Computer Graphics

1
Computer Graphics

• Week -4, Chapter three, Section 3-14-----3-24

2
Goals

• Fill-area primitives
• Pixel-array primitives and OpenGL pixel-array
  functions
• Character primitives
• OpenGL character functions
• OpenGL display lists
• OpenGL display-window reshape function

3
Fill-area primitives

• A fill-area is an area that is considered as
  having interior.
• Triangle plate is a fill-area
• Triangle made of three ropes is not
• Fill-are primitives are those geometric drawings
  to form triangles, polygons, etc.
• Primitive fill areas are in general considered as
  planar surfaces.
• Other fill areas or surface can be approximated
  by polygon surfaces. It is called surface
  tessellation, or fitting the surface with
  polygon mesh. Display of a surface in polygon
  mesh is called wire-frame.

4
Surface tessellation
Surface tessellation
Wire-frame of the surface
The whole collection of Polygons is called
polygon mesh
5
OpenGL fill-area functions (3-16)

• Triangles
• Independent traingles
• glBegin(GL_TRIANGLES)
• //requires 3, 6, 9, etc. many
  vertices.
• glEnd()
• Triangle strip
• glBegin(GL_TRIANGLE_STRIP)
• //requires 3, or more vertices.
• glEnd()
• Triangle fan
• glBegin(GL_TRIANGLE_FAN)
• //requires 3, or more vertices.
• glEnd()
• Polygons
• glBegin(GL_POLYGON)
• // requires three or more
  vertices
• glEnd()
• Quads
• glBegin(GL_QUADS)

6
Examples
glBegin(GL_TRIANGLES) glVertex(p0)
glVertex(p1) glVertex(p2) glVertex(p3)
glVertex(p4) glVertex(p5) glEnd()
glBegin(GL_TRIANGLE_STRIP) glVertex(p0)
glVertex(p1) glVertex(p2) glVertex(p3)
glVertex(p4) glVertex(p5) glEnd()
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Example
p1
glBegin(GL_TRIANGLE_FAN) glVertex(p0)
glVertex(p1) glVertex(p2) glVertex(p3)
glVertex(p4) glVertex(p5) glEnd()
p2
p0
p3
p4
p1
glBegin(GL_POLYGON) glVertex(p0)
glVertex(p1) glVertex(p2) glVertex(p3)
glVertex(p4) glVertex(p5) glEnd()
p0
p2
p3
p4
p5
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Example
glBegin(GL_QUADS) glVertex(p0)
glVertex(p1) glVertex(p2) glVertex(p3)
glVertex(p4) glVertex(p5)
glVertex(p6) glVertex(p7) glEnd()
glBegin(GL_QUAD_STRIP) glVertex(p0)
glVertex(p1) glVertex(p2) glVertex(p3)
glVertex(p4) glVertex(p5)
glVertex(p6) glVertex(p7) glEnd()
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Draw in opengl

• Try codes

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Polygon faces

• Polygon representation
• Front face and back face
• Each can have different color
• The order can be reset

11
OpenGL vertex arrays

• Using glVertex3iv (GLint )
• Using glVertex3fv(GLfloat )
• Example Draw a cube
• Glint pt83 0,0,0, 0,1,0, 1,0,0,
  1,1,0, 0,0,1, 0,1,1, 1,0,1, 1,1,1
• void quad(GLint n1, GLint n2, GLint n3, GLint
  n4)
• glBegin(GL_QUADS)
• glVertex3iv(ptn1)
• glVertex3iv(ptn2)
• glVertex3iv(ptn3)
• glVertex3iv(ptn4)
• glEnd()
• void cube()
• quad(6,2,3,7)
• quad(5,1,0,4)
• quad(7,3,1,5)
• quad(4,0,2,6)
• quad(2,0,1,3)
• quad(7,5,4,6)

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Using Vertex Array in OpenGL

• Purpose to reduce function calls in server side.
• Steps
• Activate the vertex-array feature
• Point to the location and data format
• Display the routine
• Example
• glEnableClientState(GL_VERTEX_ARRAY)
• glVertexPointer(3, GL_INT, 0, pt)
• GLubyte vertIndex 6,2,3,7,5,1,0,
• 4,7,3,1,5,4,0,2,6,2,0,1,3,7,5,4,6)
• glDrawElements (GL_QUADS, 24,
• GL_UNSIGNED_BYTE, vertIndex)

13
Pixel-array primitives

• Display a shape defined by a rectangle array of
  color values, equivalently, display a pixmap.
• The simplest case of it is the so-called mask
  whether or not a pixel is to be assigned a preset
  color.

