Chapter 2 Graphics Programming

1
Chapter 2Graphics Programming
2
Using OpenGL in Visual C 1/3

• Opengl32.dll and glu32.dll should be in the
  system folder
• Opengl32.lib and glu32.lib should be in the lib
  folder for VC
• gl.h and glu.h should be in a folder called GL
  under the include folder for VC
• Get glut32.lib, glut32.dll and glut.h from the
  course homepage and put them in the same places
  as the other files

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Using OpenGL in Visual C 2/3

• Fire up visual studio
• Create a new project as the followingFile ? New
  ? Project (input your project name, then a
  directory (workspace) with the same name will be
  built) ? Win32 Console Application ? An Empty
  Application

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Using OpenGL in Visual C 3/3

• In the workspace click on File View to access
  (expand) the source code tree
• In Source Files, add in the source code (.c
  files)
• Select Project ? Settings ? Link andin
  Object/library modules add Opengl32.lib
  glu32.lib glut32.lib
• Press F7 to build your_project.exe

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Sierpinski Gasket 1/2

• Pick an initial point at random inside the
  triangle
• Select one of the three vertices at random
• Find the point halfway between the initial point
  and the randomly selected vertex
• Display the new point
• Replace the initial point with this new point
• Return to step 2

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Sierpinski Gasket 2/2

• main()
• initialize_the_system()
• for(some_number_of_points)
• pt generate_a_point()
• display_the_point(pt)
• cleanup()

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Programming 2D Applications

• A vertex is a location in space
• glVertex
• is in the form of nt or ntv
• n is the number of dimensions
• t denotes the data type integer (i), float (f),
  or double (d) pointer to an array (v)
• define GLfloat float

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Examples

• glVertex2i(GLint xi, GLint yi)
• glVertex3f(GLfloat x, GLfloat y, GLfloat z)
• GLfloat vertex3glVertex3fv(vertex)

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OpenGL Object Examples

• glBegin(GL_LINES) glVertex2f(x1,
  y1) glVertex2f(x2, y2)glEnd()
• glBegin(GL_POINTS) glVertex2f(x1,
  y1) glVertex2f(x2, y2)glEnd()

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Code of Sierpinski Gasket 1/3

• Typedef GLfloat point22
• void display(void)
• point2 vertices30,0,250,500,500,0
• / an arbitrary triangle /
• static point2 p 75,50
• / any desired initial point /
• int j, k
• int rand() / standard random-number generator
  /

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Code of Sierpinski Gasket 2/3

• for(k0klt5000k)
• jrand()3 / pick a random vertex from 0, 1,
  2 /
• p0(p0verticesj0)/2 / compute new
  point /
• p1(p1verticesj1)/2
• glBegin(GL_POINTS)
• glVertex2fv(p) / display new point /
• glEnd()
• glFlush()

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Code of Sierpinski Gasket 3/3
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Questions

• In what colors are we drawing
• Where on the screen does our image appear
• How large will the image be
• How do we create an area of the screen a window
  for our image?
• How long will the image remain on the screen?

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Coordinate Systems
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OpenGL API
16
Graphics Functions 1/3
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Graphics Functions 2/3

• Primitive functions points, line segments,
  polygons, pixels, text, curves, surfaces
• Attributes functionscolor, pattern, typeface
• Viewing functionsposition, orientation, clipping

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Graphics Functions 3/3

• Transformation functionsrotation, translation,
  scaling
• Input functionskeyboards, mice, data tablets
• Control functionscommunicate with windows,
  initialization, error handling
• Inquiry functions number of colors, camera
  parameters/values

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Graphics Pipeline and State Machine

• Functionalities of graphics functions
• Define primitive glvertex
• Change opengl state (most of them!)
• Present color, current matrix

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OpenGL Interface

• GL (OpenGL in Windows)
• GLU (graphics utility library)uses only GL
  functions, creates common objects (such as
  spheres)
• GLUT (GL Utility Toolkit)interfaces with the
  window system
• GLX glue between OpenGL and Xwindow, used by GLUT

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OpenGL Library Organization
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API Dilemma for Primitives

• Minimal or maximal?
• Convenience versus portability
• OpenGLs intermediate approach
• GL contains a small set of basic primitives
• GLU contains richer set of objects

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OpenGL Primitives

• Geometric primitives and raster primitives

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Points and Line Segments
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Polygon Basics 1/3
Filled objects
Methods of displaying a polygon
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Polygon Basics 2/3

