Download The display callback

Programming with OpenGL
Part 3: Three Dimensions
Ed Angel
Professor of Computer Science,
Electrical and Computer
Engineering, and Media Arts
University of New Mexico
Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005
1
Objectives
• Develop a more sophisticated threedimensional example
- Sierpinski gasket: a fractal
• Introduce hidden-surface removal
Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005
2
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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3
Objectives
• Learn to build interactive programs using
GLUT callbacks
- Mouse
- Keyboard
- Reshape
• Introduce menus in GLUT
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4
Objectives
• Learn to build more sophisticated
interactive programs using
- Picking
• Select objects from the display
• Three methods
- Rubberbanding
• Interactive drawing of lines and rectangles
- Display Lists
• Retained mode graphics
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5
Three-dimensional Applications
• In OpenGL, two-dimensional applications
are a special case of three-dimensional
graphics
• Going to 3D
- Not much changes
- Use glVertex3*( )
- Have to worry about the order in which
polygons are drawn or use hidden-surface
removal
- Polygons should be simple, convex, flat
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6
Sierpinski Gasket (2D)
• Start with a triangle
• Connect bisectors of sides and remove central
triangle
• Repeat
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Example
• Five subdivisions
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8
The gasket as a fractal
• Consider the filled area (black) and the
perimeter (the length of all the lines around
the filled triangles)
• As we continue subdividing
- the area goes to zero
- but the perimeter goes to infinity
• This is not an ordinary geometric object
- It is neither two- nor three-dimensional
• It is a fractal (fractional dimension) object
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9
Gasket Program
#include <GL/glut.h>
/* initial triangle */
GLfloat v[3][2]={{-1.0, -0.58},
{1.0, -0.58}, {0.0, 1.15}};
int n; /* number of recursive steps */
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Draw one triangle
void triangle( GLfloat *a, GLfloat *b,
GLfloat *c)
/* display one triangle
{
glVertex2fv(a);
glVertex2fv(b);
glVertex2fv(c);
}
*/
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11
Triangle Subdivision
void divide_triangle(GLfloat *a, GLfloat *b, GLfloat *c,
int m)
{
/* triangle subdivision using vertex numbers */
point2 v0, v1, v2;
int j;
if(m>0)
{
for(j=0; j<2; j++) v0[j]=(a[j]+b[j])/2;
for(j=0; j<2; j++) v1[j]=(a[j]+c[j])/2;
for(j=0; j<2; j++) v2[j]=(b[j]+c[j])/2;
divide_triangle(a, v0, v1, m-1);
divide_triangle(c, v1, v2, m-1);
divide_triangle(b, v2, v0, m-1);
}
else(triangle(a,b,c));
/* draw triangle at end of recursion */
}
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12
display and init Functions
void display()
{
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_TRIANGLES);
divide_triangle(v[0], v[1], v[2], n);
glEnd();
glFlush();
}
void myinit()
{
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-2.0, 2.0, -2.0, 2.0);
glMatrixMode(GL_MODELVIEW);
glClearColor (1.0, 1.0, 1.0,1.0)
glColor3f(0.0,0.0,0.0);
}
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13
main Function
int main(int argc, char **argv)
{
n=4;
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowSize(500, 500);
glutCreateWindow(“2D Gasket");
glutDisplayFunc(display);
myinit();
glutMainLoop();
}
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14
Efficiency Note
By having the glBegin and glEnd in the
display callback rather than in the function
triangle and using GL_TRIANGLES
rather than GL_POLYGON in glBegin,
we call glBegin and glEnd only once for
the entire gasket rather than once for
each triangle
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15
Moving to 3D
• We can easily make the program threedimensional by using
GLfloat v[3][3]
glVertex3f
glOrtho
• But that would not be very interesting
• Instead, we can start with a tetrahedron
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3D Gasket
• We can subdivide each of the four faces
• Appears as if we remove a solid
tetrahedron from the center leaving four
smaller tetrahedra
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Example
after 5 iterations
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triangle code
void triangle( GLfloat *a, GLfloat *b,
GLfloat *c)
{
glVertex3fv(a);
glVertex3fv(b);
glVertex3fv(c);
}
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subdivision code
void divide_triangle(GLfloat *a, GLfloat *b,
GLfloat *c, int m)
{
GLfloat v1[3], v2[3], v3[3];
int j;
if(m>0)
{
for(j=0; j<3; j++) v1[j]=(a[j]+b[j])/2;
for(j=0; j<3; j++) v2[j]=(a[j]+c[j])/2;
for(j=0; j<3; j++) v3[j]=(b[j]+c[j])/2;
divide_triangle(a, v1, v2, m-1);
divide_triangle(c, v2, v3, m-1);
divide_triangle(b, v3, v1, m-1);
}
else(triangle(a,b,c));
}
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tetrahedron code
void tetrahedron( int m)
{
glColor3f(1.0,0.0,0.0);
divide_triangle(v[0], v[1],
glColor3f(0.0,1.0,0.0);
divide_triangle(v[3], v[2],
glColor3f(0.0,0.0,1.0);
divide_triangle(v[0], v[3],
glColor3f(0.0,0.0,0.0);
divide_triangle(v[0], v[2],
}
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v[2], m);
