OpenGL Basic Drawing

1. OpenGL Basic Drawing Jian-Liang Lin 2002

2. A Smidgen of OpenGL Code #include <whateverYouNeed.h> main() { InitializeAWindowPlease(); glClearColor (0.0, 0.0, 0.0, 0.0); glClear (GL_COLOR_BUFFER_BIT); glColor3f (1.0, 1.0, 1.0); glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0); glBegin(GL_POLYGON); glVertex3f (0.25, 0.25, 0.0); glVertex3f (0.75, 0.25, 0.0); glVertex3f (0.75, 0.75, 0.0); glVertex3f (0.25, 0.75, 0.0); glEnd(); glFlush(); UpdateTheWindowAndCheckForEvents(); }

3. OpenGL Command Syntax -1 • OpenGL constants prefix by GL_. • OpenGL routines prefixed by gl and suffiexed by the number of arguments and type. • glVertex2i, glNormal3f, glTexCoord2fv…etc

4. OpenGL Command Syntax -2

5. OpenGL as a State Machine -1 • OpenGL is a state machine. • You put it into various states (or modes) that then remain in effect until you change them. • As you've already seen, the current color is a state variable • Many state variables refer to modes that are enabled or disabled with the command glEnable() or glDisable().

6. OpenGL as a State Machine -2 • Each state variable or mode has a default value, and at any point you can query the system for each variable's current value. • Typically, you use one of the six following commands to do this: glGetBooleanv(), glGetDoublev(), glGetFloatv(), glGetIntegerv(), glGetPointerv(), or glIsEnabled().

7. GLUT: Overview • A very useful framework for rapid OpenGL program development. • Portable through lots of platforms. • Provide basic window, input, and event management. • Callback event handler. • Idle routine and timer. • A simple, cascading popup menu facility. • Offers lots of basic object. (wireframe/solid) • …

8. GLUT: Initialize and creating a window -1 • glutInit( int* argc, char** argv ); • Initialize GLUT and process any command line arguments. • This should be called before any GLUT routines. • glutInitDisplayMode( unsigned int mode ); • Specify whether to apply RGBA mode or color-indexed mode. • GLUT_RGBA, GLUT_INDEX… • Single or double buffering. • GLUT_SINGLE, GLUT_DOUBLE… • Other buffers. • GLUT_DEPTH, GLUT_STENCIL, GLUT_ACCUM…

9. GLUT: Initialize and creating a window -2 • glutInitWindowPosition( int x, int y ); • Specify the screen location of your window • glutInitWindowSize( int w, int h ); • Specify the size of your window in pixels. • glutCreateWindow( char* pTitle ); • Create a window with OpenGL context for rendering. • The window will appear until glutMainLoop() is called.

10. GLUT: Callback function -1 • Callback functions are not called by you. They are called by GLUT. • You can register some functions to GLUT, and they can be called when meeting the situation. • Some types of callback • Window event handler • IO event handler • Global handler

11. GLUT: Callback function -2 • glutDisplayFunc( void (*pfn)(void) ); • Display callback; GLUT called it when the content of the window needed to be updated. • You call manually call glutPostRedisplay() to arbitrarily ask GLUT to call that callback function. • glutReshapeFunc( void (*pfn)( int w, int h ) ); • GLUT called it when the size of window is changed. • Recalculate view frustum & viewport…

12. GLUT: Callback function -3 • glutKeyboardFunc( void(*pfn)(unsigned char key, int x, int y )); and glutMouseFunc( void(*pfn)( int button, int state, int x, int y )); • User Input event handler. • glutMotionFunc( void(*pfn)( int x, int y )); • GLUT call this registered callback function when the mouse is moved while a mouse button is also pressed. • glutIdleFunc( void(*pfn)(void)); • GLUT call this registered callback function when idling. • You can simply call: • glutIdleFunc( glutPostRedisplay );

13. Start GLUT framework • glutMainLoop(); • Like Win32’s message loop. • Start all GLUT operations.

14. GLUT: Complete sample -1 #include <GL/glut.h> void display(void) { /* clear all pixels */ glClear (GL_COLOR_BUFFER_BIT); /* draw white polygon (rectangle) with corners at * (0.25, 0.25, 0.0) and (0.75, 0.75, 0.0) */ glColor3f (1.0, 1.0, 1.0); glBegin(GL_POLYGON); glVertex3f (0.25, 0.25, 0.0); glVertex3f (0.75, 0.25, 0.0); glVertex3f (0.75, 0.75, 0.0); glVertex3f (0.25, 0.75, 0.0); glEnd(); /* don't wait! * start processing buffered OpenGL routines */ glFlush (); }