• Simple, convex, and flat

Simple
Nonsimple
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Polygon Basics 3/3
Convexity
Convex objects
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Polygon Types in OpenGL 1/2
In OpenGL, edges and interior must be drawn
separately!
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Polygon Types in OpenGL 2/2
Use triangles if possible because of efficiency!
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Drawing a Sphere 1/5
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Drawing a Sphere 2/5
z
x
?
y
?
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Drawing a Sphere 3/5

• cM_PI/180.0 // degrees to radians,
  M_PI3.14159
• for(phi-80.0 philt80.0 phi20.0)
• glBegin(GL_QUAD_STRIP)
• for(theta-180.0 thetalt180.0 theta20.0)
• xsin(ctheta)cos(cphi)
• ycos(ctheta)cos(cphi)
• zsin(cphi)
• glVertex3d(x, y, z)
• xsin(ctheta)cos(c(phi20.0))
• ycos(ctheta)sin(c(phi20.0))
• zsin(c(phi20.0))
• glVertex3d(x, y, z)
• glEnd()

Drawing the portion of lower latitudes
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Drawing a Sphere 4/5

• xy0
• z1
• glBegin(GL_TRIANGLE_FAN)
• glVertex3d(x, y, z)
• cM_PI/180.0
• zsin(c80.0)
• for(theta-180.0 thetalt180.0 theta20.0)
• xsin(ctheta)cos(c80.0)
• ycos(ctheta)sin(c80.0)
• glVertex3d(x, y, z)
• glEnd()

Drawing the portion around the north pole
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Drawing a Sphere 5/5

• xy0
• z-1
• glBegin(GL_TRIANGLE_FAN)
• glVertex3d(x, y, z)
• z-sin(c80.0)
• for(theta-180.0 thetalt180.0 theta20.0)
• xsin(ctheta)cos(c80.0)
• ycos(ctheta)sin(c80.0)
• glVertex3d(x, y, z)
• glEnd()

Drawing the portion around the south pole
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Text Stroke or Raster
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Attributes for Lines, Texts
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Color 1/4
CT1RT2GT3B, T1, T2, T3 are the tristimulus
values
Additive color matching
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Color 2/4

• Basic tenet of three-color theoryIf two colors
  produce the same tristimulus values, then they
  are visually indistinguishable
• The range of colors that we can produce on a
  given system is called that systems color gamut

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Color 3/4
Color Solid
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Color 4/4
Subtractive Color
Additive Color
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RGB Color

• glColor3f(1.0, 0.0, 0.0)Can be used to specify
  24 bits true colors
• Alpha channelUsed in OpenGL as an opacity or
  transparency value
• glClearColor(1.0, 1.0,1.0,1.0)
• glPointSize(2.0)

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Indexed Color
Color-lookup table
Example km8 pick 256 out of 16M colors
43
Two-dimensional Viewing
Objects before clipping
Image after clipping
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Viewing Volume
OpenGL default 2?2?2 cube
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Orthographic View
void glOrtho(GLdouble left, GLdouble right, )
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Matrix Mode

• There are two matrices in OpenGL
• Model-view defines COP and orientation
• Projection defines the projection matrix
• glMatrixMode(GL_PROJECTION)glLoadIdentity()glu
  Ortho2D(0.0, 500.0, 0.0, 500.0)glMatrixMode(GL_M
  ODELVIEW)

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Control Functions

• OpenGL assumes origin is bottom left
• glutInit(int argcp, char argv)
• glutCreateWindow(char title)
• glutInitDisplayMode(GLUT_RGB GLUT_DEPTH
  GLUT_DOUBLE)
• glutInitWindowSize(480,640)
• glutInitWindowPosition(0,0)
• OpenGL default RGB color, no hidden-surface
  removal, single buffering

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Aspect Ratio
Viewing rectangle glOrtho()
Display window glutInitWindowSize()
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Viewports
void glViewport(GLint x, GLint y, GLsizei w,
GLsizei h) Viewport is part of the state.
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main, display, myinit functions

• void glutMainLoop(void)Why do we need this?
• void glutDisplayFunc(void (func)(void))When is
  the function invoked?

