Input and Interaction Chapter 3. CS 480/680 2Chapter 3 -- Input and Interaction Introduction:...

Input and Input and InteractionInteraction

Chapter 3Chapter 3

CS 480/680 2Chapter 3 -- Input and Interaction

Introduction:Introduction: We now turn to the development of We now turn to the development of

interactive graphics programs.interactive graphics programs.

Our discussion has three main partsOur discussion has three main parts First, we consider the variety of devices available First, we consider the variety of devices available

for interactionfor interaction We then consider client-server networks and We then consider client-server networks and

client-server graphicsclient-server graphics Finally we develop a paint program that Finally we develop a paint program that

demonstrates the important features of demonstrates the important features of interactive graphics programming.interactive graphics programming.


1. Interaction1. Interaction One of the most important advances in One of the most important advances in

computer technology was enabling users to computer technology was enabling users to interact with computer displaysinteract with computer displays

Ivan Sutherland’s Ivan Sutherland’s Project SketchpadProject Sketchpad launched the present era of interactive launched the present era of interactive computer graphicscomputer graphics

Since 1963 when this work was published there Since 1963 when this work was published there have been many advances in both hardware and have been many advances in both hardware and software, but the viewpoint ideas he introduced software, but the viewpoint ideas he introduced still dominate computer graphics.still dominate computer graphics.


OpenGL does not support interaction directlyOpenGL does not support interaction directly windowing and input functions were left out of the windowing and input functions were left out of the

API for portability reasons.API for portability reasons. We can avoid potential difficulties by using a We can avoid potential difficulties by using a

simple library, or toolkit, as we did in Chapter simple library, or toolkit, as we did in Chapter 2.2.

Our use of the GLUT toolkit will enable us to avoid Our use of the GLUT toolkit will enable us to avoid the complexities inherent in the interactions among the complexities inherent in the interactions among the windowing systemthe windowing system

Our outline for this chapter looks like this:Our outline for this chapter looks like this: We start by describing several interactive devices We start by describing several interactive devices

and how to interact with themand how to interact with them We then put these devices in the setting of a client-We then put these devices in the setting of a client-

server network.server network. Then, we introduce an API for minimal interaction.Then, we introduce an API for minimal interaction. Finally, we shall generate sample programs.Finally, we shall generate sample programs.


2. Input Devices2. Input Devices

From the physical perspective, each From the physical perspective, each device has properties that make it more device has properties that make it more suitable for certain tasks than for suitable for certain tasks than for others.others.

There are two primary types of physical There are two primary types of physical devices:devices: Pointing DevicesPointing Devices Keyboard DevicesKeyboard Devices


2.1 Physical Input Devices2.1 Physical Input Devices

The The pointing devicepointing device allows the user to allows the user to indicate a position on the screen, indicate a position on the screen,

and almost always incorporates one or more and almost always incorporates one or more buttonsbuttons

The The keyboard devicekeyboard device is almost always a is almost always a physical keyboard, physical keyboard,

but can be generalized to include any device but can be generalized to include any device that returns character codes to a programthat returns character codes to a program


Pointing devices can be broken into two Pointing devices can be broken into two subcategoriessubcategories

relative-position devicesrelative-position devices

absolute- position devicesabsolute- position devices


Other Pointing DevicesOther Pointing Devices JoystickJoystick

SpaceBallsSpaceBalls


2.2 Logical Devices2.2 Logical Devices Two major characteristics describe the Two major characteristics describe the

logical behavior of an input devicelogical behavior of an input device What measurements the device returns to What measurements the device returns to

the user programthe user program When the device returns those When the device returns those

measurements.measurements.

Some API’s such as PHIGS and GKS Some API’s such as PHIGS and GKS consider six classes of logical input consider six classes of logical input devices.devices.

