1. 1 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Programming with OpenGL Part 3: Three Dimensions Ed Angel Professor of Computer Science, Electrical and Computer Engineering, and Media Arts University of New Mexico

2. 2 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Objectives Develop a more sophisticated three- dimensional example ­Sierpinski gasket: a fractal Introduce hidden-surface removal

3. 3 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

4. 4 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Objectives Learn to build interactive programs using GLUT callbacks ­Mouse ­Keyboard ­Reshape Introduce menus in GLUT

5. 5 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

6. 6 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

7. 7 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Sierpinski Gasket (2D) Start with a triangle Connect bisectors of sides and remove central triangle Repeat

8. 8 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Example Five subdivisions

9. 9 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

10. 10 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Gasket Program #include /* initial triangle */ GLfloat v[3][2]={{-1.0, -0.58}, {1.0, -0.58}, {0.0, 1.15}}; int n; /* number of recursive steps */

11. 11 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Draw one triangle void triangle( GLfloat *a, GLfloat *b, GLfloat *c) /* display one triangle */ { glVertex2fv(a); glVertex2fv(b); glVertex2fv(c); }

12. 12 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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 */ }

13. 13 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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); }

14. 14 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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(); }

15. 15 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

16. 16 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Moving to 3D We can easily make the program three- dimensional by using GLfloat v[3][3] glVertex3f glOrtho But that would not be very interesting Instead, we can start with a tetrahedron

17. 17 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

18. 18 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Example after 5 iterations

19. 19 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 triangle code void triangle( GLfloat *a, GLfloat *b, GLfloat *c) { glVertex3fv(a); glVertex3fv(b); glVertex3fv(c); }

20. 20 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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)); }

21. 21 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 tetrahedron code void tetrahedron( int m) { glColor3f(1.0,0.0,0.0); divide_triangle(v[0], v[1], v[2], m); glColor3f(0.0,1.0,0.0); divide_triangle(v[3], v[2], v[1], m); glColor3f(0.0,0.0,1.0); divide_triangle(v[0], v[3], v[1], m); glColor3f(0.0,0.0,0.0); divide_triangle(v[0], v[2], v[3], m); }

22. 22 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

23. 23 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

24. 24 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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)

25. 25 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

26. 26 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Volume Subdivision

27. 27 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

28. 28 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

29. 29 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Physical Devices mousetrackball light pen data tablet joy stick space ball

30. 30 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

31. 31 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

32. 32 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

33. 33 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

34. 34 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

35. 35 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

36. 36 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

37. 37 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

38. 38 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

39. 39 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

40. 40 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

41. 41 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

42. 42 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

43. 43 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

44. 44 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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() }

45. 45 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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(); }

46. 46 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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 }

47. 47 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

48. 48 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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) h w

49. 49 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

50. 50 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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); }

51. 51 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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(){}

52. 52 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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(); }

53. 53 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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)

54. 54 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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); }

55. 55 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

56. 56 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

57. 57 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Reshape possiblities original reshaped

58. 58 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

59. 59 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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 */ }

60. 60 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

61. 61 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

62. 62 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Defining a simple menu In main.c menu_id = glutCreateMenu(mymenu); glutAddmenuEntry(“clear Screen”, 1); gluAddMenuEntry(“exit”, 2); glutAttachMenu(GLUT_RIGHT_BUTTON); entries that appear when right button depressed identifiers clear screen exit

63. 63 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Menu actions ­Menu callback ­Note each menu has an id that is returned when it is created ­Add submenus by glutAddSubMenu(char *submenu_name, submenu id) void mymenu(int id) { if(id == 1) glClear(); if(id == 2) exit(0); } entry in parent menu

64. 64 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Other functions in GLUT Dynamic Windows ­Create and destroy during execution Subwindows Multiple Windows Changing callbacks during execution Timers Portable fonts ­glutBitmapCharacter ­glutStrokeCharacter

65. 65 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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?

66. 66 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

67. 67 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

68. 68 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

69. 69 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

70. 70 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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)

71. 71 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Using Regions of the Screen Many applications use a simple rectangular arrangement of the screen ­Example: paint/CAD program Easier to look at mouse position and determine which area of screen it is in than using selection mode picking drawing area tools menus

72. 72 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

73. 73 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Writing Modes frame buffer application ‘ bitwise logical operation

74. 74 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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!

75. 75 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

76. 76 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 Rubberband Lines initial display draw line with mouse in XOR mode mouse moved to new position first point second point original line redrawn with XOR new line drawn with XOR

77. 77 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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)

78. 78 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

79. 79 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

80. 80 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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 ); }

81. 81 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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

82. 82 Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005 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(); 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();