Introduction to OpenGL Programming

1
Introduction to OpenGL Programming
2
What is OpenGL

• OpenGL is a software API to graphics hardware.
• designed as a streamlined, hardware-independent
  interface to be implemented on many different
  hardware platforms
• Intuitive, procedural interface with c binding
• No windowing commands !
• No high-level commands for describing models of
  three-dimensional objects
• The OpenGL Utility Library (GLU) provides many of
  the modeling features, such as quadric surfaces
  and NURBS curves and surfaces

3
SGI and GL

• Silicon Graphics (SGI) revolutionized the
  graphics workstation by implementing the pipeline
  in hardware (1982)
• To access the system, application programmers
  used a library called GL
• With GL, it was relatively simple to program
  three dimensional interactive applications

4
OpenGL

• The success of GL lead to OpenGL (1992), a
  platform-independent API that was
• Easy to use
• Close enough to the hardware to get excellent
  performance
• Focus on rendering
• Omitted windowing and input to avoid window
  system dependencies

5
OpenGL Evolution

• Controlled by an Architectural Review Board (ARB)
• Members include SGI, Microsoft, Nvidia, HP,
  3DLabs, IBM,.
• Relatively stable (present version 2.0)
• Evolution reflects new hardware capabilities
• 3D texture mapping and texture objects
• Vertex and fragment programs
• Allows for platform specific features through
  extensions

6
OpenGL Libraries

• OpenGL core library
• OpenGL32 on Windows
• GL on most unix/linux systems (libGL.a)
• OpenGL Utility Library (GLU)
• Provides functionality in OpenGL core but avoids
  having to rewrite code
• Links with window system
• GLX for X window systems, WGL for Windows
• Cross-platform GUI libraries GLUT, SDL, FLTK,
  QT,

7
Windowing with OpenGL

• OpenGL is independent of any specific window
  system
• OpenGL can be used with different window systems
• X windows (GLX)
• MFC
• GLUT provide a portable API for creating window
  and interacting with I/O devices

8
GLUT

• OpenGL Utility Toolkit (GLUT)
• Provides functionality common to all window
  systems
• Open a window
• Get input from mouse and keyboard
• Menus
• Event-driven
• Code is portable but GLUT lacks the functionality
  of a good toolkit for a specific platform
• No slide bars, buttons,

9
Software Organization
application program
OpenGL Motif widget or similar
GLUT
GLX, AGLor WGL
GLU
OpenGL
X, Win32, Mac O/S
software and/or hardware
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OpenGL Architecture
geometry pipeline
Immediate Mode
Per Vertex Operations Primitive Assembly
Polynomial Evaluator
DisplayList
Per Fragment Operations
Frame Buffer
Rasterization
CPU
Texture Memory
Pixel Operations
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OpenGL as a state machine

• GL State Variables- can be set and queried by
  OpenGL. Remains unchanged until the next change.
• Projection and viewing matrix
• Color and material properties
• Lights and shading
• Line and polygon drawing modes
• OpenGL functions are of two types
• Primitive generating
• Can cause output if primitive is visible
• How vertices are processed and appearance of
  primitive are controlled by the state
• State changing
• Transformation functions
• Attribute functions

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OpenGL Syntax

• Functions have prefix gl and initial capital
  letters for each word
• glClearColor(), glEnable(), glPushMatrix()
• glu for GLU functions
• gluLookAt(), gluPerspective()
• Constants begin with GL_, use all capital letters
  
• GL_COLOR_BUFFER_BIT, GL_PROJECTION, GL_MODELVIEW
  
• Extra letters in some commands indicate the
  number and type of variables
• glColor3f(), glVertex3f()
• OpenGL data types
• GLfloat, GLdouble, GLint, GLenum,
• Underlying storage mode is the same
• Easy to create overloaded functions in C but
  issue is efficiency

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OpenGL function format
function name
dimensions
glVertex3f(x,y,z)
x,y,z are floats
belongs to GL library
glVertex3fv(p)
p is a pointer to an array
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OpenGL defines

• Most constants are defined in the include files
  gl.h, glu.h and glut.h
• Note include ltGL/glut.hgt should automatically
  include the others
• Examples
• glBegin(GL_POLYGON)
• glClear(GL_COLOR_BUFFER_BIT)
• include files also define OpenGL data types
  GLfloat, GLdouble,.