OpenGL does not follow this approach, but OpenGL does not follow this approach, but we will look at these classes briefly and see we will look at these classes briefly and see how OpenGL provides similar functionalityhow OpenGL provides similar functionality


1. String1. String a device that provides ASCII strings to the a device that provides ASCII strings to the

user program.user program. Most windowing systems and OpenGL do Most windowing systems and OpenGL do

not distinguish between a logical string not distinguish between a logical string device and the keyboard.device and the keyboard.

2. Locator2. Locator a device that provides a position in world a device that provides a position in world

coordinates to the user program. coordinates to the user program. It is usually implemented via a pointing device, It is usually implemented via a pointing device,

such as a mouse or a trackball.such as a mouse or a trackball. In OpenGL we usually have to do the In OpenGL we usually have to do the

conversion from screen coordinates to conversion from screen coordinates to world coordinates within our own world coordinates within our own programsprograms


3. Pick3. Pick a device that returns the identifier of an a device that returns the identifier of an

object to the user program.object to the user program. It is usually implemented with the same It is usually implemented with the same

device as the locator, but has a separate device as the locator, but has a separate software interfacesoftware interface

In OpenGL, we can use a process called In OpenGL, we can use a process called selectionselection to accomplish picking. to accomplish picking.

4. Choice4. Choice a device that allows the user to select a device that allows the user to select

one of a discrete number of options.one of a discrete number of options. In OpenGL we can use various widgets In OpenGL we can use various widgets

provided by the windowing system.provided by the windowing system.


5. Dial5. Dial Dials provide analog input to the user Dials provide analog input to the user

programprogram Here again widgets provide this facility Here again widgets provide this facility

through graphical devices, such as slidebarsthrough graphical devices, such as slidebars

6. Stroke6. Stroke A device that returns an array of locations.A device that returns an array of locations. Although we can think of a stroke as similar Although we can think of a stroke as similar

to multiple locators, it is often implemented to multiple locators, it is often implemented such that an action, such as pushing down a such that an action, such as pushing down a mouse button, starts the transfer of data into mouse button, starts the transfer of data into the specified array, and a second action, the specified array, and a second action, such as the releasing of the button, ends the such as the releasing of the button, ends the transfertransfer


2.3 Measure and Trigger2.3 Measure and Trigger The manner by which physical and The manner by which physical and

logical input devices provide input to logical input devices provide input to an application program can be an application program can be described in terms of two entities:described in terms of two entities: Measure -- what is returnedMeasure -- what is returned Trigger -- when it is returnedTrigger -- when it is returned

Measure can also include status Measure can also include status informationinformation


2.4 Input Modes2.4 Input Modes We can obtain the measure of a We can obtain the measure of a

device in three distinct modes.device in three distinct modes. Each mode is defined by the relationship Each mode is defined by the relationship

between the measure process and the between the measure process and the trigger.trigger.

The three modes are:The three modes are: Request ModeRequest Mode Sample-ModeSample-Mode Event ModeEvent Mode


Request Mode:Request Mode: The measure of the device is not The measure of the device is not

returned to the program until the device returned to the program until the device is triggered.is triggered.

This mode is standard in non-graphical This mode is standard in non-graphical applications such as typical C programsapplications such as typical C programs

input is buffered until the trigger is hitinput is buffered until the trigger is hit for scanf( ) the trigger is the “enter” key.for scanf( ) the trigger is the “enter” key.


Sample Mode:Sample Mode: The input is immediateThe input is immediate As soon as the function call in the user As soon as the function call in the user

program is encountered, the measure is program is encountered, the measure is returned.returned.

Both request mode and sample mode are Both request mode and sample mode are useful for situations where the program useful for situations where the program guides the user, but are not useful in guides the user, but are not useful in applications where the user controls the flow applications where the user controls the flow of the program. (Flight Simulator)of the program. (Flight Simulator)


Event Mode:Event Mode: Each time a device is triggered, an event is Each time a device is triggered, an event is

generated and placed into an event queue. generated and placed into an event queue. From here there are two approachesFrom here there are two approaches

1) The program can then get events off 1) The program can then get events off the queue, and “service” them.the queue, and “service” them.

