Getting Started With OpenGL

1
Getting Started With OpenGL

• August 24, 2005

2
Primitives

• OpenGL supports a number of primitives that
  include
• Points (GL_POINTS)
• Lines (GL_LINES), Line Strips or Polylines
  (GL_LINE_STRIP), and Line Loops (GL_LINELOOP)
• Triangles (GL_TRIANGLES), Triangle Strips
  (GL_TRIANGLE_STRIP), and Triangle Fans
  (GL_TRIANGLE_FAN)
• Quadrangles (GL_QUADS) and Quadrangle Strips
  (GL_QUAD_STRIP)
• Polygons (GL_POLYGON )

3
Drawing Primitives

• All primitives are drawn in a similar fashion
• glBegin (GL_????) // starts the primitive
• glVertex3f ( xcoord, ycoord, zcoord) // defines
  the next vertex
• glEnd( ) // ends the primitive

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glVertex

• There are many glVertex calls in OpenGL. The
  digit after the glVertex can be a 2, 3, or a 4
  which indicates the number of values for the
  coordinate (2 2 D, 3 3 D, 4 Homogeneous
  Coordinates)
• The letter after the digit (an b, s, i, f, or d
  normally) indicates the type of the coordinates
• b indicates bytes
• s indicates shorts
• i indicates ints
• f indicates floats
• d indicates doubles

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Other Vertex Modifiers

• Can set the color on a per vertex basis
• glColor34bsifd
• glColor3bsifd gives color in terms of red,
  green, and blue components.
• glColor4bsifd gives color in terms of red,
  green, blue, and alpha components. You could
  consider alpha to be a "transparency" factor.

6
Color Cubes

• red, green, and blue are between 0.0 and 1.0 for
  each component with floats and doubles.
• 0 to 255 for bytes
• 0 to 32,767 for ints

7
Normals for the Cornerscan also be specified

• glNormal3bsifd
• For several reasons OpenGL would prefer unit
  normals but doesn't require them.

8
Vector Calls

• Much easier to pass an array (or vector) of
  values for some of these. Some OpenGL calls
  support Vector versions as well that take a 1 D
  array as the parameter.
• Examples
• glVertex234sifdv
• glColor34bsifdv
• glNormal3bsifdv

9
Different Ways to Draw an Object.

• Let us assume we want to draw a cone where the
  base and the point are aligned on the Y axis.
• We can draw approximations of it using a number
  of methods and primitives.

10
One Version -- Lines

• void drawCone ( double baseRadius, double height,
  int numberOfSections) double
  angle glBegin( GL_LINES) for ( angle 0
  angle lt 2 3.14159 angle 2.0
  3.14159 / numberOfSections) glVertex3d (
  sin(angle)baseRadius, 0.0,
  cos(angle)baseRadius, 0.0) glVertex3d ( 0.0,
  0.0, height)

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• glEnd( )glBegin(GL_LINE_LOOP)for (angle
  0.0 angle lt 3.14159 angle 3.14159 /
  numberOfSections) glVertex3d(sin(angle)baseRad
  ius, 0.0, cos(angle)baseRadius)glEnd()
  return

12
Version 2 Solids (Triangles)

• void drawCone (double baseRadius, double
  height, int nbrOfSections) double
  angleglBegin(GL_TRIANGLE_FAN)glVertex3d(0.0,
  height, 0.0)for (angle 0.0 angle lt 2.0
  3.14159 angle 3.14159 2.0 /
  nbrOfSections) glVertex3d(sin(angle)baseRadius
  , 0.0, cos(angle) baseRadius)glEnd(
  )

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• glBegin(GL_TRIANGLE_FAN)glVertex3d(0.0, 0.0,
  0.0)for (angle 0.0 angle lt 2.0 3.14159
  angle 2.0 3.14159 / nbrOfSections)
  glVertex3d(sin(angle) baseRadius, 0.0,
  cos(angle) baseRadius)glEnd(
  )return

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How would we integrate this into a GLUT Program?

