTONGLUT_RIGHT_BUTTON

GLUT_UPGLUT_DOWN

Systems with only one mouse button can only generatea GLUT_LEFT_BUTTON callback.

GLUT Reshape Event

Trigger: Active window is resized

Callback function form:

void reshape_func(GLsizei w, GLsizei h);

Registration:

glutReshapeFunc(reshape_func);

GLUT Move Event

Trigger: The mouse is moved while one or more mousebuttons are pressed.

Callback function form:

void motion_func(int x, int y);

Registration:

glutMotionFunc(motion_func);

GLUT Keyboard Event

Trigger: Any key is depressed.

Callback function form:

void keyboard_function(unsigned char key, int x, int y);

Registration:

glutKeyboardFunc(keyboard_function);

Other Defined Events in GLUT

glutPassiveMotionFuncglutVisibilityFuncglutEntryFuncglutSpecialFuncglutSpaceballMotionFuncglutSpaceballRotateFuncglutSpaceballButtonFuncglutButtonBoxFuncglutDialsFuncglutTabletMotionFuncglutTabletButtonFuncglutMenuStatusFunc

Example: Simple Square Drawing Program

• Open a window.• Clear it to black.• Draw a box at location of the mouse each time the left

button is clicked. Color of box should be randomly selected from RGB space.

• Clear window when resized.• Quit when right button is clicked.

Square Program Source Code Slide 1

/* This program illustrates the use of the glut library forinterfacing with a Window System */

/* The program opens a window, clears it to black,then draws a box at the location of the mouse each time theleft button is clicked. The right button exits the program

The program also reacts correctly when the window ismoved or resized by clearing the new window to black*/

#include <GL/gl.h>#include <GL/glut.h>

/* globals */

GLsizei wh = 500, ww = 500; /* initial window size */GLfloat size = 3.0; /* half side length of square */

Square Program Source Code Slide 2

void drawSquare(int x, int y){

y = wh-y; glColor3f( (char) random()%256, (char) random()%256, (char) random()%256); glBegin(GL_POLYGON); glVertex2f(x+size, y+size);

glVertex2f(x-size, y+size); glVertex2f(x-size, y-size); glVertex2f(x+size, y-size); glEnd(); glFlush();

}

Square Program Source Code Slide 3

void myReshape(GLsizei w, GLsizei h){/* adjust clipping box */ glMatrixMode(GL_PROJECTION); glLoadIdentity(); glOrtho(0.0, (GLdouble)w, 0.0, (GLdouble)h, -1.0, 1.0); glMatrixMode(GL_MODELVIEW); glLoadIdentity();

/* adjust viewport and clear */ glViewport(0,0,w,h); glClearColor (0.0, 0.0, 0.0, 0.0); glClear(GL_COLOR_BUFFER_BIT); glFlush();

/* set global size for use by drawing routine */ww = w;

wh = h; }

Square Program Source Code Slide 4

void myinit(void){ glViewport(0,0,ww,wh);

/* Pick 2D clipping window to match size of screen window This choice avoids having to scale object coordinateseach time window is resized */ glMatrixMode(GL_PROJECTION); glLoadIdentity(); glOrtho(0.0, (GLdouble) ww , 0.0, (GLdouble) wh , -1.0, 1.0);

/* set clear color to black and clear window */ glClearColor (0.0, 0.0, 0.0, 0.0);

glClear(GL_COLOR_BUFFER_BIT);glFlush();

/* callback routine for reshape event */ glutReshapeFunc(myReshape);}

Square Program Source Code Slide 5

void mouse(int btn, int state, int x, int y){ if(btn==GLUT_RIGHT_BUTTON&state==GLUT_DOWN) exit();}

int main(int argc, char** argv){ glutInit(&argc,argv);

glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);glutCreateWindow("square");

myinit ();

glutReshapeFunc (myReshape); glutMouseFunc (mouse); glutMotionFunc(drawSquare);

glutMainLoop();}

Output of “Square” program

Implementing Choice:Menus in GLUT

•Four steps: Create menu: glutCreateMenu(menu); Define menu entries: glutAddMenuEntry Attach menu to a mouse button: gluAttachMenu

Define callback function: void menu(int id);

Creating a Menu in GLUT

int glutCreateMenu(void (*func)(int value));

Creates a new pop-up menu.

Returns a unique integer identifier for the menu.

Takes as argument a pointer to a single callback functionthat takes an integer argument. The integer argument of the callback is mapped to the menu choice.

Sets the current menu to the newly created menu.

Menu Identifier Callback Function Choice

Associating a Menu with a Mouse Key

void glutAttachMenu(int button);

Associates the selected button with the current menu.

button is selected from the GLUT defined button constants:

GLUT_LEFT_BUTTON, GLUT_MIDDLE_BUTTON, GLUT_RIGHT_BUTTON

Adding Entries to the Menu

void glutAddMenuEntry(char *name, int value);

String to appearin menu entry.

Value to be passedto callback function.

Adds a menu entry to the bottom of the current menu.

Building a Sub-menu

void glutAddSubMenu(char *name, int menu);

String to display in the menuitem from which to cascade sub-menu.

Identifier of menu to cascadefrom this sub-menu item.

Double Buffering

• Instead of one color buffer, we use two Front Buffer: one that is displayed but not written to Back Buffer: one that is written to but not displayed

• Program then requests a double buffer in main.c glutInitDisplayMode(GL_RGB | GL_DOUBLE) At the end of the display callback buffers are swapped

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void mydisplay(){

glClear(GL_COLOR_BUFFER_BIT|….)./* draw graphics here */.

glutSwapBuffers()}

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Using the idle callback

• The idle callback is executed whenever there are no events in the event queue glutIdleFunc(myidle) Useful for animations

void myidle() {/* change something */

t += dtglutPostRedisplay();

}

Void mydisplay() {glClear();

/* draw something that depends on t */glutSwapBuffers();

}

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Using globals

• The form of all GLUT callbacks is 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}