2) associate a “callback” function with a 2) associate a “callback” function with a specific type of event and when that event specific type of event and when that event comes to the front of the queue, that comes to the front of the queue, that function is automatically executed.function is automatically executed.


3. Clients and Servers3. Clients and Servers Def: ServerDef: Server

A Server is something A Server is something that can perform tasks for that can perform tasks for clients.clients.

Clients and Servers can Clients and Servers can be distributed over a be distributed over a network, or contained network, or contained entirely within a entirely within a computational unit.computational unit. OpenGL programs are OpenGL programs are

clients that use the clients that use the graphics servergraphics server


4. Display Lists4. Display Lists

We saw in Chapter 1 the history of We saw in Chapter 1 the history of graphics hardwaregraphics hardware


Today:Today: The display processor has become the The display processor has become the

graphics server, and the user program graphics server, and the user program has become a client.has become a client.

The issue is not the refresh rate, but the The issue is not the refresh rate, but the amount of traffic between the client and amount of traffic between the client and the serverthe server

Now we can send graphical entities to a Now we can send graphical entities to a display in one of two ways:display in one of two ways:

immediate modeimmediate mode retained-moderetained-mode


Immediate Mode:Immediate Mode: As soon as the program executes a As soon as the program executes a

statement that defines a primitive, that statement that defines a primitive, that primitive is sent to the server for display primitive is sent to the server for display and no memory of it is retained.and no memory of it is retained.

To redisplay it, you must redefine it and To redisplay it, you must redefine it and resend itresend it

For complex objects in highly interactive For complex objects in highly interactive applications, this process can cause applications, this process can cause considerable quantity of data to pass considerable quantity of data to pass from the client to the serverfrom the client to the server


Retained Mode:Retained Mode: We define an object once,We define an object once,

then put its description into a display listthen put its description into a display list

The display list is stored in the server and The display list is stored in the server and redisplayed by a simple function call redisplayed by a simple function call issued from the client to the server.issued from the client to the server.

Advantages:Advantages: makes good use of hardware makes good use of hardware

(compute and graphics)(compute and graphics)

Disadvantages:Disadvantages: memory on the servermemory on the server overhead of creating the listsoverhead of creating the lists


4.1 4.1 Definition and Execution of Display Definition and Execution of Display ListsLists

Display lists have much in common with Display lists have much in common with ordinary files.ordinary files.

There must be a mechanism to define/create There must be a mechanism to define/create them as well as and add-to/manipulate them.them as well as and add-to/manipulate them.

OpenGL has a small set of functions to OpenGL has a small set of functions to manipulate display lists and placed only a few manipulate display lists and placed only a few restrictions on display-list contents.restrictions on display-list contents.

We will look at several examples to show their We will look at several examples to show their use.use.


Display lists are defined similarly to Display lists are defined similarly to geometric primitivesgeometric primitives

Each display list must have a unique identifier Each display list must have a unique identifier

#define BOX 1#define BOX 1

// GL_COMPILE // GL_COMPILE sayssays

glNewList(BOX, GL_COMPILE); // send to server glNewList(BOX, GL_COMPILE); // send to server

glBegin(GL_POLYGON); // but don’t displayglBegin(GL_POLYGON); // but don’t display

glColor3f(1.0,0.0,0.0);glColor3f(1.0,0.0,0.0);

glVertex2f(-1.0, -1.0);glVertex2f(-1.0, -1.0);




glEnd();glEnd();

glEndList();glEndList();


If we want an immediate display of the contents, If we want an immediate display of the contents, we can use the flagwe can use the flag

GL_COMPIL_AND_EXECUTEGL_COMPIL_AND_EXECUTE

Each time that we wish to draw the box on the Each time that we wish to draw the box on the server, we execute the following:server, we execute the following:

glCallList(BOX);glCallList(BOX);