• int main(int argCount, char argValues)
  glutInit( argCount, argValues) glutInitDi
  splayMode(GLUT_RGBA) glutInitWindowSize(600,
  600) glutCreateWindow("Main Window") glutDis
  playFunction(display) init( ) glutMainLoop()
  return EXIT_SUCCESS

15
display Routine

• void display( ) glMatrixMode(GL_MODELVIEW)
  glClear(GL_COLOR_BUFFER_BIT) drawCone(1.0,
  3.0, 36) glFlush()

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init routine

• void init( ) glClearColor(0.0, 0.0, 0.0,
  0.0)glMatrixMode(GL_PROJECTION)glLoadIdentity(
  )glOrtho(-3.0, 3.0, -1.0, 5.0, -3.0,
  3.0)glMatrixMode (GL_MODELVIEW)

17
Other Features

• People also like to have handlers to capture
  keystrokes (to quit a program for example) in
  their code. This can be added as
• glutKeyboardFunc( processKeys )

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processKeys Code

• void processKeys ( unsigned char keyCode, int x,
  int y)
• if (keyCode 'q') exit(0) else if
  (keyCode '?')
• // whatever.

19
Attributes

• We mentioned Attributes as an item to change the
  appearance of primitives. The easiest one is the
  one to set the color.
• glColor3usifd( red, green, blue)

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Attributes Lines

• For lines and edges we can set the width and
  style of the line.
• glLineWidth( width )
• glLineStipple ( scale, pattern)
• glEnable (GL_LINE_STIPPLE) // need to enable
  the system to use stippilng
• The argument for width is a scale factor. The
  bigger the number the wider the line.
• The pattern argument for the line stipple is a 16
  bit pattern where the line is drawn for 1's and
  blank areas for 0's.
• The scale argument for the line stipple is an
  integer to enlarge the pattern by. (2 times, 3
  times, 10 times, etc)

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Attributes Of Polygons, Triangles, and
Quadrilaterals

• What gets Drawn?
• glPolygonMode ( GLenum face, GLenum mode)
• face is one of GL_FRONT, GL_BACK, and
  GL_FRONT_AND_BACK.
• mode is one of GL_POINT, GL_LINE, GL_FILL.

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Stippling Polygons

• glEnable (GL_POLYGON_STIPPLE)
• glPolygonStipple (stipplePattern)
• The stipple pattern must be an array of unsigned
  bytes that holds a 32 x 32 bitmap of the stipple.
  1's are drawn, 0's are not.

23
Where are we at?

• We should now be able to draw what we need using
  OpenGL calls.
• We don't have all the viewing options we would
  like.
• There is no shading/lighting specified yet so
  things look a little flat.

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Complex Objects

• We have a few options if we wish to draw a
  complex object.
• Write a series of glBegin's, glEnd's, and
  glVertex's
• Write a procedure to generate the shape given
  some parameters.
• Both of these store the information in the
  program could we read it in from a file.
• We could.
• Some advantages in that it may be easier to
  change.

25
How big can Objects Be?

• They could be thousands of polygons in size.
  (that gets rid of coding into the program!)
• Even reading it in that's a lot of calls!
• Vector Calls (Vertex Arrays)
• OpenGL has some calls that work on groups of
  points stored in arrays. These are the vertex
  calls.

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This is a model that contains 69451 triangles!
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How Does a Vertex Array Work?

• double points 3 0.0, 0.0, 0.0,
  1.0, 0.0, 0.0,
• 1.0, 1.0, 0.0, 0.0, 1.0,
  0.0
• glEnableClientState(GL_VERTEX_ARRAY)
• glVertexPointer(3, GL_DOUBLE, 0, points)
• glDrawArrays(GL_QUAD, 0, 4)

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Why Do That?

• Fewer calls
• May be faster due to call overhead.

29
Saving and Restoring Attributes

• glPopAttrib()
• glPushAttrib(mask)
• The mask may be or'd between a whole slew of
  values but the ones that would be most useful at
  the moment are
• GL_LINE_BIT
• GL_COLOR_BUFFER_BIT
• GL_POINT_BIT
• GL_POLYGON_BIT
• GL_POLYGON_STIPPLE_BIT
• GL_ALL_ATTRIB_BITS

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What Primitives Are We Missing?

• One primitive that OpenGL does not directly
  support is text!
• Glut has a facility for generating text if
  needed.
• glutStrokeCharacter( font, character)
• font is one of GLUT_STROKE_ROMAN and
  GLUT_STROKE_MONO_ROMAN
• character is a single character value (not a
  string!)
• glutBitmapCharacter(font, character)
• Same idea, different fonts.