15
GLUT

• Developed by Mark Kilgard
• Hides the complexities of differing window system
  APIs
• Default user interface for class projects
• Glut routines have prefix glut
• glutCreateWindow()
• Has very limited GUI interface
• Glui is the C extension of glut

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Glut Routines

• Initialization glutInit() processes (and
  removes) commandline arguments that may be of
  interest to glut and the window system and does
  general initialization of Glut and OpenGL
• Must be called before any other glut routines
• Display Mode The next procedure,
  glutInitDisplayMode(), performs initializations
  informing OpenGL how to set up the frame buffer.
• Display Mode Meaning
• GLUT_RGB Use RGB colors
• GLUT_RGBA Use RGB plus alpha (for transparency)
• GLUT_INDEX Use indexed colors (not recommended)
• GLUT_DOUBLE Use double buffering (recommended)
• GLUT_SINGLE Use single buffering (not
  recommended)
• GLUT_DEPTH Use depthbuffer (for hidden surface
  removal.)

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Glut Routines

• Window Setup
• glutInitWindowSize(int width, int height)
• glutInitWindowPosition(int x, int y)
• glutCreateWindow(char title)

18
A Simple Program

• Generate a square on a solid background

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simple.c
include ltGL/glut.hgt void mydisplay()
glClear(GL_COLOR_BUFFER_BIT) glBegin(GL_POLYGON
) glVertex2f(-0.5, -0.5)
glVertex2f(-0.5, 0.5)
glVertex2f(0.5, 0.5)
glVertex2f(0.5, -0.5) glEnd() glFlush()
int main(int argc, char argv) glutInit(a
rgc, argv) glutInitDisplayMode (GLUT_SINGLE
GLUT_RGB) glutInitWindowSize(500,500) glutIn
itWindowPosition(0,0) glutCreateWindow("simple")
glutDisplayFunc(mydisplay) init()
glutMainLoop()
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Closer Look at the main()
includes gl.h

• include ltGL/glut.hgt
• int main(int argc, char argv)
• glutInit(argc,argv)
• glutInitDisplayMode(GLUT_SINGLEGLUT_RGB)
• glutInitWindowSize(500,500)
• glutInitWindowPosition(0,0)
• glutCreateWindow("simple")
• glutDisplayFunc(mydisplay)
• init()
• glutMainLoop()

define window properties
display callback
set OpenGL state
enter event loop
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init.c
black clear color

• void init()
• glClearColor (0.0, 0.0, 0.0, 1.0)
• glColor3f(1.0, 1.0, 1.0)
• glMatrixMode (GL_PROJECTION)
• glLoadIdentity ()
• glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0)

opaque window
fill/draw with white
viewing volume
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Event Handling

• Virtually all interactive graphics programs are
  even driven
• GLUT uses callbacks to handle events
• Windows system invokes a particular procedure
  when an event of particular type occurs.
• MOST IMPORTANT display event
• Signaled when window first displays and whenever
  portions of the window reveals from blocking
  window
• glutDisplayFunc(void (func)(void)) registers
  the display callback function
• Running the program glutMainLoop()
• Main event loop. Never exit()

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More Callbacks

• glutReshapeFunc(void (func)(int w, int h))
  indicates what action should be taken when the
  window is resized.
• glutKeyboardFunc(void (func)(unsigned char key,
  int x, int y)) and glutMouseFunc(void (func)(int
  button, int state, int x, int y)) allow you to
  link a keyboard key or a mouse button with a
  routine that's invoked when the key or mouse
  button is pressed or released.
• glutMotionFunc(void (func)(int x, int y))
  registers a routine to call back when the mouse
  is moved while a mouse button is also pressed.
• glutIdleFunc(void (func)(void)) registers a
  function that's to be executed if no other events
  are pending - for example, when the event loop
  would otherwise be idle

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Compilation on Windows

• See class web site on how to set up a project
  with OpenGL
• Visual C
• Get glut.h, glut32.lib and glut32.dll from web
• Create a console application
• Add opengl32.lib, glut32.lib, glut32.lib to
  project settings (under link tab)