Because the call to set the drawing color to Red Because the call to set the drawing color to Red will still be in effect, you usually see display lists will still be in effect, you usually see display lists with the following at the beginning:with the following at the beginning:

glPushAttrib(GL_ALL_ATTRIB_BITS);glPushAttrib(GL_ALL_ATTRIB_BITS); glPushMatrix();glPushMatrix();

and the following at the endand the following at the end glPopAttrib();glPopAttrib(); glPopMatrix();glPopMatrix();


5. 5. Programming Event-Programming Event-Driven InputDriven Input

In this section, we develop event-driven In this section, we develop event-driven input through a number of simple examples input through a number of simple examples that use the callback mechanism we that use the callback mechanism we introduced earlier.introduced earlier.

We will examine various events that are We will examine various events that are recognized by the window system and write recognized by the window system and write callback functions that govern how the callback functions that govern how the application responds to those events.application responds to those events.


5.1 Using the Pointing Device5.1 Using the Pointing Device Two types of events are associated Two types of events are associated

with the pointing devicewith the pointing device A move eventA move event

this is generated if the mouse is moved with this is generated if the mouse is moved with one of the buttons depressedone of the buttons depressed

A passive eventA passive event this is generated if the mouse is moved this is generated if the mouse is moved

without a button being held down.without a button being held down.

A mouse event occurs when one of A mouse event occurs when one of the mouse buttons is either the mouse buttons is either depressed or releaseddepressed or released


We specify the mouse callback function in main We specify the mouse callback function in main with the GLUT callwith the GLUT call

glutMouseFunc(mouse_callback_func);glutMouseFunc(mouse_callback_func);

The mouse callback function must have the formThe mouse callback function must have the form void mouse_callback_func(int button, int state, int x, int void mouse_callback_func(int button, int state, int x, int

y);y);

The code for this would look likeThe code for this would look likevoid mouse_callback_function(int button, int state, int x, void mouse_callback_function(int button, int state, int x,

int y)int y)

{{

if(button==GLUT_LEFT_BUTTON && if(button==GLUT_LEFT_BUTTON &&

state==GLUT_DOWN)state==GLUT_DOWN)

exit();exit();

}}


Note:Note: with this code, if any other mouse event had with this code, if any other mouse event had

occurred, no response action would occur occurred, no response action would occur because no callbacks corresponding to the because no callbacks corresponding to the events has been defined (or registered).events has been defined (or registered).

Our next example Our next example is going to draw a box at the mouse location is going to draw a box at the mouse location

when the left button is pushedwhen the left button is pushed and terminate when the middle button is and terminate when the middle button is

pushed.pushed.


int main(int argc, char **argv)int main(int argc, char **argv)

{{

glutInit(&argc, argv);glutInit(&argc, argv);

glutInitDisplay(GLUT_SINGLE | glutInitDisplay(GLUT_SINGLE | GLUT_RGB);GLUT_RGB);

glutCreateWindow(“square”);glutCreateWindow(“square”);

myinit();myinit();

glutReshapeFunc(myReshape);glutReshapeFunc(myReshape);

glutMouseFunc(mouse);glutMouseFunc(mouse);

glutDisplayFunc(display);glutDisplayFunc(display);

glutMainLoop( );glutMainLoop( );

}}


Reshape eventReshape event is generated whenever the window is is generated whenever the window is

resized.resized. This is typically from user interaction.This is typically from user interaction. We will discuss this later.We will discuss this later.

