More Primitives

1
More Primitives Attributes

• September 1, 2004

2
This Example

• We will try to do a fair amount with a very
  simple basic object a box.
• The width, depth, height, and position of the box
  can be specified.
• We will also look at how to "move around" the box
  and how to get the hidden surfaces to look
  "right".

3
Preamble

• /
• This example will build a Table and Chair
  using only boxes
• and will use a STL list to hold onto the
  objects to be
• displayed.
• The main reason for this example is to give
  you an idea of
• what can be done with a simple type of object
  and a little
• imagination.
• Author Jerry Heuring
• Date August 31, 2004
• /
• include ltcmathgt
• include "glut.h"
• include ltlistgt
• using namespace std

4
Box Class

• class Box private double location3
  float color3 bool filled double
  width, height, depthpublic Box(double x
  0.0, double y 0.0, double z 0.0,
• double width 1.0, double height 1.0, double
  depth 1.0,
• float red 1.0, float green 1.0, float blue
  1.0) void setFilled(bool f) bool
  isFilled() void setLocation(double x, double
  y, double z) void setColor(float red, float
  green, float blue) void display()

5
Box Constructor

• BoxBox (double x, double y, double z, double
  width, double height, double depth, float red,
  float green, float blue)
• location0 x
• location1 y
• location2 z
• color0 red
• color1 green
• color2 blue
• this-gtwidth width
• this-gtheight height
• this-gtdepth depth
• filled true

6
Get/Set Methods

• void BoxsetFilled(bool f) filled f
• bool BoxisFilled() return filled
• void BoxsetLocation ( double x, double y,
  double z) location0 x location1
  y location2 z
• void BoxsetColor( float r, float g, float b)
  color0 r color1 g color2
  b

7
display method

• void Boxdisplay () glMatrixMode(GL_MODELVI
  EW) glPushAttrib(GL_ALL_ATTRIB_BITS)
  glColor3f( color0, color1, color2 ) if
  (filled) glPolygonMode(GL_FRONT_AND_BACK,
  GL_FILL) else glPolygonMode(GL_FRONT_AN
  D_BACK, GL_LINE) glLineWidth(4.0) // Wide
  Lines so we can see them
  glBegin(GL_QUAD_STRIP) glVertex3d(
  location0, location1, location2) glVertex
  3d( location0, location1 height,
  location2) glVertex3d( location0width,
  location1, location2) glVertex3d(
  location0width, location1 height,
  location2) glVertex3d( location0width,
  location1, location2depth )

8

• glVertex3d( location0width, location1
  height, location2 depth) glVertex3d(
  location0, location1, location2
  depth) glVertex3d( location0, location1
  height, location2depth) glVertex3d(
  location0, location1,
  location2) glVertex3d( location0,
  location1 height, location2) glEnd()
  glBegin(GL_QUADS) glVertex3d (location0,
  location1, location2) glVertex3d
  (location0width, location1,
  location2) glVertex3d (location0,
  location1, location2depth)

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• glVertex3d (location0width, location1,
  location2depth)
• glVertex3d (location0, location1height,
  location2)
• glVertex3d (location0width,
  location1height, location2)
• glVertex3d (location0, location1height,
  location2depth)
• glVertex3d (location0width,
  location1height, location2depth)
• glEnd()
• glPopAttrib()

10
Prototypes and Globals

• //
• // Prototypes...
• //
• void buildTable()
• void buildChair()
• void initialize()
• void display()
• void updatePosition(int)
• void handleKey(unsigned char ch, int x, int y)
• /
• All our objects will be stored in a STL List
• /
• static listltBoxgt objectList
• static double angle 0.0
• const double M_PI atan(1.0) 4.0

11
main()

• /
• The main program builds the objects and hooks
  up routines
• for handling redisplay, keystrokes and a
  timer.
• /
• int main (int argCount, char argValues)
  glutInit(argCount, argValues)
  glutInitDisplayMode(GLUT_RGBA GLUT_DEPTH)
  glutInitWindowSize(700, 700)
  glutCreateWindow("Table Example") initialize()
  buildTable() buildChair()
  glutDisplayFunc(display) glutTimerFunc(500.0,upd
  atePosition, 1) glutKeyboardFunc(handleKey)
  glutMainLoop() return EXIT_SUCCESS

12
initialize()

• void initialize()
• /
• this routine is largely lifted from the
  hoops example.
• I've added a statement to enable depth
  testing so that
• Hidden surfaces are handled correctly.
• / glEnable (GL_DEPTH_TEST)
  glClearColor(1.0, 1.0, 1.0, 1.0) // White for
  the clear color glMatrixMode(GL_PROJECTION)
  glLoadIdentity() glOrtho(-70.0, 70.0,
  -40.0, 100.0, -100.0, 100.0)
  glMatrixMode(GL_MODELVIEW)

13
display()

• void display()
• / Use an iterator to step through the list
  and display each piece... We need to do a
  few things here. We will clear a Depth
  Buffer. It is necessary to do the hidden
  surfaces. Also, have added a call to
  gluLookAt to have us circle the object for
  inspection purposes. / listltBoxgtiterator
  iter glMatrixMode(GL_MODELVIEW)
  glClear(GL_COLOR_BUFFER_BIT GL_DEPTH_BUFFER_BIT)
  glLoadIdentity() gluLookAt(sin(angle M_PI
  / 180.0)3525.0, 15.0, cos (angleM_PI /
  180.0)-40.0, // eyepoint shifts 25.0,
  15.0, 0.0, // look at point 0.0, 1.0, 0.0)
  // up vector

14

• for (iter objectList.begin() iter !
  objectList.end() iter)
• iter-gtdisplay()
• glFlush()