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Simple Animation

• Animation
• Redraw swap buffers
• What looks like if using single buffer
• Example program
• More on the glut documentation
• Chapter 2 of textbook
• OpenGL redbook
• Links in the class resources page

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OpenGL Drawing

• We have learned how to create a window
• Steps in the display function
• Clear the window
• Set drawing attributes
• Send drawing commands
• Swap the buffers
• OpenGL coordinate system has different origin
  from the window system
• Uses lower left corner instead of upper left
  corner as origin

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Clear the Window

• glClear(GL_COLOR_BUFFER_BIT)
• clears the frame buffer by overwriting it with
  the background color.
• Background color is a state set by
  glClearColor(GLfloat r, GLfloat g, GLfloat b,
  GLfloat a) in the init().

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Drawing Attributes Color

• glColor3f(GLfloat r, GLfloat g, GLfloat b) sets
  the drawing color
• glColor3d(), glColor3ui() can also be used
• Remember OpenGL is a state machine
• Once set, the attribute applies to all subsequent
  defined objects until it is set to some other
  value
• glColor3fv() takes a flat array as input
• There are more drawing attributes than color
• Point size glPointSize()
• Line width glLinewidth()
• Dash or dotted line glLineStipple()
• Polygon pattern glPolygonStipple()

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Drawing Commands

• Simple Objects glRectf()
• Complex Objects
• Use construct glBegin(mode) and glEnd() and a
  list of vertices in between
• glBegin(mode)
• glVertex(v0)
• glVertex(v1)
• ...
• glEnd()
• Some other commands can also be used between
  glBegin() and glEnd(), e.g. glColor3f().
• Example

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Projection and Viewport
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Orthographic projection

• Orthographic View
• glOrtho(left, right, bottom, top, front, back)
• Specifies the coordinates of 3D region to be
  projected into the image space.
• Any drawing outside the region will be
  automatically clipped away.

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Viewports

• Do not have use the entire window for the image
  glViewport(x,y,w,h)
• Values in pixels (screen coordinates)

33
Window to Viewport mapping
Aspect Ratio Height/Width If the aspect ratio of
the window Is different from that of the
viewport, the picture will be distorted.
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Sierpinski Gasket (2D)

• Start with a triangle
• Connect bisectors of sides and remove central
  triangle
• Repeat

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Example

• Five subdivisions

36
Gasket Program

• include ltGL/glut.hgt
• / initial triangle /
• GLfloat v32-1.0, -0.58,
• 1.0, -0.58, 0.0, 1.15
• int n / number of recursive steps /
• void triangle( GLfloat a, GLfloat b, GLfloat
  c)
• / display one triangle /
• glVertex2fv(a)
• glVertex2fv(b)
• glVertex2fv(c)

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Triangle Subdivision

• void divide_triangle(GLfloat a, GLfloat b,
  GLfloat c, int m)
• / triangle subdivision using vertex numbers /
• point2 v0, v1, v2
• int j
• if(mgt0)
• for(j0 jlt2 j) v0j(ajbj)/2
• for(j0 jlt2 j) v1j(ajcj)/2
• for(j0 jlt2 j) v2j(bjcj)/2
• divide_triangle(a, v0, v1, m-1)
• divide_triangle(c, v1, v2, m-1)
• divide_triangle(b, v2, v0, m-1)
• else(triangle(a,b,c))
• / draw triangle at end of recursion /

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Gasket Display Functions

• void display()
• glClear(GL_COLOR_BUFFER_BIT)
• glBegin(GL_TRIANGLES)
• divide_triangle(v0, v1, v2, n)
• glEnd()
• glFlush()
• By having the glBegin and glEnd in the display
  callback rather than in the function triangle and
  using GL_TRIANGLES rather than GL_POLYGON in
  glBegin, we call glBegin and glEnd only once for
  the entire gasket rather than once for each
  triangle

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Example 3D Gasket
after 5 iterations
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Tetrahedron Code

• void tetrahedron( int m)
• glColor3f(1.0,0.0,0.0)
• divide_triangle(v0, v1, v2, m)
• glColor3f(0.0,1.0,0.0)
• divide_triangle(v3, v2, v1, m)
• glColor3f(0.0,0.0,1.0)
• divide_triangle(v0, v3, v1, m)
• glColor3f(0.0,0.0,0.0)
• divide_triangle(v0, v2, v3, m)