DisplayDisplay we don’t need the display function since we don’t need the display function since

things are only drawn when the mouse things are only drawn when the mouse button is pushed, button is pushed,

but it is required by GLUT, so our code will but it is required by GLUT, so our code will be:be:

void display ( ) { }void display ( ) { }


MouseMouse

void mouse(int btn, int state, int x, int y)void mouse(int btn, int state, int x, int y)

{{

if(btn==GLUT_LEFT_BUTTON && if(btn==GLUT_LEFT_BUTTON &&

state=GLUT_DOWN) state=GLUT_DOWN)

draw_square(x,y);draw_square(x,y);

if(btn==GLUT_MIDDLE_BUTTON &&if(btn==GLUT_MIDDLE_BUTTON &&

state==GLUT_DOWN) state==GLUT_DOWN)

exit( );exit( );

}}


Glsizei wh = 500, ww = 500;Glsizei wh = 500, ww = 500;

Glfloat size = 3.0;Glfloat size = 3.0;

void myinit(void)void myinit(void)

{{

glViewport(0,0,ww,wh);glViewport(0,0,ww,wh);

glMatrixMode(GL_PROJECTION);glMatrixMode(GL_PROJECTION);

glLoadIdentity( );glLoadIdentity( );

gluOrtho2D(0.0, (Gldouble)ww, 0.0, (Gldouble)wh);gluOrtho2D(0.0, (Gldouble)ww, 0.0, (Gldouble)wh);

glMatrixMode(GL_MODELVIEW);glMatrixMode(GL_MODELVIEW);

glClearColor(0.0,0.0,0.0,0.0);glClearColor(0.0,0.0,0.0,0.0);

glClear(GL_COLOR_BUFFER_BIT);glClear(GL_COLOR_BUFFER_BIT);

glFlush( );glFlush( );

}}


void drawSquare(int x, int y)void drawSquare(int x, int y)

{{

y=wh-y;y=wh-y;

glColor3ub( (char)rand()%256, (char)rand()%256,glColor3ub( (char)rand()%256, (char)rand()%256,

(char)rand()%256); (char)rand()%256);

glBegin(GL_POLYGON);glBegin(GL_POLYGON);

glVertex2f( x+size, y+size);glVertex2f( x+size, y+size);

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

glVertex2f( x-size, y-size);glVertex2f( x-size, y-size);

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

glEnd( );glEnd( );


}}

Note: The position returned by the mouse is in the Note: The position returned by the mouse is in the window’s system (origin is in the top left), hence we have window’s system (origin is in the top left), hence we have to flip itto flip it


5.2 Window Events5.2 Window Events If the window size changes, we have If the window size changes, we have

to consider three questions:to consider three questions: 1. Do we redraw all the objects that were 1. Do we redraw all the objects that were

in the window before it was resized?in the window before it was resized?

2. What do we do if the aspect ratio of the 2. What do we do if the aspect ratio of the new window is different from that of the new window is different from that of the old window?old window?

3. Do we change the size or attributes of 3. Do we change the size or attributes of new primitives if the size of the new new primitives if the size of the new window is different from that of the old?window is different from that of the old?


In our example:In our example: We ensure that squares of the same size We ensure that squares of the same size

are drawn, regardless of the size or are drawn, regardless of the size or shape of the window.shape of the window.

Therefore, we clear the screen each time it is Therefore, we clear the screen each time it is resized and use the entire new window as resized and use the entire new window as our drawing area.our drawing area.

The reshape event returns the height The reshape event returns the height and width of the new windowand width of the new window

We use these to create a new OpenGL We use these to create a new OpenGL clipping window using gluOrtho2D,clipping window using gluOrtho2D,

As well as a new viewport with the same As well as a new viewport with the same aspect ratio,aspect ratio,

We then clear the window to black.We then clear the window to black.


void myReshape(Glsizei w, Glsizei h)void myReshape(Glsizei w, Glsizei h)

{{

glMatrixMode(GL_PROJECTION);glMatrixMode(GL_PROJECTION);


gluOrtho2D(0.0, (Gldouble)ww, 0.0, gluOrtho2D(0.0, (Gldouble)ww, 0.0, (Gldouble)wh);(Gldouble)wh);

glMatrixMode(GL_MODELVIEW); glMatrixMode(GL_MODELVIEW);


glViewport(0,0,w,h);glViewport(0,0,w,h);

glClearColor(0.0,0.0,0.0,0.0);glClearColor(0.0,0.0,0.0,0.0);

glClear(GL_COLOR_BUFFER_BIT);glClear(GL_COLOR_BUFFER_BIT);


}}


5.3 Keyboard Events5.3 Keyboard Events Keyboard events are generated when the Keyboard events are generated when the

mouse is in the window and one of the mouse is in the window and one of the keys is depressed.keys is depressed.