v[1], m);
v[1], m);
v[3], m);
21
Almost Correct
• Because the triangles are drawn in the order
they are defined in the program, the front
triangles are not always rendered in front of
triangles behind them
get this
want this
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22
Hidden-Surface Removal
• We want to see only those surfaces in front of
other surfaces
• OpenGL uses a hidden-surface method called
the z-buffer algorithm that saves depth
information as objects are rendered so that only
the front objects appear in the image
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23
Using the z-buffer algorithm
• The algorithm uses an extra buffer, the z-buffer, to
store depth information as geometry travels down the
pipeline
• It must be
- Requested in main.c
• glutInitDisplayMode
(GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH)
- Enabled in init.c
• glEnable(GL_DEPTH_TEST)
- Cleared in the display callback
• glClear(GL_COLOR_BUFFER_BIT |
GL_DEPTH_BUFFER_BIT)
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Surface vs Volume Subdvision
• In our example, we divided the surface of
each face
• We could also divide the volume using the
same midpoints
• The midpoints define four smaller
tetrahedrons, one for each vertex
• Keeping only these tetrahedrons removes
a volume in the middle
• See text for code
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Volume Subdivision
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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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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
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Physical Devices
mouse
data tablet
trackball
joy stick
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light pen
space ball
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Incremental (Relative) Devices
• Devices such as the data tablet return a
position directly to the operating system
• Devices such as the mouse, trackball, and
joy stick return incremental inputs (or
velocities) to the operating system
- Must integrate these inputs to obtain an
absolute position
•
•
•
•
Rotation of cylinders in mouse
Roll of trackball
Difficult to obtain absolute position
Can get variable sensitivity
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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
• 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 (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
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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
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Input Modes
• 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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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
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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 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)
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
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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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The display callback
• The display callback is executed whenever
GLUT determines that the window should be
refreshed, for example
-
When the window is first opened
When the window is reshaped
When a window is exposed
When the user program decides it wants to change the
display
• In main.c
-glutDisplayFunc(mydisplay) identifies the
function to be executed
- Every GLUT program must have a display callback
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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 using
glutPostRedisplay();
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
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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
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 += dt
glutPostRedisplay();
}
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
}
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The mouse callback
glutMouseFunc(mymouse)
void mymouse(GLint button, GLint
state, GLint x, GLint y)
• Returns
- which button (GLUT_LEFT_BUTTON,
GLUT_MIDDLE_BUTTON,
GLUT_RIGHT_BUTTON) caused event
- state of that button (GLUT_UP, GLUT_DOWN)
- Position in window
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Positioning
• The position in the screen window is usually measured
in pixels with the origin at the top-left corner
• Consequence of refresh done from top to bottom
• OpenGL uses a world coordinate system with origin at
the bottom left
• Must invert y coordinate returned by callback by
height of window
• y = h – y;
(0,0)
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h
w
48
Obtaining the window size
• To invert the y position we need the
window height
- Height can change during program execution
- Track with a global variable
- New height returned to reshape callback that
we will look at in detail soon
- Can also use query functions
• glGetIntv
• glGetFloatv
to obtain any value that is part of the state
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Terminating a program
• In our original programs, there was no
way to terminate them through OpenGL
• We can use the simple mouse callback
void mouse(int btn, int state, int x, int y)
{
