CS430 Computer Graphics

1. CS430 Computer Graphics Graphics Programming and OpenGL Chi-Cheng Lin, Winona State University

2. Topics • Get Started with Graphics Programming • OpenGL • Drawing Basic Primitives • Interaction

3. Get Started with Graphics Programming • What are required? • Software • Graphics library (libraries) • Hardware • Display to show graphics • Input device(s) for interaction • Programmer’s passion (and patient :) • willing to spend time writing and testing programs to produce different graphics with different parameters (i.e., doing experiments) to master graphics programming

4. Get Started with Graphics Programming • Steps • Initialization • Set up display mode (graphics mode) • Set up coordinate system • Coordinates x and y in (x, y) measured in pixels • Basic primitives • point(x,y,color), line(x1,y1,x2,y2), moveto(x,y), lineto(x,y), and cp: current pen position

5. Device-Independent Programming • Why device-independent programming? • It’s hard to port one graphics application from one environment to another • Different commands required to drive different displays • Different libraries • Device-independent programming make the task of porting (same program: compile  run  identical output) easy

6. OpenGL • Tool to support device-independent graphics programming • Same program works on a different environment with OpenGL library installed • Examples • X-Windows – UNIX – SUN • X-Windows – Linux – PC (MESA) • MS-Windows - PC • API, underlying hardware/OS details hidden

7. OpenGL • Powerful 3D graphics drawing • Suitable for 2D graphics, too • Unified approach to producing pictures • Libraries • GLU: OpenGL Utility Library • Routines using lower-level OpenGL commands • Part of OpenGL • GLUT: OpenGL Utility Toolkit • Window-system-independent toolkit • Written by Mark Kilgard

8. Window-Based Programming • Event-driven programming • Event queue receives messages • Callback functions created for events and executed when events occur

9. Window-Based Programming • Skeleton of even-driven OpenGL program void main() { initialize things Create a screen window glutDisplayFunc(myDisplay);//register the redraw function glutReshapeFunc(myReshape);//register the reshape function glutMouseFunc(myMouse);//register the mouse action function glutKeyboardFunc(myKeyboard);//register the keyboard action function perhaps initialize other things glutMainLoop();//enter the unending main loop } all of the callback functions are defined here

10. Opening a window for drawing //appropriate #includes go here -see Appendix 1 void main(int argc, char** argv) { glutInit(&argc, argv);//initialize the toolkit glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB); //set the display mode glutInitWindowSize(640, 480); //set window size glutInitWindowPosition(100, 150); //set the window position on screen glutCreateWindow("my first attempt"); //open the screen window //register the callback functions glutDisplayFunc(myDisplay); glutReshapeFunc(myReshape); glutMouseFunc(myMouse); glutKeyboardFunc(myKeyboard); myInit(); //additional initializations as necessary glutMainLoop(); //go into a perpetual loop }

11. Window-Based Programming • “my first attempt” window • Note that (0, 0) is at the lower left corner

12. Drawing Basic Primitives • Vertices glBegin(GL_POINTS); glVertex2i(100, 50); glVertex2i(100, 130); glVertex2i(150, 130); glEnd();

13. Format of OpenGL Commands • Example: • A GLU library routine begin with glu • A GLUT library routine begins with glut

14. Command Suffixes and Data Types

15. Example on Data Type • Encapsulating OpenGL details in the generic function drawDot() void drawDot(int x,int y)  danger: passes ints { //draw dot at integer point (x,y) glBegin(GL_POINTS); glVertex2i(x,y); glEnd(); } void drawDot(GLint x,GLint y) // OK here { //draw dot at integer point (x,y) glBegin(GL_POINTS); glVertex2i(x,y); glEnd(); }

16. States (Attributes) • Set vertex size • glPointSize(size) • Set primitive (foreground) color • glColor3f(red, green, blue) • E.g., glColor3f(0.0, 0.0, 0.0) • Set background color • glClearColor(red, green, blue, alpha) • Clear entire window to background color • glClear(GL_COLOR_BUFFER_BIT)

17. Coordinate System • Set up in myInit() void myInit(void) { glClearColor(1.0, 1.0, 1.0, 0.0) glColor3f(0.0, 0.0, 0.0) glPointSize(4.0) glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluOrtho2D(0,640.0,0,480.0); }

18. What should be in myDisplay? void myDisplay(void) { glClear(GL_COLOR_BUFFER_BIT) glBegin(GL_POINTS); glVertex2i(100, 50); glVertex2i(100, 130); glVertex2i(150, 130); glEnd(); glFlush(); // send all output to display }

19. More Primitives • glBegin(constant) vertices glEnd(); defines various primitives with different values of constant • GL_POLYGON: filled polygon • GL_TRIANGLES: triangles • GL_QUAS: quadrilaterals :

20. Interaction • Mouse • glutMouseFunc(myMouse) • Motion • glutMotionFunc(myMovedMouse) • Keyboard • glutKeyboardFunc(myKeyboard)

21. Mouse • Prototype of myMouse • void myMouse(int button, int state, int x, int y) • button • GLUT_LEFT_BUTTON • GLUT_MIDDLE_BUTTON • GLUT_RIGHT_BUTTON • state • GLUT_UP • GLUT_DOWN • (x, y) • Location measured in pixel from upper left corner

22. Motion • Prototype of myMotion • void myMovedMouse(int x, int y) • (x, y) • Location measured in pixel from upper left corner

23. Keyboard • Prototype of myKeyboard • void myKeyboard(unsigned int key, int x, int y) • key • ASCII value of the key pressed • (x, y) • Location of the mouse from upper left corner when the key is pressed