GLUT ignores key releasesGLUT ignores key releases

All the keyboard callbacks are registered in a All the keyboard callbacks are registered in a single callback function such as:single callback function such as:

glutKeyboardFunc(keyboard);glutKeyboardFunc(keyboard);

For example:For example: void keyboard(unsigned char key, int x, int void keyboard(unsigned char key, int x, int

y)y) {{ if(key==‘q’ || key ==‘Q’) exit();if(key==‘q’ || key ==‘Q’) exit(); }}


5.4 The Display and Idle Callbacks5.4 The Display and Idle Callbacks glutDisplayFunc(display); glutDisplayFunc(display);

display is invoked when GLUT determines display is invoked when GLUT determines that the window should be redisplayedthat the window should be redisplayed

window initially opened, window initially opened, We can also tell GLUT to call it withWe can also tell GLUT to call it with

glutPostDisplay();glutPostDisplay();

idle idle The idle callback is invoked when there are The idle callback is invoked when there are

no other events.no other events. Its default is the null function.Its default is the null function.


5.5 Window Management5.5 Window Management GLUT also supports both multiple windows GLUT also supports both multiple windows

and subwindowsand subwindows

We can open a second top-level window byWe can open a second top-level window by id=glutCreateWindow(“second window”);id=glutCreateWindow(“second window”);

The id allows us to select which window to work in The id allows us to select which window to work in byby

glutSetWindow(id);glutSetWindow(id);

We can make this window have different We can make this window have different properties by invoking the glutInitDisplayMode properties by invoking the glutInitDisplayMode before glutCreateWindow.before glutCreateWindow.

And each window can have its own set of callback And each window can have its own set of callback functions, because callback specifications refer to functions, because callback specifications refer to the present window.the present window.


6. Menus6. Menus

GLUT provides pop-up menus that we GLUT provides pop-up menus that we can use with the mouse to create can use with the mouse to create sophisticated interactive applications.sophisticated interactive applications.

Using the menus involves taking a few Using the menus involves taking a few simple stepssimple steps define the entriesdefine the entries link the menu to a mouse buttonlink the menu to a mouse button define a callback for each menu entry.define a callback for each menu entry.


glutCreateMenu(demo_menu);glutCreateMenu(demo_menu);

glutAddMenuEntry(“quit”,1);glutAddMenuEntry(“quit”,1);

glutAddMenuEntry(“increase square size”,2);glutAddMenuEntry(“increase square size”,2);

glutAddMenuEntry(“decrease square size”,3);glutAddMenuEntry(“decrease square size”,3);

glutAttachMenu(GLUT_RIGHT_BUTTON);glutAttachMenu(GLUT_RIGHT_BUTTON);

Our callback function is:Our callback function is:void demo_menu(int id)void demo_menu(int id)

{{

if(id==1) exit( );if(id==1) exit( );

else if(id==2)size = 2*size;else if(id==2)size = 2*size;

else if (size>1) size = size/2;else if (size>1) size = size/2;

glutPostRedisplay( );glutPostRedisplay( );

}}


GLUT also supports hierarchical menusGLUT also supports hierarchical menus

sub_menu = glutCreateMenu(size_menu);sub_menu = glutCreateMenu(size_menu);

glutAddMenuEntry(“increase square size”,2);glutAddMenuEntry(“increase square size”,2);

glutAddMenuEntry(“decrease square size”,3);glutAddMenuEntry(“decrease square size”,3);

glutCreateMenu(top_menu);glutCreateMenu(top_menu);

glutAddMenuEntry(“quit”,1);glutAddMenuEntry(“quit”,1);

glutAddSubMenu(“Resize”, sub_menu);glutAddSubMenu(“Resize”, sub_menu);

glutAttachMenu(GLUTRIGHTBUTTON);glutAttachMenu(GLUTRIGHTBUTTON);


7. Picking7. Picking

Picking is an input operation that allows Picking is an input operation that allows the user to identify an object on the the user to identify an object on the display.display.

