OpenGL Basic Drawing

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OpenGL Basic Drawing

• Jian-Liang Lin
• 2002

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A Smidgen of OpenGL Code

• include ltwhateverYouNeed.hgt
• 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()

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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, glTexCoord2fvetc

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OpenGL Command Syntax -2
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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().

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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().

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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)

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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

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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.

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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

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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

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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 )

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Start GLUT framework

• glutMainLoop()
• Like Win32s message loop.
• Start all GLUT operations.

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GLUT Complete sample -1

• include ltGL/glut.hgt
• 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 ()

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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.
• /

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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.
  /

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GLUT Complete sample result
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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.

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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()

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A Drawing Survival Kit -1

• Clearing the window
• glClear( GLbitfield mask )
• GL_COLOR_BUFFER_BIT -gt glClearColor( )
• GL_DEPTH_BUFFER_BIT -gt 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.

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A Drawing Survival Kit -2

• Specifying Vertices
• glVertex2,3,4sifdv( )
• Ex glVertex3f( 1.0f, 0.5f, 2.0f )
• Vertex array
• Not introduced here.

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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()

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A Drawing Survival Kit -4
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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)

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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 )

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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 )

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A Drawing Survival Kit -8

• Specifying a Shading Model
• glShadeModel( GLenum mode )
• mode GL_SMOOTH(default), GL_FLAT

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Any Question?

• ?