15
buildTable()

• / Build a table out of boxes. Just a top and
  4 legs. /
• void buildTable()
• // Table top
• objectList.push_back(Box( 0.0, 30.0, 0.0,
  60.0, 2.0, 20.0, 1.0, 0.5, 0.5))
• // The 4 legs at the corners in a different
  color
• objectList.push_back(Box( 0.0, 0.0, 0.0,
  2.0, 30.0, 2.0, 0.5, 1.0, 0.5))
• objectList.push_back(Box( 58.0, 0.0, 0.0,
  2.0, 30.0, 2.0, 0.5, 1.0, 0.5))
• objectList.push_back(Box( 0.0, 0.0, 18.0,
  2.0, 30.0, 2.0, 0.5, 1.0, 0.5))
• objectList.push_back(Box( 58.0, 0.0, 18.0,
  2.0, 30.0, 2.0, 0.5, 1.0, 0.5))

16
buildChair()

• void buildChair() / Creates the
  boxes that make up a chair. Note that the
  chair is always in one place. It might be nice
  to make it possible to set a location...
  / // The seat of the chair
  objectList.push_back(Box(-10.0, 16.0, 2.0,
  16.0, 1.0, 16.0, 0.5, 0.5,
  1.0)) // The two Back legs/supports
  objectList.push_back(Box(-10.0, 0.0, 2.0,
  2.0, 40.0, 2.0, 0.5, 1.0, 1.0))
  objectList.push_back(Box(-10.0, 0.0, 16.0,
  2.0, 40.0, 2.0, 0.5, 1.0, 1.0))

17

• // The two Front legs (under the table)
• objectList.push_back(Box( 4.0, 0.0, 2.0,
  2.0, 16.0, 2.0, 0.5, 1.0, 1.0))
  objectList.push_back(Box( 4.0, 0.0, 16.0,
  2.0, 16.0, 2.0, 0.5, 1.0, 1.0))
  // 3 slats up the back of the chair
  objectList.push_back(Box(-10.0, 24.0, 2.0,
  2.0, 3.0, 16.0, 0.5, 1.0, 1.0))
  objectList.push_back(Box(-10.0, 32.0, 2.0,
  2.0, 3.0, 16.0, 0.5, 1.0, 1.0))
  objectList.push_back(Box(-10.0, 40.0, 2.0,
  2.0, 3.0, 16.0, 0.5, 1.0, 1.0))

18
updatePosition()

• / The timer routine. It shifts the viewing
  angle by 5 degrees and redraws the display.
  Hooking up the Timer function again is
  necessary in windows but not in Unix /
• void updatePosition(int value) angle
  5.0 if (angle gt 360.0) angle -
  360.0 glutTimerFunc(500.0, updatePosition,
  1) glutPostRedisplay()

19
handleKey()

• / handle the keystrokes -- currently only 3
  characters are recognized x -- exit
  w -- wireframe f -- filled /void
  handleKey(unsigned char ch, int x, int
  y) listltBoxgtiterator iter if (ch 'x')
  exit(0) else if (ch 'w') for (iter
  objectList.begin() iter !
  objectList.end() iter) iter-gtsetFilled(fal
  se) glutPostRedisplay() else if (ch
  'f') for (iter objectList.begin() iter
  ! objectList.end() iter) iter-gtsetFilled(
  true) glutPostRedisplay()

20
gluLookAt

• Used to change the viewpoint
• gluLookAt (double eyex, double eyey, double
  eyez, double xcenter, double ycenter, double
  zcenter, double upx, double upy, double upz)
• Common Problems
• forget to load the identity matrix (reset the
  current transform)
• change it while in Projection Mode

21
Depth Buffer

• The depth buffer compares the distance each pixel
  is from the eye while drawing objects. If the
  object is closer than the current pixel's depth
  it updates it with the new one. If it is further
  it remains unchanged.
• 3 Pieces
• glut needs to allocate a buffer
• depth processing must be enabled
• need to clear the depth buffer when we clear the
  screen.

22
Call Lists

• Lists of primitives and certain changes can be
  combined into a Call List in OpenGL.
• Advantages
• Can save time for certain classes of operations
• Matrix Operations
• Raster bitmaps and images
• Lights, Materials, and Lighting Methods
• Polygon Stipples

23
Basic Calls

• glGenLists(1)
• glNewList (GLuint listID, GLenum mode)
• listID is the id for the list
• mode is either GL_COMPILE or GL_COMPILE_AND_EXECUT
  E
• glEndList()
• glCallList(GLuint listID)

24
Example Snippet

• GLuint listID
• listID glGenLists(1)glNewList(listID,
  GL_COMPILE)
• glBegin(GL_QUAD_STRIP) glVertex3d()
• glEnd()
• glEndList()
• glCallList (listID)

25
What is Really Happening?

• The system is generating some code for OpenGL to
  output to the screen. It does not keep track of
  any or your variables.
• For items where OpenGL may need to do a fair
  amount of calculation it may be more efficient to
  store in a call list rather than recompute.

26
Does it Help Us?

• This really doesn't help us -- yet.
• Multiple copies of something drawn in the same
  location aren't helpful.
• Haven't dealt with Materials and Lights.
• Not working with images

27
When Will it Help Real Soon Now

• We would like to transform objects
• Basic Affine Transformations
• Translation (moving something)
• Rotation (turning something)
• Scaling (shrinking or enlarging something)

28
How Does This Help With Call Lists?

• With Transformations call lists are useful since
  our table could all be 1 cube scaled and
  translated appropriately.
• Also, transformations can be stored in call
  lists. As we will see a transformation often
  involves a matrix multiplication that could be
  done ahead of time.