Although the action uses the pointing device, Although the action uses the pointing device, the information the user wants is not a the information the user wants is not a position.position.

A pick device is considerably more difficult to A pick device is considerably more difficult to implement on a modern system than is a implement on a modern system than is a locatorlocator

The text gives some ideasThe text gives some ideas


8. A Simple Paint 8. A Simple Paint ProgramProgram

Any paint program should demonstrate Any paint program should demonstrate most of the following features:most of the following features: ability to work with geometric objectsability to work with geometric objects ability to manipulate pixelsability to manipulate pixels provide control of attributes (color, ...)provide control of attributes (color, ...) include menus for controlling the include menus for controlling the

applicationapplication behave correctly when the window is moved behave correctly when the window is moved

or resized.or resized.


9. Animating Interactive 9. Animating Interactive ProgramsPrograms

So far our programs have been staticSo far our programs have been static Once a primitive was placed on the display Once a primitive was placed on the display

its image did not change until the screen its image did not change until the screen was cleared.was cleared.

Suppose that we want to create a Suppose that we want to create a picture in which one or more objects picture in which one or more objects are changing or moving, and thus their are changing or moving, and thus their images must change.images must change.


9.1 The Rotating Square9.1 The Rotating Square Consider the rotating two dimensional Consider the rotating two dimensional

point wherepoint where x = cos x = cos ,, y = sin y = sin


void display( )void display( )

{{

glClear( );glClear( );

glBegin(GL_POLYGON); glBegin(GL_POLYGON);

thetar = theta/((2*3.14159)/360.0);thetar = theta/((2*3.14159)/360.0);

glVertex2f(cos(thetar), sin(thetar));glVertex2f(cos(thetar), sin(thetar));

glVertex2f(sin(thetar), cos(thetar));glVertex2f(sin(thetar), cos(thetar));

glVertex2f(-cos(thetar), -sin(thetar));glVertex2f(-cos(thetar), -sin(thetar));

glVertex2f(sin(thetar), -cos(thetar));glVertex2f(sin(thetar), -cos(thetar));

glEnd( );glEnd( );

}}


Now suppose we want to increase theta by Now suppose we want to increase theta by a fixed amount whenever nothing else is a fixed amount whenever nothing else is happening.happening.

glutIdleFunc(idle);glutIdleFunc(idle);

void idle( )void idle( )

{{

theta+=2;theta+=2;

if(theta >=360.0) theta-=360.0;if(theta >=360.0) theta-=360.0;

glutPostRedisplay( );glutPostRedisplay( );

}}


So, what would this do?So, what would this do?

glutMouseFunc(mouse);glutMouseFunc(mouse);

void mouse(int button, int state, int x, int y)void mouse(int button, int state, int x, int y)

{{

if(button==GLUT_LEFT && if(button==GLUT_LEFT && state==GLUT_DOWN) state==GLUT_DOWN)

glutIdleFunc(idle);glutIdleFunc(idle);

if(button==GLUT_MIDDLE && if(button==GLUT_MIDDLE && state==GLUT_DOWN) state==GLUT_DOWN)

glutIdleFunc(NULL);glutIdleFunc(NULL);

}}


9.2 Double Buffering9.2 Double Buffering

Why do we want double buffering?Why do we want double buffering?