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Sample main program

• Void main(int argc, char argv)
• glutInit(argc,argv)
• glutInitDisplayMode(GLUT_SINGLE
  GLUT_RGB)glutInitWindowSize(500, 500)
• glutInitWindowPosition(0, 0)glutCreateWindow(S
  imple OpenGL example)
• glutDisplayFunc(display)
• myinit()
• glutMainLoop()

For most non-interactive applications
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Gasket Program 1/3

• void myinit(void)
• / attributes /
• glClearColor(1.0, 1.0, 1.0, 1.0) / white
  background /
• glColor3f(1.0, 0.0, 0.0) / draw in red /
• / set up viewing /glMatrixMode(GL_PROJECTION)
  glLoadIdentity()gluOrtho2D(0.0, 500.0, 0.0,
  500.0)glMatrixMode(GL_MODELVIEW)

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Gasket Program 2/3

• void display(void)
• typedef GLfloat point22 / define a point
  data type /point2 vertices30.0,0.0,250.0,
  500.0,500.0,0.0 / triangle /
• int i, j, kint rand()
• point2 p75.0, 50.0 / arbitrary point
  inside triangle /
• glClear(GL_COLOR_BUFFER_BIT) / clear the
  window /

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Gasket Program 3/3

• / compute and output 5000 new points /
• for(k0 klt5000 k)
• jrand()3 / compute point halfway between
  vertex and old point / p0(p0vertexj0)/
  2.0 p1(p1vertexj1)/2.0
• glBegin(GL_POINTS) / plot point
  / glVertex2fv(p) glEnd()
• glFlush()

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Using Recursion 1/3

• void triangle(point2 a, point2 b, point2 c)
• glBegin(GL_TRIANGLES)
• glVertex2fv(a)
• glVertex2fv(b)
• glVertex2fv(c)
• glEnd()

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Using Recursion 2/3

• void divide_triangle(point2 a, point2 b, point2
  c, int k)
• point2 ab, ac, bc
• int j
• if(kgt0)
• for(j0 jlt2 j) abj(ajbj)/2
• for(j0 jlt2 j) acj(ajcj)/2
• for(j0 jlt2 j) bcj(bjcj)/2
• divide_triangle(a, ab, ac, k-1)
• divide_triangle(c, ac, bc, k-1)
• divide_triangle(b, bc, ab, k-1)
• else triangle(a, b, c) / draw triangle at end
  of of recursion /

a
ab
ac
b
c
bc
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Using Recursion 3/3

• Void display(void)
• glClear(GL_COLOR_BUFFER_BIT)
• divide_triangle(v0, v1, v2, n)
• glFlush()

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Three-Dimensional Gasket 1/2

• typedef Glfloat point33
• point3 vertices4 0,0,0,0,250,500,100,500
  ,250,250,250,100,250
• Point3 new,old250, 100, 250
• jrand()4
• new0(old0verticesj0)/2
• new1(old1verticesj1)/2
• new2(old2verticesj2)/2

tetrahedron
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Three-Dimensional Gasket 2/2

• / plot point /
• glBegin(GL_POINTS)
• glVertex3fv(new)
• glEnd()
• / replace old point by new /
• old0new0
• old1new1
• old2new2

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3D Recursive Gasket 1/3

• void triangle(point3 a, point3 b, point3 c)
• glBegin(GL_POLYGON)
• glVertex3fv(a)
• glVertex3fv(b)
• glVertex3fv(c)
• glEnd()

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3D Recursive Gasket 2/3

• void divide_triangle(point3 a, point3 b, point3
  c, int k)
• point3 ab, ac, bc
• int j
• if(kgt0)
• for(j0 jlt3 j) abj(ajbj)/2
• for(j0 jlt3 j) acj(ajcj)/2
• for(j0 jlt3 j) bcj(bjcj)/2
• divide_triangle(a, ab, ac, k-1)
• divide_triangle(c, ac, bc, k-1)
• divide_triangle(b, ab, ab, k-1)
• else triangle(a, b, c) / draw triangle at end
  of recursion/

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3D Recursive Gasket 3/3

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

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Hidden-Surface Removal 1/3
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Hidden-Surface Removal 2/3

• Z-Buffer
• glutInitDisplayMode(GLUT_SINGLE GLUT_RGB
  GLUT_DEPTH)
• glEnable(GL_DEPTH_TEST)
• glClear(GL_COLOR_BUFFER_BIT GL_DEPTH_BUFFER_BIT)
  

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Hidden-Surface Removal 3/3

• void display()
• glClear(GL_COLOR_BUFFER_BIT
  GL_DEPTH_BUFFER_BIT)
• tetrahedron(n)
• glFlush()

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Summary and Notes

• Coding OpenGL in Visual C
• OpenGL API
• Graphics (primitives, attributes), viewing,
  control, communications with windows
• Sample codes