if(btn==GLUT_RIGHT_BUTTON && state==GLUT_DOWN)
exit(0);
}
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Using the mouse position
• In the next example, we draw a small square
at the location of the mouse each time the left
mouse button is clicked
• This example does not use the display
callback but one is required by GLUT; We can
use the empty display callback function
mydisplay(){}
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Drawing squares at cursor
location
void mymouse(int btn, int state, int x, int y)
{
if(btn==GLUT_RIGHT_BUTTON && state==GLUT_DOWN)
exit(0);
if(btn==GLUT_LEFT_BUTTON && state==GLUT_DOWN)
drawSquare(x, y);
}
void drawSquare(int x, int y)
{
y=w-y; /* invert y position */
glColor3ub( (char) rand()%256, (char) rand )%256,
(char) rand()%256); /* a random color */
glBegin(GL_POLYGON);
glVertex2f(x+size, y+size);
glVertex2f(x-size, y+size);
glVertex2f(x-size, y-size);
glVertex2f(x+size, y-size);
glEnd();
}
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Using the motion callback
• We can draw squares (or anything else)
continuously as long as a mouse button is
depressed by using the motion callback
-glutMotionFunc(drawSquare)
• We can draw squares without depressing
a button using the passive motion
callback
-glutPassiveMotionFunc(drawSquare)
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Using the keyboard
glutKeyboardFunc(mykey)
void mykey(unsigned char key,
int x, int y)
- Returns ASCII code of key depressed and
mouse location
void mykey()
{
if(key == ‘Q’ | key == ‘q’)
exit(0);
}
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Special and Modifier Keys
• GLUT defines the special keys in glut.h
- Function key 1: GLUT_KEY_F1
- Up arrow key: GLUT_KEY_UP
• if(key == ‘GLUT_KEY_F1’ ……
• Can also check of one of the modifiers
-GLUT_ACTIVE_SHIFT
-GLUT_ACTIVE_CTRL
-GLUT_ACTIVE_ALT
is depressed by
glutGetModifiers()
- Allows emulation of three-button mouse with one- or
two-button mice
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Reshaping the window
• We can reshape and resize the OpenGL
display window by pulling the corner of
the window
• What happens to the display?
- Must redraw from application
- Two possibilities
• Display part of world
• Display whole world but force to fit in new window
– Can alter aspect ratio
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Reshape possiblities
original
reshaped
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The Reshape callback
glutReshapeFunc(myreshape)
void myreshape( int w, int h)
- Returns width and height of new window (in pixels)
- A redisplay is posted automatically at end of
execution of the callback
- GLUT has a default reshape callback but you
probably want to define your own
• The reshape callback is good place to put
viewing functions because it is invoked when
the window is first opened
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Example Reshape
• This reshape preserves shapes by making the viewport
and world window have the same aspect ratio
void myReshape(int w, int h)
{
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION); /* switch matrix mode */
glLoadIdentity();
if (w <= h)
gluOrtho2D(-2.0, 2.0, -2.0 * (GLfloat) h / (GLfloat) w,
2.0 * (GLfloat) h / (GLfloat) w);
else gluOrtho2D(-2.0 * (GLfloat) w / (GLfloat) h, 2.0 *
(GLfloat) w / (GLfloat) h, -2.0, 2.0);
glMatrixMode(GL_MODELVIEW); /* return to modelview mode */
}
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Toolkits and Widgets
• Most window systems provide a toolkit or library
of functions for building user interfaces that use
special types of windows called widgets
• Widget sets include tools such as
-
Menus
Slidebars
Dials
Input boxes
• But toolkits tend to be platform dependent
• GLUT provides a few widgets including menus
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Menus
• GLUT supports pop-up menus
- A menu can have submenus
• Three steps
- Define entries for the menu
- Define action for each menu item
• Action carried out if entry selected
- Attach menu to a mouse button
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Defining a simple menu
• In main.c
menu_id = glutCreateMenu(mymenu);
glutAddmenuEntry(“clear Screen”, 1);
gluAddMenuEntry(“exit”, 2);
glutAttachMenu(GLUT_RIGHT_BUTTON);
clear screen
exit
entries that appear when
right button depressed
identifiers
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Menu actions
- Menu callback
void mymenu(int id)
{
if(id == 1) glClear();
if(id == 2) exit(0);
}
- Note each menu has an id that is returned when it is
created
- Add submenus by
glutAddSubMenu(char *submenu_name, submenu id)
entry in parent menu
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Other functions in GLUT
• Dynamic Windows
- Create and destroy during execution
• Subwindows
• Multiple Windows
• Changing callbacks during execution
• Timers
• Portable fonts
-glutBitmapCharacter
-glutStrokeCharacter
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Picking
• Identify a user-defined object on the display
• In principle, it should be simple because the
mouse gives the position and we should be able
to determine to which object(s) a position
corresponds
• Practical difficulties
- Pipeline architecture is feed forward, hard to go from
screen back to world
- Complicated by screen being 2D, world is 3D
- How close do we have to come to object to say we
selected it?