In main we request a double buffered In main we request a double buffered display by:display by:

glutInitDisplayMode(GLUT_RGB | glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE)GLUT_DOUBLE)

After we have finished drawing, we call After we have finished drawing, we call glutSwapBuffers( );glutSwapBuffers( );


9.3 Other Buffering 9.3 Other Buffering ProblemsProblems Suppose we want to Suppose we want to

add an elapsed time add an elapsed time clockclock single buffer causes single buffer causes

clock to flickerclock to flicker double buffer causes double buffer causes

drawing to flickerdrawing to flicker answer is to draw answer is to draw

the items to both the items to both buffers buffers

See Chapter 9 for See Chapter 9 for detailsdetails


10. Design of Interactive 10. Design of Interactive ProgramsPrograms

A good interactive program should A good interactive program should have:have:

A smooth display (no flicker or artifacts of refresh)A smooth display (no flicker or artifacts of refresh) A variety of interactive devices on the displayA variety of interactive devices on the display A variety of methods for entering and displaying A variety of methods for entering and displaying

informationinformation An easy to use interface that does not require An easy to use interface that does not require

substantial effort to learn.substantial effort to learn. Feedback to the user.Feedback to the user. Tolerance for user errorsTolerance for user errors A design that incorporates consideration of both A design that incorporates consideration of both

visual and motor properties of humans.visual and motor properties of humans.


10.1 10.1 Toolkits, Widgets, and the Frame Toolkits, Widgets, and the Frame BufferBuffer

The paint program used interactive tools, The paint program used interactive tools, such as pop-up menus that were provided such as pop-up menus that were provided by GLUT.by GLUT.

There are many more possibilities, such as There are many more possibilities, such as slide-bars, dials, hot areas on the screen, slide-bars, dials, hot areas on the screen, sound, and iconssound, and icons

Usually these tools are supplied with various Usually these tools are supplied with various toolkits, although there is nothing to prevent you toolkits, although there is nothing to prevent you from writing your own.from writing your own.

Designing our own widget set will be much easier Designing our own widget set will be much easier after pixel operations are introduced in Chapter 9after pixel operations are introduced in Chapter 9

most of these operations are described in terms most of these operations are described in terms of the contents of the frame buffer of the contents of the frame buffer

bit-block transfer operations (bitblt)bit-block transfer operations (bitblt)


Rubberbanding is one Rubberbanding is one of those techniquesof those techniques

Note that before each Note that before each new line segment is new line segment is drawn, the previous drawn, the previous line segment must be line segment must be erased.erased.

Usually the Usually the rubberbanding begins rubberbanding begins when the mouse when the mouse button is depressed button is depressed and continues until the and continues until the button is released.button is released.

Other objects that are Other objects that are drawn interactively drawn interactively with rubberbanding with rubberbanding include rectangles and include rectangles and circles.circles.


12. Summary12. Summary In this chapter, we have touched on a In this chapter, we have touched on a

number of topics related to interactive number of topics related to interactive computer graphics computer graphics

These interactive aspects make the field exciting These interactive aspects make the field exciting and fun.and fun.

We handled simple interaction using GLUTWe handled simple interaction using GLUT We have been heavily influenced by the We have been heavily influenced by the

client-server perspectiveclient-server perspective This will be crucial in Chapter 8This will be crucial in Chapter 8 The overhead of setting up a program to run in The overhead of setting up a program to run in

this environment is small. Each program is this environment is small. Each program is structured the same.structured the same.


13. Suggested Readings13. Suggested Readings

Sutherland’s Sutherland’s Project SketchpadProject Sketchpad Xerox PARC Research of the 70’sXerox PARC Research of the 70’s

windows-icons-menus-pointing interfaceswindows-icons-menus-pointing interfaces Human Computer InterfaceHuman Computer Interface

Foley, Van Dam, et.alFoley, Van Dam, et.al UI ToolkitsUI Toolkits

GLUT, GLUT, Scripting ones -- tcl/tkScripting ones -- tcl/tk ... ...

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