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Defining pixel-array patterns

• Defining a bitmap pattern (or mask)
• glBitmap(width, height, x0, y0, xOffset, yOffset,
  bitShape)
• Example
• GLubytes bitshape20 0x1c, 0x00, 0x1c, 0x00,
  0x1c, 0x00, 0x1c, 0x00, 0x1c, 0x00, 0xff, 0x80,
  0x7f, 0x00, 0x3e, 0x00, 0x1x, 0x00, 0x08, 0x00)
• glPixelStorei(GL_UNPACK_ALIGNMENT, 1)
• glRasterPos2i(3,4)
• glBitmap(9, 10, 0.0, 0.0, 20.0, 15.0, bitshape)

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A bit pattern, defined in an array with 10 rows
and 9 columns, Is stored in 8-bit blocks of 10
rows With 16 bit values per row. It is displayed
on the monitor At (3,3).
15
Buffers

• Storing colors (or color buffers)
• When supporting stereoscopic view Left, right
• When supporting double buffer animation Front,
  back pair for both left and right.
• When none is supported, the default is front-left
  color buffer.
• Symbolic constants GL_FRONT_LEFT,
  GL_FRONT_RIGHT, GL_BACK_LEFT, GL_BACK_RIGHT,
  GL_FRONT, GL_BACK, GL_LEFT, GL_RIGHT,
  GL_FRONT_AND_BACK.
• Storing other pixel information
• Depth buffer----object distances from the viewing
  position
• Stencil buffer----boundary patterns for a scene.
• Symbolic constants GL_DEPTH_COMPONENT,
  GL_STENCIL_INDEX.

16
Defining pixel-array patterns

• Using glDrawPixels and glDrawBuffer function
• glDrawPixels(width,height, dataFormat, dataType,
  pixMap)
• glDrawPixels(128,128,GL_RGB, GL_UNSIGNED_BYTE,colo
  rShape)
• glDrawBuffer(buffer)
• Using glReadPixels and glReadBuffer function
• glReadPixels(xmin,ymin,width,height,dataFormat,dat
  aType,array)
• glReadBuffer(buffer)

17
Logic operator

• glEnable(GL_COLOR_LOGIC_OP)
• glLogicOp(logicOp)
• Check out this example

18
Character primitives

• Bitmap font
• glutBitmapCharacter (font, character)
• Fast to render, directly put into the frame
  buffer.
• Example
• glRasterPosition2i(x,y)
• Char text H, e, l, l, o, ,,a,
  s, u, r, a, m, s
• For(k0 klt12 k)
• glutBitmapCharacter(GLUT_BITMAP_9_BY_15,textk
  )
• Stroke font
• glutStrokeCharacter(font, character)
• The size and position is adjusted by the
  transfomration.
• slower to render, draw it into the frame buffer.
• Example
• char str Hello, Golden Rams!
• for(i0 iltstrlen(str) i)
• glutStrokeCharacter(GLUT_STROKE_ROMAN,
  stri)

19
OpenGL display lists

• To save an object description of fixed shape and
  size (We can still transform it later).
• The commends are stored in the server-side, which
  reduces the transmit of those commands when the
  object is needed.
• In a display list, no uncertain information can
  be included. Hence no variables and no commands
  that may change during execution.

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Example for using display list

• Refer to the example on Page 153
• Const double TWO_PI 6.2831853
• GLuint regHex
• GLdouble theta
• GLint x,y,k
• regHex glGenLists(1)
• glNewList(regHex, GL_COMPILE)
• glBegin(GL_POLYGON)
• for(k0 klt6 k)
• theta TWO_PIk/6.0
• x 200150cos(theta)
• y 200150sin(theta)
• glVertex2i(x,y)
• glEnd()
• glEndList()
• glCallList(regHex)

21
Other features of Display lists

• List base
• Execute several lists
• Delete lists

22
OpenGL reshape function

• Adjust the display-window when it is reshaped.
• Using function glutReshapeFunc(functionName) to
  define the callback.
• Put it in main function.
• Using the example in Page 156.