OpenGL Basic Drawing

OpenGL Basic Drawing

Jian-Liang Lin2002

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();

}

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

OpenGL Command Syntax -2

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 variableMany state variables refer to modes that are enabled or disabled with the command glEnable() or glDisable().

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().

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)– …

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…

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.

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


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…


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 );

Start GLUT framework

glutMainLoop();– Like Win32’s message loop.– Start all GLUT operations.

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 ();

}


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. */


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. */

}

GLUT: Complete sample result

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.

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();

}

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.


Specifying Vertices– glVertex{2,3,4}{sifd}[v]( … );

• Ex: glVertex3f( 1.0f, 0.5f, 2.0f );

– Vertex array• Not introduced here.


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();


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)


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 );


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 );


Specifying a Shading Model– glShadeModel( GLenum mode );

• mode: GL_SMOOTH(default), GL_FLAT

Any Question?

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OpenGL Basic Drawing

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