15. GLUT: Complete sample -2 void init (void) { /* select clearing (background) color */ glClearColor (0.0, 0.0, 0.0, 0.0); /* initialize viewing values */ glMatrixMode(GL_PROJECTION); glLoadIdentity(); glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0); } /* * Declare initial window size, position, and display mode * (single buffer and RGBA). Open window with "hello" * in its title bar. Call initialization routines. * Register callback function to display graphics. * Enter main loop and process events. */

16. GLUT: Complete sample -3 int main(int argc, char** argv) { glutInit(&argc, argv); glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB); glutInitWindowSize (250, 250); glutInitWindowPosition (100, 100); glutCreateWindow ("hello"); init (); glutDisplayFunc(display); glutMainLoop(); return 0; /* ISO C requires main to return int. */ }

17. GLUT: Complete sample result

18. GLUT: Simple 3D objects drawing support -1 • GLUT includes several 3D objects drawing: • Cone, cube, sphere, dodecahedron, icosahedron, octahedron, teapot, tetrahedron, torus. • Both wireframe and solid. • glutWireCube, glutSolidSphere … etc. • These objects are drawn centered at the origin of the world coordinate system.

19. GLUT: Simple 3D objects drawing support -2 • You can try this simple display callback function and see the result. void display(void) { glClear( GL_COLOR_BUFFER_BIT ); glutWireCube( 1.0 ); glFlush(); }

20. A Drawing Survival Kit -1 • Clearing the window • glClear( GLbitfield mask ); • GL_COLOR_BUFFER_BIT -> glClearColor( … ); • GL_DEPTH_BUFFER_BIT -> glClearDepth( … ); • Specifying a color • glColor3f( GLfloat r, GLfloat g, GLfloat b ); • glIndexi( int i ); • glutSetColor( int i, GLfloat r, GLfloat g, GLfloat b ); for setting indexed color.

21. A Drawing Survival Kit -2 • Specifying Vertices • glVertex{2,3,4}{sifd}[v]( … ); • Ex: glVertex3f( 1.0f, 0.5f, 2.0f ); • Vertex array • Not introduced here.

22. A Drawing Survival Kit -3 • OpenGL Geometric Drawing Primitives • glBegin( GLenum mode ); • mode is GL_POLYGON, GL_LINES … etc • glEnd(); • All primitives should be placed between these two OpenGL routines • Ex glBegin(GL_POLYGON); glVertex2f(0.0, 0.0); glVertex2f(0.0, 3.0); glVertex2f(4.0, 3.0); glVertex2f(6.0, 1.5); glVertex2f(4.0, 0.0); glEnd();

23. A Drawing Survival Kit -4

24. A Drawing Survival Kit -5 • Valid calls between glBegin(…); & glEnd(); • glVertex*(); glNormal*(); glColor*(); glIndex*(); glTexCoord*(); glMaterial*(); … • Forcing Completion of Drawing • glFlush(void); • Asynchronous • glFinish(void); • Synchronous • glutSwapBuffers(void); • Swap back buffer and front buffer. (Double buffering for animation)

25. A Drawing Survival Kit -6 • Basic State Management • glEnable( … ); • Enable some state. Ex: glEnable( GL_LIGHTING ); • glDisable( … ); • Disable some state. Ex. glDisable( GL_TEXTURE_2D ); • glIsEnabled( … ); • Query if the specific state is enabled. Ex: glIsEnabled( GL_BLEND ); • glGetBooleanv(…); glGetIntegerv(…); glGetFloatv(…); glGetDoublev( … ); glGetPointerv(…); • Query the specific state value. • Ex: glGetIntegerv( GL_VIEWPORT, vp );

26. A Drawing Survival Kit -7 • Polygon Detail • glPolygonMode( GLenum face, GLenum mode ); • face: GL_FRONT, GL_BACK, GL_FRONT_AND_BACK • mode: GL_POINT, GL_LINE, GL_FILL • glFrontFace( GLenum mode ); • mode: GL_CCW(default), GL_CW • glCullFace( GLenum mode ); • mode: GL_FRONT, GL_BACK, GL_FRONT_AND_BACK • glEnable( GL_CULL_FACE );

27. A Drawing Survival Kit -8 • Specifying a Shading Model • glShadeModel( GLenum mode ); • mode: GL_SMOOTH(default), GL_FLAT

28. Any Question? ?