Chi-Cheng Lin, Winona State University

CS430 Computer Graphics

Graphics Programming and

OpenGL

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Topics

Get Started with Graphics Programming

OpenGL Drawing Basic Primitives Interaction

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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

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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

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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

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OpenGL

Tool to support device-independent graphics programmingSame program works on a different

environment with OpenGL library installedExamples

X-Windows – UNIX – SUN X-Windows – Linux – PC (MESA) MS-Windows - PC

API, underlying hardware/OS details hidden

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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

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Window-Based Programming

Event-driven programmingEvent queue receives messagesCallback functions created for events

and executed when events occur

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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

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//appropriate #includes go here -see Appendix 1void 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}

• Opening a window for drawing

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Window-Based Programming

“my first attempt” window

Note that (0, 0) is at the lower left corner

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Drawing Basic Primitives Vertices

glBegin(GL_POINTS);glVertex2i(100, 50);glVertex2i(100, 130);glVertex2i(150, 130);

glEnd();

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Format of OpenGL Commands

Example:

A GLU library routine begin with glu A GLUT library routine begins with

glut

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Command Suffixes and Data Types

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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();}

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States (Attributes)

Set vertex sizeglPointSize(size)

Set primitive (foreground) colorglColor3f(red, green, blue)E.g., glColor3f(0.0, 0.0, 0.0)

Set background colorglClearColor(red, green, blue, alpha)

Clear entire window to background colorglClear(GL_COLOR_BUFFER_BIT)

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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);

}

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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

}

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More Primitives

glBegin(constant)vertices

glEnd();defines various primitives with different values of constantGL_POLYGON: filled polygonGL_TRIANGLES: trianglesGL_QUAS: quadrilaterals

:

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Interaction

MouseglutMouseFunc(myMouse)

MotionglutMotionFunc(myMovedMouse)

KeyboardglutKeyboardFunc(myKeyboard)

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Mouse

Prototype of myMousevoid 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

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Motion

Prototype of myMotionvoid myMovedMouse(int x, int y)(x, y)

Location measured in pixel from upper left corner

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Keyboard

Prototype of myKeyboardvoid 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