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Three Approaches
• Hit list
- Most general approach but most difficult to
implement
• Use back or some other buffer to store
object ids as the objects are rendered
• Rectangular maps
- Easy to implement for many applications
- See paint program in text
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Rendering Modes
• OpenGL can render in one of three modes
selected by glRenderMode(mode)
-GL_RENDER: normal rendering to the frame buffer
(default)
-GL_FEEDBACK: provides list of primitives rendered
but no output to the frame buffer
-GL_SELECTION: Each primitive in the view volume
generates a hit record that is placed in a name
stack which can be examined later
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Selection Mode Functions
•glSelectBuffer(): specifies name buffer
•glInitNames(): initializes name buffer
•glPushName(id): push id on name buffer
•glPopName(): pop top of name buffer
•glLoadName(id): replace top name on
buffer
• id is set by application program to identify
objects
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Using Selection Mode
• Initialize name buffer
• Enter selection mode (using mouse)
• Render scene with user-defined identifiers
• Reenter normal render mode
- This operation returns number of hits
• Examine contents of name buffer (hit
records)
- Hit records include id and depth information
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Selection Mode and Picking
• As we just described it, selection mode
won’t work for picking because every
primitive in the view volume will generate
a hit
• Change the viewing parameters so that
only those primitives near the cursor are
in the altered view volume
- Use gluPickMatrix (see text for details)
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Using Regions of the Screen
• Many applications use a simple rectangular
arrangement of the screen
- Example: paint/CAD program
tools
drawing area
menus
• Easier to look at mouse position and determine
which area of screen it is in than using selection
mode picking
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Using another buffer and
colors for picking
• For a small number of objects, we can assign a
unique color (often in color index mode) to each
object
• We then render the scene to a color buffer other
than the front buffer so the results of the
rendering are not visible
• We then get the mouse position and use
glReadPixels() to read the color in the buffer
we just wrote at the position of the mouse
• The returned color gives the id of the object
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Writing Modes
application
bitwise logical operation
‘
frame buffer
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XOR write
• Usual (default) mode: source replaces
destination (d’ = s)
- Cannot write temporary lines this way because
we cannot recover what was “under” the line in
a fast simple way
• Exclusive OR mode (XOR) (d’ = d  s)
- x  y  x =y
- Hence, if we use XOR mode to write a line, we
can draw it a second time and line is erased!
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Rubberbanding
• Switch to XOR write mode
• Draw object
- For line can use first mouse click to fix one
endpoint and then use motion callback to
continuously update the second endpoint
- Each time mouse is moved, redraw line which
erases it and then draw line from fixed first
position to to new second position
- At end, switch back to normal drawing mode
and draw line
- Works for other objects: rectangles, circles
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Rubberband Lines
second point
first point
initial display
draw line with mouse
in XOR mode
mouse moved to original line redrawn new line drawn
with XOR
new position
with XOR
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XOR in OpenGL
• There are 16 possible logical operations
between two bits
• All are supported by OpenGL
- Must first enable logical operations
• glEnable(GL_COLOR_LOGIC_OP)
- Choose logical operation
• glLogicOp(GL_XOR)
• glLogicOp(GL_COPY) (default)
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Immediate and Retained Modes
• Recall that in a standard OpenGL program, once
an object is rendered there is no memory of it
and to redisplay it, we must re-execute the code
for it
- Known as immediate mode graphics
- Can be especially slow if the objects are complex and
must be sent over a network
• Alternative is define objects and keep them in
some form that can be redisplayed easily
- Retained mode graphics
- Accomplished in OpenGL via display lists
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Display Lists
• Conceptually similar to a graphics file
- Must define (name, create)
- Add contents
- Close
• In client-server environment, display list is
placed on server
- Can be redisplayed without sending primitives
over network each time
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Display List Functions
• Creating a display list
GLuint id;
void init()
{
id = glGenLists( 1 );
glNewList( id, GL_COMPILE );
/* other OpenGL routines */
glEndList();
}
• Call a created list
void display()
{
glCallList( id );
}
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Display Lists and State
• Most OpenGL functions can be put in
display lists
• State changes made inside a display list
persist after the display list is executed
• Can avoid unexpected results by using
glPushAttrib and glPushMatrix upon
entering a display list and glPopAttrib
and glPopMatrix before exiting
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Hierarchy and Display Lists
• Consider model of a car
- Create display list for chassis
- Create display list for wheel
glNewList( CAR, GL_COMPILE );
glCallList( CHASSIS );
glTranslatef( … );
glCallList( WHEEL );
glTranslatef( … );
glCallList( WHEEL );
…
glEndList();
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