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Sources of Information

• www.opengl.org

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Windows Example

• include ltwindows.hgtinclude ltGL/gl.hgtinclude
  ltGL/glu.hgtinclude ltwingdi.hgt
• char applicationName "OpenGL With Windows"
• / set up the pixel format for a given device
  context. Need to have a format that is
  appropriate for an OpenGL Rendering
  Context./

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• void SetDrawingContextPixelFormat( HDC
  handleDeviceContext )
• int pixelFormat static PIXELFORMATDESCRIPTOR
  pfdesc sizeof(PIXELFORMATDESCRIPTOR), //
  Size of the structure 1, // version
  PFD_DRAW_TO_WINDOW PFD_SUPPORT_OPENGL
  PFD_DOUBLEBUFFER, PFD_TYPE_RGBA, // set up as
  Red-green-blue-alpha 32, // 32 bit
  color 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 16, // Depth buffer depth in bits 0,
  0, 0, 0, 0, 0, 0

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• // Choose a pixel format supported by the
  machine// that matches the given description as
  closely as// possible.
• pixelFormat ChoosePixelFormat(handleDeviceConte
  xt, pfdesc )
• SetPixelFormat ( handleDeviceContext,
  pixelFormat, pfdesc)
• return

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• LRESULT CALLBACK WndProc(HWND window, UINT
  message, WPARAM wordParameter, LPARAM
  longParameter)
• static HGLRC renderingContext NULL static
  HDC deviceContext NULL static float angle
  0.0 switch (message) case WM_CREATE //
  Called on window creation deviceContext
  GetDC(window) // create the device
  and rendering SetDrawingContextPixelFormat(devic
  eContext) // contexts renderingContext
  wglCreateContext(deviceContext) wglMakeCurren
  t(deviceContext, renderingContext)
• SetTimer(window, 30000, 1, NULL) //
  set up the timer to expire periodically glClearC
  olor(0.0, 0.0, 0.0, 0.0) glMatrixMode(GL_PROJEC
  TION) glOrtho(-2.0, 2.0, -2.0, 2.0, -2.0,
  2.0) glMatrixMode(GL_MODELVIEW) break

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• case WM_DESTROY // Called when the
  application ends
• KillTimer(window, 101) // Kill the time
  and release
• wglMakeCurrent(deviceContext, NULL) // the
  contexts
• wglDeleteContext(renderingContext)
• PostQuitMessage(0) // Tell the
  application to exit
• break
• case WM_SIZE // Called on resize
• break

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• case WM_TIMER // Timer callback InvalidateRec
  t(window, NULL, FALSE) // Force a redraw
  -- the timer has rung break
• case WM_PAINT // Called on redisplay / redraw
• // Do the OpenGL drawing.... glMatrixMode(GL
  _MODELVIEW) glClear(GL_COLOR_BUFFER_BIT) g
  lLineWidth(8.0) glPolygonMode(GL_FRONT_AND_BAC
  K, GL_LINE) glColor3f(1.0, 1.0,
  1.0) glBegin(GL_TRIANGLES) glVertex3f(0.0
  , 0.0, 0.0) glVertex3f(0.0, 1.0,
  0.0) glVertex3f(1.0, 0.0, 0.0) glEnd()
  SwapBuffers(deviceContext) // Swap
  front and back buffers ValidateRect(window,
  NULL) break

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• default
• return DefWindowProc(window, message,
  wordParameter, longParameter)
• return 0L

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• int APIENTRY WinMain ( HINSTANCE instance,
  HINSTANCE previousInstance, LPSTR cmdLine, int
  cmdShow)
• MSG message WNDCLASS windowClass HWND
  window
• const unsigned int xPosition 50, yPosition
  50, initialWidth 600, initialHeight 600
• windowClass.style CS_HREDRAW
  CS_VREDRAW CS_OWNDC
• windowClass.lpfnWndProc WndProc windowClass.c
  bClsExtra 0 windowClass.cbWndExtra
  0 windowClass.hInstance instance windowCla
  ss.hIcon NULL windowClass.hCursor
  LoadCursor(NULL, IDC_ARROW) windowClass.hbrBackg
  round NULL windowClass.lpszMenuName
  NULL windowClass.lpszClassName
  applicationName

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• if (RegisterClass(windowClass) 0) return
  FALSE
• window CreateWindow ( applicationName, applic
  ationName, WS_OVERLAPPEDWINDOW
  WS_CLIPCHILDREN WS_CLIPSIBLINGS, xPosition,
  yPosition, initialWidth, initialHeight, NULL,
  NULL, instance, NULL)
• if (window NULL) return FALSE

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• ShowWindow(window, cmdShow) UpdateWindow(window
  )
• while (GetMessage(message, NULL, 0, 0))
  TranslateMessage(message) DispatchMessage(
  message)
• return message.wParam