OpenGL basics

1
OpenGL basics
2
OpenGL pipeline
3
Using the pipeline
4
Pipeline details
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OpenGL libraries

• gl - the basic OpenGL functions glVertex2i(0,
  0)
• glu - utility routines for common higher level
  operations (cubes, spheres)
• glut - a system independent library of window and
  mouse functions
• wgl (wiggle) - native Windows API
• glx - native X API

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Example simple.c
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Main routine
Initialize GLUT, create window Register event
callback routines Start main event loop
int main(int argc, char argv)
glutInit(argc, argv) glutCreateWindow("single
triangle") glutDisplayFunc(display)
glutReshapeFunc(reshape) glutMainLoop()
return 0
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Reshape function
Called when canvas is resized/moved Sets up 3d to
2d geometry
Void reshape(int w, int h) glViewport(0, 0,
w, h) glMatrixMode(GL_PROJECTION)
glLoadIdentity() glOrtho(0, w, 0, h, -1, 1)
glScalef(1, -1, 1) glTranslatef(0, -h,
0)
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Display function
void display(void) glClear(GL_COLOR_BUFFER_BIT
) glBegin(GL_TRIANGLES) glColor3f(0.0,
0.0, 1.0) / blue / glVertex2i(0, 0)
glColor3f(0.0, 1.0, 0.0) / green /
glVertex2i(200, 200) glColor3f(1.0, 0.0,
0.0) / red / glVertex2i(20, 200)
glEnd() glFlush ()
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Compiling/debugging

• cc -o simple simple.c -lglut -lGLU -lGL -lXmu
  -lXext -lX11 -lm
• Printf and scanf can be used for text I/O
  simultaneously with graphics

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Online references

• Bluebook references (man pages)
• Greenbook GLUT references (here)
• Other sources on 486 home page
• http//opengl.org/

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Lab 1

• Play with triangle
• Move it with mouse
• Animate it

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Part A. Play with the triangle

• a. Undo the glScalef and glTranslatef so the Y
  goes up, not down.
• b. Make the triangle equilateral.
• c. Use glScalef to shrink the triangle by 50.
• d. Use the glTranslate to move the triangle to
  the center of the window.
• e. Make the triangle scale with the window resize
  by making the coordinates in glVertex2i relative
  to w and h, not absolute (you need globals!)
• f. Make the simple triangle a blend of orange,
  purple and grey, not red, blue, green.
• g. Make the triangle a pentagon (use GL_POLYGON)
  with any combination of colors youd like.
• h. Turn off glClear and see what happens.
• i. Try the glBegin with GL_POINTS, GL_LINES,
  GL_LINE_LOOP, and others.

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Void reshape(int w, int h) glViewport(0, 0,
w, h) glMatrixMode(GL_PROJECTION)
glLoadIdentity() glOrtho(0, w, 0, h, -1, 1)
glScalef(1, -1, 1) glTranslatef(0, -h,
0)
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void display(void) glClear(GL_COLOR_BUFFER_BIT
) glBegin(GL_TRIANGLES) glColor3f(0.0,
0.0, 1.0) / blue / glVertex2i(0, 0)
glColor3f(0.0, 1.0, 0.0) / green /
glVertex2i(200, 200) glColor3f(1.0, 0.0,
0.0) / red / glVertex2i(20, 200)
glEnd() glFlush ()
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glVertex primitives

• GL_POINTS
• GL_LINES
• GL_LINE_STRIP
• GL_LINE_LOOP
• GL_POLYGON

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openGL conventions

• glVertex2i(100,50)
• glVertex3d(100.0,50.0,60.0)
• glColor3f, glColor4d

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Part B. Move with the mouse.

• a. Add a callback routine for the move w/ button
  down so it sets the fan center to a new position.
  Use global variables for the fan position. Do a
  glutPostRedisplay to update the screen.
• b. Modify the glTranslate call to move the fan to
  that location. Email me the source.

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Part C. Make it a rotating fan.

• a. Make the triangle a fan by making four
  triangles centered in the window.
• b. Use the idle even to advance a global variable
  Angle use Angle in a glRotatef call in the
  display callback to make the fan rotate. You will
  need to use glTranslatef to bring the fan to the
  center of the window (try translating to x w/2,
  y h/2. Set the increment of the angle to a
  small value so the fan doesnt rotate too fast.
• c. Use double buffering to make it go smoothly.
• d. Add a menu to turn the fan on and off (and, if
  you wish, slow it down and speed it up.)

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GLUT callbacks

• glutMouseFunc(myMouse)
• glutMouseMoved(myMove)
• glutKeyboardFunc(myKeyBoard)
• glutIdleFunc(myidle)
• Glut menu routines
• Display, reshape

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General pattern

• Define callback routine
• Any name, fixed parameters
• void zKeyBrd(unsigned char key, int x, int y)
• Register callback
• glutKeyboardFunc(keyboard)
• Remove registration
• glutKeyboardFunc( (void )NULL )

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Using globals

• On user event set a global variable
• In reshape and display use the value

void ClickDrag(int x, int y) printf("(d,
d)\n", x, y) xcenter x ycenter y
glutPostRedisplay() glutMotionFunc(ClickDrag
)
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Menus
void HandleMenu(int mode) switch (mode)
case 1 glutIdleFunc(myidle)
glutPostRedisplay() break case 2
glutIdleFunc((void )NULL)
glutPostRedisplay() break case 3
exit(0)
void initMenus ( void ) glutCreateMenu(HandleM
enu) glutAddMenuEntry("Start",1)
glutAddMenuEntry("Stop",2) glutAddMenuEntry("Ex
it",3) glutAttachMenu(GLUT_RIGHT_BUTTON)